Download β Relation for Local Galaxies

The Nature of the Second Parameter in the IRX-β Relation for
Local Galaxies
Kathryn Grasha1 , Daniela Calzetti1 , Janice C. Lee2 , Daniel A. Dale3
ABSTRACT
We present an analysis of 98 galaxies of low-dust content, selected from the
Spitzer Local Volume Legacy (LVL) survey, aimed at examining the dust attenuation relation in normal star-forming galaxies. The infrared-excess (IRX-β)
diagram is a technique used to correct star-forming galaxies for dust attenuation
solely from observations of the ultraviolet (UV) colors, β. The UV colors are
tightly related to the total attenuation as measured by the ratio of the total infrared (TIR) to the UV flux in starburst galaxies. Previous research has, however,
indicated that normal star-forming galaxies, when compared to their starburst
counterparts, do not follow the same dust attenuation relation and have a much
larger spread in the TIR to far-UV (FUV) luminosity for a fixed UV spectral
slope. We investigate the reason(s) for which normal star-forming galaxies deviate from the IRX-β starburst attenuation relation, examining the role that the
age of the stellar population plays as the “second parameter” responsible for the
observed deviation. We model the FUV to far-infrared spectral energy distribution (SED) of each galaxy using Starburst99 synthetic stellar spectra with a
wide range of varying parameters, including metallicity, attenuation, age, and we
include both constant star formation and instantaneous bursts. We find that,
in virtually dust-free galaxies, the stellar population age influences galaxies that
are represented with an instantaneous star formation history (SFH), where an
increase in β correlates with an increase in the stellar population age at a significance level of 5σ. We also find that dust-free galaxies represented with a
continuous star formation rate (SFR) do not appear to have any correlation between the duration of star forming activity with the observed UV color. We use
the EW(Hα) as a proxy for the birthrate parameter, where a correlation between
the equivalent width (EW) of Hα with both the UV spectral slope and the distance of a galaxy to the starburst IRX relation, is seen for continuous star forming
1
Astronomy Department, University of Massachusetts, Amherst, MA 01003, USA
2
STScI, 3700 San Martin Drive, Baltimore, MD 21218, USA
3
Department of Physics, University of Wyoming, Laramie, WY 82071, USA
–2–
galaxies at a 4σ confidence level, in agreement with previous results. For both
types of galaxies, there is an increase in degeneracy as galaxies with larger contributions from the infrared are considered, i.e. galaxies with increasing amounts
of dust. Lack of a simple relationship for all types of galaxies suggests that the
UV attenuation in normal star-forming galaxies may not be recovered with UV
color alone and is highly influenced by the SFH. As a whole, we find that our
galaxies have a tight correlation between the far-UV to near-infrared luminosity
and β, suggesting that the scatter from the “second parameter” is better defined
in terms of β as opposed to the distance from the starburst IRX relation.
Subject headings: galaxies: star formation
1.
Introduction
Detailed knowledge of the stellar populations of galaxies is one of the ways to gain key
insight into the evolution and formation of galaxies in the universe. Studying the stellar
populations that give rise to the observed spectral energy distribution (SED) of a galaxy
supplies knowledge of the type of stars within the galaxy and gives estimates of the star
formation rate (SFR) along with the star formation history (SFH). A major obstacle in
studying the intrinsic SED of a galaxy is correcting for the ultraviolet (UV) and optical flux
lost to dust attenuation and being re-emitted in the infrared (IR). UV wavelengths, while the
most susceptible to the effects of dust, provide insights on the young stellar populations, while
longer wavelengths provide information on the older and more evolved stellar populations.
Observations from the UV to the far-infrared (160 µm; FIR) constrain the amount of dust
attenuation in a galaxy, giving rise to key insights of the physical processes at play in
galaxies. Multiwavelength observations allow for an effective means of disentangling the
different stellar populations within the observed galaxy SED. A method that allows the
recovery of UV flux lost to dust will improve knowledge on the evolution of all types of
galaxies at all redshifts. This becomes vitally important as surveys explore deeper into
the universe, discovering galaxies that occupy the fainter end of the luminosity function.
Understanding the physical properties of how stars form in these high-redshift systems plays
a vital role in our knowledge of galaxy evolution.
At high-redshift, where multiwavelength information is often limited, correcting for dust
attenuation in galaxies is commonly done with the infrared (IR) excess (IRX-β) relation,
which relates the observed UV colors (β) to the fraction of UV stellar emission absorbed
by dust and re-emitted in the FIR. These two quantities are correlated in local starburst
galaxies (Meurer et al. 1999) and in high-redshift systems (Reddy et al. 2010, 2012). The
–3–
IRX was originally defined by Meurer et al. (1999) as the ratio of the total IR luminosity to
the FUV luminosity as
LFIR
,
(1)
IRX ≡ log
L1600
where LFIR (erg s−1 cm−2 ) is defined as the integrated dust luminosity between 20 − 100 µm
from IRAS (Infrared Astronomical Satellite) observations, and L1600 is the luminosity in the
FUV from IU E (International Ultraviolet Explorer) data at 1600 Å. The value of the IRX
diagram correlates with the UV spectral index β, defined as the power-law
f λ ∝ λβ ,
(2)
where fλ is the flux per unit wavelength (erg s−1 cm−2 Å−1 ), used to estimate the attenuation
in the UV (Calzetti et al. 1994). This result gives an empirical relationship between β and
a correction for dust attenuation, allowing the recovery of the intrinsic UV flux solely from
the UV colors and completely independent of the distribution and properties of the dust.
Kong et al. (2004) introduced a different definition of the IRX,
LTIR
IRX ≡ log
,
LFUV
(3)
where LTIR (erg s−1 cm−2 ) is the total integrated IR luminosity between 8 − 1000 µm from,
e.g., Spitzer (Spitzer Space T elescope) IR observations and the flux in the FUV is from
GALEX (GALaxy Evolution eXplorer; Martin et al. 2005) observations. The spectral slope
βGLX is found with FUV and NUV photometric data from GALEX as
βGLX =
log fFUV − log fNUV
,
log λFUV − log λNUV
(4)
where fFUV and fNUV (erg s−1 cm−2 Å−1 ) is the flux density per unit wavelength of the FUV
(λef f = 1520 Å) and NUV (λef f = 2310 Å) bands. In some literature, the UV spectral
slope is given simply as the difference between the FUV and NUV magnitudes; we adopt
the GALEX definition of the UV spectral slope in our work from here onward in this paper.
We also adopt the IRX definition as defined by Kong et al. (2004), employing the total IR
luminosity LTIR , calculated from Spitzer observations.
While the IRX relationship provides accurate dust attenuation estimates for starburst
galaxies, it does not apply to quiescent, normal star-forming galaxies, where the older stellar
population contaminates the observed UV SED (Kong et al. 2004). Since the seminal work of
Meurer et al. (1999), defining the relationship between the UV and IR properties for starburst
galaxies, a great deal of effort has been dedicated to studying the IRX diagram from the
global flux of galaxies (Kong et al. 2004; Siebert et al. 2005; Buat et al. 2005; Johnson et al.
–4–
2007; Dale et al. 2007, 2009) and spatially resolved galactic regions (Bell et al. 2002; Gordon
et al. 2004; Calzetti et al. 2005; Boquien et al. 2009, 2012; Mao et al. 2012). The quiescent
galaxies on the IRX show a considerable amount of scatter compared to the starburst IRX
relation. Currently, no single relation for normal star-forming galaxies exists between βGLX
the UV attenuation nor is there agreement as to the underlying physical reason that causes
the spread. It is necessary to unravel the reasons for which the normal star-forming galaxies
deviate from the starburst IRX relation to better understand the nature of the relationship
between the UV and IR properties of galaxies.
Much work has been done to try and account for this deviation, where Kong et al. (2004)
explains the offset of normal star-forming galaxies in terms of the birthrate parameter b,
which accounts for present to past-averaged SFRs, where normal star-forming galaxies have
a much lower ratio of the b-parameter compared to starburst galaxies. However, the results of
Boquien et al. (2009, 2012), and Siebert et al. (2005) do not support the birthrate parameter
result; instead, they suggest that the offset may merely be a result of the difference in dust
geometry between normal star-forming and starburst galaxies. However, works by Burgarella
et al. (2005); Cortese et al. (2006, 2008); Dale et al. (2009), and Reddy et al. (2012) show a
connection between the age of the stellar population, and hence the SFH, and the observed
spread in the IRX-β, where the b-parameter influences a galaxy on the IRX diagram and
lower birthrate systems are generally located further from the starburst IRX relationship.
The small aperture of the IU E photometry used in Meurer et al. (1999) for the formulation of the IRX also may have applied a systematic offset, where UV flux densities may have
been severely underestimated, resulting in an overestimation of the IRX values for starburst
galaxies and further impeding the reconciliation of the relation between normal star-forming
and starburst galaxies. Recent work of Takeuchi et al. (2012) has attempted to account for
the small aperture size of the IU E to reconcile the deviation of normal star-forming galaxies
from that of starburst galaxies. They showed that when the work of Meurer et al. (1999)
is corrected for the aperture effect, normal star-forming galaxies have less of an offset from
the starburst relation, however, there is still large scatter present and a second parameter is still required to account for the scatter. Work by Burgarella et al. (2005); Johnson
et al. (2007), and Boquien et al. (2012) showed that galaxies with a higher β corresponds to
steeper attenuation, allowing for the possibility that variations in the adopted attenuation
curve could cause the spread in the IRX diagram. A steepening in the attenuation curve as
a galaxy moves toward higher β values suggests a transition exists in the attenuation law
from highly efficient SFRs (starburst-type galaxies) to more quiescent, normal star-forming
galaxies. Work by Calzetti (2001); Boquien et al. (2009), and Reddy et al. (2010, 2012)
showed that adopting different extinction curves and dust geometries can impact the IRX
diagram, where the age of the recovered stellar population is determined by the adopted
–5–
extinction curve. Finally, Hao et al. (2011) found that β is a poor predictor of the amount
of dust attenuation present in normal star-forming galaxies.
The literature so far has not presented a clear case for the “second parameter” in the
IRX-β relation that drives most of the scatter. Starburst galaxies are well-fit with a single attenuation relation because they are dominated in the UV wavelengths by the young
stellar populations formed in the most recent burst. More quiescent galaxies have a larger
presence of older/aging stellar populations, giving rise to a non-negligible flux contribution
in the optical, IR and, primarily, the UV regime. It is necessary to disentangle the SEDs
of non-starburst galaxies into the young stellar population, dust, and evolved stellar populations in order to tackle the scatter of the IRX relation for normal star-forming galaxies.
Multiwavelength data are required in order to properly investigate the parameters responsible for the deviation between the UV attenuation and β in normal, star-forming galaxies.
In this paper, we will attempt to address how the age of the stellar population influences
the IRX diagram with a combination of UV, optical, and IR photometric data in normal
star-forming galaxies that are selected to have virtually no dust effects. This will allow us
to quantify whether the mean stellar population age is a viable second parameter, and can
be responsible for most of the deviation of normal star-forming galaxies from the starburst
dust attenuation relationship. We do not focus on converting LTIR /LFUV into an estimate
of the UV attenuation AFUV .
Throughout this paper, we adopt a ΛCMD concordance cosmology model of Ωm = 0.27,
ΩΛ = 0.73, and H◦ = 70 km s−1 Mpc−1 (Komatsu et al. 2011). All numbers taken from the
literature are re-calculated (if necessary) to this cosmological model.
2.
Sample Selection
Our sources are selected in order to best answer the question of why normal starforming galaxies deviate from the IRX relation for starburst galaxies. With this in mind,
we have selected a subset of virtually dust-free galaxies from the Spitzer Local Volume
Legacy (LVL; Dale et al. 2009) survey, which has captured predominantly low-metallicity,
low-luminosity dwarf and irregular galaxies. All 258 galaxies in the LVL survey are local
(D < 11 Mpc) galaxies that avoid the Galactic plane (|b| > 20◦ ), and are brighter than
B = 15.5 magnitude (Lee et al. 2011). The LVL sample is built on UV, Hα, and HST
(Hubble Space T elescope) imaging from the 11 Mpc Hα and Ultraviolet Galactic Survey
(11HUGS; Kennicutt et al. 2008) and the Advanced Camera for Surveys (ACS) Nearby
Galactic Survey Treasury (ANGST; Dalcanton et al. 2009), where the galaxies from the ACS
data set are |b| > 20◦ and D < 3.5 Mpc and the Hα images are |b| > 30◦ and D < 11 Mpc,
providing a statistically robust and complete sample of the nearest galaxies to the Milky
–6–
Way (MW). The LVL provides an exceptional way to study the star formation activity in
a sample of low-mass, low-surface brightness systems that are not flux-limited. We require
that all the galaxies in our analysis must have readily available observations at the FUV,
NUV, U, B, V, J, H, Ks, IRAC 3.5 µm, 4.5 µm, 5.8 µm, 8.0 µm, MIPS 24 µm, 70 µm, and
160 µm wavelengths.
For the FUV and NUV bands, we use GALEX observations. For the optical bands of
U, B, and V, we use photometric data from, in order of preference; the Third Reference
Catalogue of Bright Galaxies (RC3; de Vaucoulers et al. 1995; Corwin et al. 1994), the
Vatican Advanced Technology Telescope (VATT; Taylor et al. 2005), and the Sloan Digital
Sky Survey (SDSS; Abazajian et al. 2009). The SDSS u′ , g ′ , r′ photometry is converted to the
Johnson U, B, V magnitude system according to Jester et al. (2005). U-band observations
are required for each galaxy as the U-band gives the ability to characterize star formation
bursts over the age-range of 0.1-1 Gyr. The U − B color is also a strong age discriminator,
similar to the D(4000) Å break. J, H, and Ks bands are taken from the 2 Micron All Sky
Survey (2MASS; Skrutskie et al. 2006) catalog. 3.6 µm to 160µm photometric observations
are taken from the Spitzer/MIPS (Multiband Imaging Photometry for Spitzer; Rieke et al.
2004) and Spitzer/IRAC (Infrared Array Camera; Fazio et al. 2004) archival catalogs.
After all the photometry is collected for every galaxy, we correct all the data for foreground Galactic extinction (Schlegel et al. 1998) assuming Aλ /E(B − V ) = 3.1 for all optical
and IR data and, corrected as
Fλ,corr = Fλ,i × 10−0.4Aλ ,
(5)
where Aλ is the extinction corrections at the wavelengths of U, B, V, J, H, and Ks. For the
GALEX filter bandpasses, AF U V = 8.016 E(B − V )M W and AN U V = 8.087 E(B − V )M W .
These flux values Fλ,corr are the values that we compare to our models to investigate the
best-fit parameters that give rise to the observed SEDs (Section 4.3).
We further segregate the galaxies in the LVL sample by excluding all galaxies with a
value of log LTIR /LFUV > 0.5, allowing us to study only virtually dust-free galaxies. 175
galaxies in the LVL survey met the requirement of log LTIR /LFUV < 0.5, where 98 of those
have the necessary optical band photometry. The galaxies in the LVL survey represent a truly
unbiased, representative, and statistically robust sample of nearby star-forming galaxies.
Furthermore, by restricting our analysis to galaxies with little dust content, we remove one
degree of freedom and can more accurately than in previous studies, pin down the role of
stellar population ages and history for normal star-forming galaxies on the IRX-β relation.
Some of our photometric data have upper limits as a result of non-detections in the IR
bands (J band to 160 µm); we have treated upper limits as appropriate. Non-detections
–7–
imply that the measures flux density is below the 5σ upper limit. Table 1 lists all 98 sources
used in our analysis of the IRX diagram.
3.
3.1.
Definitions
Bolometric Infrared Luminosity
The TIR luminosity is the aggregate emission from all dust grains over the wavelength
range 8 − 1000 µm. We estimate the total IR flux emission FTIR from MIPS 24, 70, and
160 µm bands using the recipe in Dale et al. (2002):
FTIR = 1.559 ν fν (24 µm) + 0.7686 ν fν (70 µm) + 1.347 ν fν (160 µm),
(6)
where fν is the measured flux density (Jy) at each wavelength. The total IR luminosity is
LTIR = 4πD2 FTIR ,
(7)
where D is the distance to each galaxy and LTIR is used in the IRX calculation for the
ratio of the total IR flux to the FUV flux. For each model, the luminosity lost due to
dust absorption in the UV and optical regime must be equal to the bolometric luminosity
recovered in the IR regime. We assume that the stellar light lost in the UV/optical regime
to dust is fully recovered in the IR as dust emission, implying that the galaxy-average dust
scattering observed is negligible.
For sources that have any combination of upper limits for the flux density of 24, 70,
or 160 µm, we treat the estimate of the IR luminosity as an upper limit, with calculated
luminosities and their uncertainties listed in Table 1.
3.2.
The IRX Diagram
Figure 1 shows the IRX relation of log LTIR /LFUV as a function of β for the 98 galaxies
in our sample, calculated according to Eq. 3. Meurer et al. (1999) found a correlation
between LTIR /LFUV and β (and hence an AFUV and measured SFR) for starburst galaxies,
as shown shown in Figure 1, where the scatter greatly increases for the normal star-forming
galaxies that make up our sample.
The value of β, defined by Calzetti et al. (1994), is derived from a power-law fit of the
form fλ ∝ λβ in the range 1268 ≤ λ ≤ 2580 Å. We approximate our UV spectral index value
with βGLX (Eq. 4), according to Kong et al. (2004), using GALEX observations. The IRX
attenuation relation for starburst galaxies, as determined by Meurer et al. (1999), is
log IRX = log(101.77+0.796βGLX − 1) + 0.076 ± 0.044,
and can be seen in comparison to our galaxies in Figure 1.
(8)
–8–
Fig. 1.— The IRX diagram, showing the ratio log LTIR /LFUV as a function of βGLX , the
UV spectral index. All downward pointing arrows represent galaxies with upper limit values
of the total bolometric IR luminosity, LTIR . The solid black line shows the starburst IRX
attenuation relation, determined by a least-squares fit to the starburst galaxies from (Meurer
et al. 1999). The perpendicular distance dp represents the shortest distance between each
galaxy to the starburst attenuation curve. The horizontal dotted line represents the IRX
value where we consider all sources below that to be free from dust effects. The angled
dashed line in the upper right-hand corner represents the completeness level for sources with
dp analysis; the three sources that lie rightward are excluded from all perpendicular distance
results. The average error bar size is shown in the bottom right corner.
–9–
3.3.
Perpendicular Distance dp
If the stellar population is a parameter responsible for the observed deviation of normal
star-forming galaxies from the starburst IRX relation, the distance of an individual galaxy
from the starburst relation should give insight into the role age plays in determining the
location of any galaxy on the diagram. The shortest distance from a galaxy to the starburst
relation is the perpendicular distance, given as
p
(9)
dp = (xi − xIRX )2 + (yi − yIRX )2 ,
where (xi , yi ) are the βGLX and IRX coordinates for an individual galaxy, (xIRX , yIRX ) are the
βGLX and IRX values of the starburst relationship that is closest to the galaxy coordinates,
and dp is a dimensionless distance on the IRX diagram. We will be following the convention
as defined in Kong et al. (2004) by assigning a positive dp distance for a galaxy that has a
value of LTIR /LFUV lying above the starburst IRX relation and a negative dp for galaxies
that lie below the starburst LTIR /LFUV value for a fixed β. All of the galaxies in our sample
lie below the starburst IRX relation, giving all of our distances to be negative.
Despite having a sample size of 98 galaxies in the range of log LTIR /LFUV < 0.5, there
are more dusty galaxies present in the LVL survey that we did not include as they contained
too much dust to be considered in our analysis. In order to guarantee complete sampling
and sound statistics of any correlation with dp , we exclude galaxies that are right-ward
of the dotted line drawn in the right-hand corner of Figure 1. This guarantees that any
galaxies above log LTIR /LFUV > 0.5 not present in our sample does not make our dp results
incomplete.
4.
4.1.
Modeling
Generating the Synthetic Spectral Energy Distributions
The origin of the IRX scatter demands an understanding of the exact nature and origin
of not only the overall dust content of each galaxy, but knowledge on the distribution of
the stellar populations ages as well. Since we desire to better understand the underlying
parameter that best accounts for the offset and spread of normal star-forming galaxies from
the IRX relation for starburst galaxies, we want to explore and examine the parameters of
each model that gives rise to the structure in each observed SED. We model the observed
SED for each observed galaxy with synthetic models from Starburst99 (Leitherer et al.
1999). Our Starburst99 models draw from a range in metallicities of Z = 0.0004, 0.004, 0.008,
and 0.020 using Padova stellar evolutionary model tracks or Padova tracks extended to
include thermally pulsating asymptotic giant branch stars to the Starburst99 models. Our
– 10 –
models cover the age range of 10 Myr to 5 Gyr and are drawn from a Kroupa stellar initial
mass function (IMF) from 0.1 M⊙ to 100 M⊙ , with exponents of 1.3 in the mass range of
0.1 − 0.5 M⊙ and 2.3 over the mass range 0.5 − 100 M⊙ for the stellar populations. The
SED models use either a continuous SFR of 1 M⊙ yr−1 or a fixed mass (instantaneous burst)
SFR with a total stellar mass of 106 M⊙ , using time steps of 0.1 × 106 yr. Our generated
SEDs cover the wavelength range from 90 Å to 160 µm. We keep the star formation history
very simple on purpose as we need only to divide the galaxies according to the two possible
extremes of either instantaneous or constant star formation.
Luminosities and colors are determined by convolving the generated synthetic SED with
the spectral response function of the filters for all of our passbands from the FUV to 160 µm.
The synthetic stellar spectra is convolved with the response function of each passband as
R
fλ Rλ dλ
,
(10)
fλ,conv = Rλ
Rλ dλ
λ
where Rλ is the filter response function and fλ is the flux density per unit wavelength
(erg s−1 cm−2 Å−1 ) of the synthetic spectra at the effective wavelength for each passband, as
listed in Table 2. It is these convolved flux values that we compare to the galaxy observations.
The IR luminosity is calculated assuming that all attenuated stellar light is re-emitted by
dust in the infrared.
4.2.
Dust Attenuation Models
The observed SED of galaxies in the UV and optical regime is determined by the intrinsic
spectrum of the stellar population in addition to any reddening by presence of dust. While
we specifically preselected our galaxies to have minimal dust effects, we have applied a range
of color excess values from E(B − V ) = [0, 0.10] with steps of ∆E(B − V ) = 0.01 and values
of E(B − V ) = [0.1, 0.3] in steps of ∆E(B − V ) = 0.05 to account for any non-negligible
dust attenuation effects. We apply the attenuation to our unextinguished models according
to the prescription,
Fλ = Fλ,i × 10−0.4 k(λ) E(B−V ) ,
(11)
where Fλ is the observed (reddened) flux (erg s−1 cm−2 ), Fλ,i is the intrinsic flux, E(B − V )
is the color excess of the stellar population, and k(λ) is the dust attenuation per wavelength
model, where we adopt as a default the starburst attenuation curve (Calzetti et al. 2000).
All models with color excess values of E(B − V ) > 0.1 produced too much IR emission that
was not recovered in the IR; no model with the coarser sampling of ∆E(B − V ) = 0.05 above
E(B − V ) = 0.1 was able to produce the observed IR emission. We do not add intrinsic
– 11 –
extinction effects to the galaxy observations; instead we use the extinguished models to
estimate the dust content of each galaxy.
4.2.1. Variations in the Dust Extinction Models
In this section, we explore the effect of adopting different extinction models on the IRX
diagram. In addition to the starburst attenuation curve, we adopt a Small Magellanic Cloud
(SMC; Bouchet et al. 1985) and a Milky Way (MW, with Rv = 3.1; Cardelli et al. 1989)
extinction curve, with both foreground and mixed dust geometry. The foreground case is
described by equation 11, while the homogeneously dust/stars mixed case is described as
Fλ = Fλ,i
1 − e−τ
,
τ
(12)
where τ = 0.921 k(λ) E(B − V ), Fλ,i is the intrinsic flux, and Fλ is the observed (reddened)
flux.
Figure 2 shows the IRX diagram of a bursting galaxy at four different ages (100, 300,
700, 1000 Myr) with a fixed metallicity of Z = 0.0004, with an adopted starburst, MW,
SMC, or a mixed-dust MW/SMC extinction curve. Since we are primarily showing how the
value of βGLX changes with the age of the model, we have arbitrarily offset all the data to be
consistent with log LTIR /LFUV = −1. Our findings agree with the results of Calzetti (2001);
Johnson et al. (2007) that the MW extinction curve does a poor job at recreating the IRX
diagram. When adopting the SMC extinction curve, galaxies with the same color excess value
E(B − V ), age, and metallicity are both redder in color and have smaller FUV contributions
compared to observations using the starburst attenuation curve. This shows the importance
of adopting an accurate attenuation; selecting different extinction curves can significantly
change the location of a galaxy on the IRX diagram. Changing the assumed extinction
model changes the fraction of younger galaxies recovered, as adopting a SMC extinction
curve results in more optical to near-IR light contributed from an aging stellar population as
opposed to the amount contributed from dust, resulting in older populations recovered. This
result is consistent with the work of Reddy et al. (2010, 2012), which showed that adopting
a SMC extinction curve, compared to a starburst attenuation curve, does generally yield
galaxies with older stellar populations. Mixed dust models trace out the same locations in
the IRX diagram as their foreground dust counterparts, however there is smaller range in
both βGLX and the IRX values for the mixed dust models.
– 12 –
Fig. 2.— Top panel: The IRX diagram, showing how a galaxy with a bursting star formation
at a fixed metallicity evolves from 100 Myr to 1 Gyr. The shapes of the colors represent
the age of the galaxy model, where all blue points represent galaxies with an adopted MW
extinction curve, black shapes represent galaxies with an assumed starburst (SB) attenuation
curve, red shapes represent galaxies with an adopted SMC extinction curve, pink shapes
represent galaxies with a mixed dust (MD) SMC extinction curve, and the purple points
represent galaxies with a MD MW extinction curve. The size of each shape represents the
color excess value E(B − V ) from 0.0 to 0.07 in steps of 0.01. Each model has been given
an offset, normalized to log LTIR /LFUV = −1. Bottom panel: A zoom in of the evolution
of 100 Myr with an adopted SB attenuation model, where the colored points represent
models with different metallicities, with black representing a metallicity of Z = 0.0004, green
representing Z = 0.004, magenta representing Z = 0.008, and cyan representing Z = 0.020.
– 13 –
Since the UV spectral slope βGLX is dependent on the adopted extinction law, it is
expected to find βGLX changing with variations in the extinction curve. Since the FUV flux
is predominately supplied by the very young stellar population and the flux in the NUV has
a larger contribution from older stars, the UV colors are primarily a measurement of the
obscuration of the stellar populations as opposed to supplying information on dust effects,
as found by Kong et al. (2004).
4.3.
Fitting the SED and Estimations of the Stellar Population Age
In order to determine how well the models compare to the observed data, each model is
allowed an offset c that minimizes χ2 between the fluxes of the model and the observed data
points. The physical properties of the best-fit models allows us to describe the properties of
the observed galaxies. Since there are generally at least two stellar populations contributing
to the observed SED, we do the fitting of each galaxy from the FUV to Ks bands, where
the 3.0-160 µm bands are used purely to estimate the dust luminosity and not the age of
the stellar population giving rise to the observed UV and optical stellar continuum. For
each galaxy i in our sample, the best-fit model is determined with a χ2 minimization in the
following way:
X Fobs,B − ci Fmod ,B i
2
,
(13)
χi =
σ(Fobs,B )
B
where we sum over the bands FUV, NUV, U, B, V, J, H, and Ks, giving us seven degrees
of freedom in determining the scale factor c. Fobs,B is the observed flux at each band pass,
σ(Fobs,B ) is accompanying 1σ errors, Fmodi ,B is the flux of each individual model, and ci is
the offset that best matches the model and the observed galactic fluxes, calculated as
X Fobs,B Fmod ,B . X Fobs,B 2
i
.
ci =
2 (F
σ
σ(Fobs,B )
obs,B )
B
B
(14)
The best-fit model is determined by plugging in the value of ci for each model into χ2i . Each
χ2i value for each model is then assigned a weight wi = exp(−χ2i /2), giving a probability
distribution function (PDF) for the galaxy parameters of interest. We do not automatically
assign the physical properties associated with χ2best to describe the observed SED. Instead,
the average of the models that lie within a factor of three of χ2best is accepted as an initial
estimate of the specific galaxy parameter of interest (age, metallicity, color excess E(B − V ),
and star formation type). This process makes sure that we have not selected a single model
that fits the galaxy observations by chance. The range of acceptable parameter values for
each galaxy, as determined from the PDF, is the range allowed for each parameter and is
– 14 –
represented as an error bar in all the following figures. Our χ2 routine is performed using
YAFITS in the Java programming language.
Even with the best-fit model that minimizes χ2 with n input parameters, there is often
times an arbitrarily large number of acceptable models that all provide reasonable fits to
the data. We further refine the total number of acceptable models for each galaxy through
conservation of energy: The total amount of light absorbed due to dust attenuation in the
range 912 − 22, 000 Å has to equal the aggregate emission from all the dust grains, LTIR .
Any model that does not meet this criterion is rejected. Table 1 lists the best-fit model
and range for each galaxy. Some galaxies are satisfied with both bursting and continuous
star forming models; these galaxies have both models listed in Table 1 but we only show
the bursting model in all of our plots. Figure 3 shows an example of a best-fit Starburst99
model to a set of observations from the FUV to the Ks band for IC 1574.
5.
5.1.
Results and Analysis
The Color Excess E(B − V )
We apply extinction to the stellar population of our synthetic models according to Eq.
11, where the color excess values range from E(B −V ) = [0, 0.10] with steps of ∆E(B −V ) =
0.01 and coarser sampling, with values of E(B − V ) = [0.1, 0.3] with steps of ∆E(B − V ) =
0.05. Figure 4 shows E(B − V ) as a function of βGLX and Figure 5 shows E(B − V ) as
a function of dp ; as expected, galaxies in the range 0 < IRX < 0.5 have a higher average
E(B − V ) value than galaxies at IRX< 0. We find no correlation between the color excess
of the best-fit model of each galaxy to the UV spectral index βGLX . This leads us to believe
that the UV spectral index is not a good indicator of the dust content as traced by the
virtually dust-free galaxies in our study. When examining the relation between E(B − V )
with dp , we find that there is a general trend for an increase in the observed color excess
of the stellar population as a galaxy lies closer to the starburst IRX relation, as seen in
Figure 5. While this suggests a general trend toward redder colors with an decrease in the
perpendicular distance (galaxies that are located closer to the starburst IRX relation), the
relation is not significant enough for us to conclude that the galaxies with the least amount
of dust are located further away from the starburst IRX relation than more dusty galaxies.
We have listed both the Spearman rank correlation coefficient ρ and the Kendall correlation
coefficient τ for nearly every variable in our study as a function of both βGLX and dp in Table
5 for the entire sample of galaxies, Table 3 for bursting galaxies and Table 4 for continuous
galaxies, where the significance of the correlation is generally 3.5σ or smaller. In addition,
we separate into subsets on the IRX diagram, where we perform statistics on the galaxies
below and above log LTIR /LFUV < 0 for each table.
– 15 –
Fig. 3.— A best-fit Starburst99 SED from the FUV to the Ks bands for IC 1574. The galaxy
observations are represented with crosses and accompanying 1σ error bars. The solid circles
represent the convolved flux values at each wavelength. The value of χ2best is 0.5.
– 16 –
Fig. 4.— The color excess E(B − V ) of the best-fit model for each galaxy as a function of the
UV spectral index βGLX . The solid squares represent galaxies that are best-fit with bursting
SFRs and open circles represent galaxies that are best-fit with continuous SFRs. The colors
represent the location of each galaxy on the IRX diagram, where red points represent galaxies
with an IRX value of 0 < log LTIR /LFUV < 0.5 and blue points represent galaxies with an
IRX value of log LTIR /LFUV < 0.
– 17 –
Fig. 5.— The color excess E(B − V ) of the best-fit model for each galaxy as a function of
the perpendicular distance dp . The solid squares represent galaxies that are best-fit with
bursting SFRs and open circles represent galaxies that are best-fit with continuous SFRs.
The colors represent the location of each galaxy on the IRX diagram, where red points
represent galaxies with an IRX value of 0 < log LTIR /LFUV < 0.5 and blue points represent
galaxies with an IRX value of log LTIR /LFUV < 0.
– 18 –
The Birthrate b-Parameter
5.2.
The birthrate parameter (b-parameter) is the ratio of the current star formation to
its overall lifetime average (Kennicutt et al. 2008) and is independent of distance. The
b-parameter is often also denoted as a measure of the SFR per stellar mass, written as
b=
SFR
SFR t0 (1 − R)
=
,
hSFRipast
Mstar
(15)
where where t0 is the age of the galaxy (usually taken to be ∼12 Gyr), Mstar is the total
stellar mass of the of the galaxy, and R is the fraction of gas that stars reinjected through
stellar winds into the interstellar medium during their lifetime. We use both the ratio of
the FUV to near-IR (NIR) luminosities and the equivalent width (EW) of Hα as proxies for
the birthrate parameter to investigate if the age of the stellar population and the SFH is
responsible for the deviation of normal star-forming galaxies from the starburst IRX relation.
5.2.1. The FUV to NIR Luminosity Ratio
We examine the ratio of the FUV (1520 Å) to NIR (3.6 µm) as a tracer of the bparameter as the FUV traces star formation activity over very recent times (∼100 Myr)
while the NIR traces the total stellar mass built up over much longer timescales. The ratio
roughly gives the SFR per unit stellar mass, providing a normalized measure of the star
formation activity. The FUV/NIR ratio is very sensitive to extinction affects, and we have
corrected our observed FUV luminosities for extinction effects according to Eq. 11, where
the color excess values E(B − V ) are taken from the best-fit SED models to the galaxy
observations. The FUV/NIR ratio is calculated as
FUV/NIR =
νLν (1520 Å)
.
νLν (3.6 µm)
(16)
Figure 6 and Figure 7 shows the FUV/NIR ratio as a function of βGLX and dp , respectively.
There is considerable spread between FUV/NIR and dp ; we do not find any correlation
between the FUV/NIR ratio and dp , in disagreement with the results of Dale et al. (2009).
However, when taking into consideration the b-parameter and βGLX , there is an increasing
redness in the UV colors with lower values of the birthrate parameter, where the statistics
between the type of galaxy and and FUV/NIR ratio is listed in Table 3 for bursting galaxies,
Table 4 for continuous galaxies, and Table 5 for the entire sample of galaxies. The correlation
in this case is very significant, especially for bursting galaxies, significant at the 5σ level.
– 19 –
Fig. 6.— The log of the FUV/NIR ratio of each galaxy as a function of the UV spectral
index βGLX . The solid squares represent galaxies that are best-fit with bursting SFRs and
open circles represent galaxies that are best-fit with continuous SFRs. The size of the
circles/squares represents the age for the best-fit models, shown in the right-hand panel.
The colors represent the location of each galaxy on the IRX diagram, where red points
represent galaxies with an IRX value of 0 < log LTIR /LFUV < 0.5 and blue points represent
galaxies with an IRX value of log LTIR /LFUV < 0.
– 20 –
Fig. 7.— The log of the FUV/NIR ratio of each galaxy as a function of the UV spectral index dp , the perpendicular distance to the starburst IRX relation. The solid squares
represent galaxies that are best-fit with bursting SFRs and open circles represent galaxies that are best-fit with continuous SFRs. The size of the circles/squares represents the
age for the best-fit models, shown in the right-hand panel. The colors represent the location of each galaxy on the IRX diagram, where red points represent galaxies with an IRX
value of 0 < log LTIR /LFUV < 0.5 and blue points represent galaxies with an IRX value of
log LTIR /LFUV < 0.
– 21 –
5.2.2. Hα Equivalent Width
We use the EW(Hα) emission line to estimate the intensity of the current star formation
over the past average as another proxy for the b-parameter, where the EW(Hα) is much less
sensitive to extinction effects compared to the FUV/NIR ratio. The EW(Hα) is the ratio
of the luminosity of the Hα emission line to the continuum luminosity at λHα = 6563 Å,
where the emission line is primarily produced by young, massive stars (> 10M⊙ ) over short
timescales and the red continuum luminosity at 6365 Å traces the total mass built up from
older stars at much longer timescales (Kennicutt et al. 1994). The EW(Hα) measurements
are taken from Kennicutt et al. (2008), where the Hα flux is corrected for contamination
from [NII] as
EW(Hα + [NII])
,
(17)
EW (Hα) =
1 + [NII]/Hα
where we have EW(Hα) measurements for 93 out of our 98 galaxies. The EW(Hα) allows
us to circumvent the problem of lack of correlation between the current duration of star
formation as a function of βGLX for continuous galaxies. Figure 8 and Figure 9 show the
EW(Hα) of our galaxies as a function of βGLX and dp for both continuous and bursting star
forming galaxies, respectively. The relation between the EW(Hα) and βGLX is only slightly
more significant. The significance of both the Spearman and Kendall correlations are given in
Table 3 and Table 4 for both EW(Hα) as a function of perpendicular distance and EW(Hα)
as a function of βGLX . In all cases, the correlation is marginally significant, between 3σ and
4σ.
It is important to note that an increase in the amount of dust can reduce the observed
EW(Hα) if there is differential attenuation in gas emission and stellar continuum (Hao et al.
2011). We would expect an increase in the scatter between the EW(Hα) and dp /βGLX as
galaxies with brighter LTIR /LFUV ratios are examined. However, for our low-dust, normal
star-forming galaxies, we believe this effect to be negligible.
5.3.
Stellar Population Age Estimators
We employ the U − B colors to place additional constraints on the effects of the age
of the stellar populations on the IRX diagram. We use the log (LU /LB ) color ratio as an
age indicator as this ratio is straddles the 4000 Å region, which is sensitive to the age of
the galactic stellar populations. The narrow bands of the D(4000) spectral discontinuity,
the ratio of the flux densities in the bands of 3850 − 3950 Å and 4000 − 4100 Å, allow it
to be fairly insensitive to dust and highly correlates with the b-parameter (Kauffmann et al.
2003). This makes D(4000) a valuable indicator of the mean stellar population age. The
log (LU /LB ) colors are more sensitive to dust attenuation than the D(4000) break, but are
– 22 –
Fig. 8.— The Hα equivalent width (EW) of each galaxy as a function of the UV spectral
index βGLX . The solid squares represent galaxies that are best-fit with bursting SFRs and
open circles represent galaxies that are best-fit with continuous SFRs. The size of the
circles/squares represents the age for the best-fit models, shown in the right-hand panel.
The colors represent the location of each galaxy on the IRX diagram, where red points
represent galaxies with an IRX value of 0 < log LTIR /LFUV < 0.5 and blue points represent
galaxies with an IRX value of log LTIR /LFUV < 0. The solid black line represent the leastsquares fit to the galaxies best represented with bursting SFRs and the dotted line represents
the least-squares fit to the galaxies represented with continuous SFRs.
– 23 –
Fig. 9.— The Hα equivalent width (EW) of each galaxy as a function of the perpendicular
distance dp . The solid squares represent galaxies that are best-fit with bursting SFRs and
open circles represent galaxies that are best-fit with continuous SFRs. The size of the
circles/squares represents the age for the best-fit models, shown in the right-hand panel.
The colors represent the location of each galaxy on the IRX diagram, where red points
represent galaxies with an IRX value of 0 < log LTIR /LFUV < 0.5 and blue points represent
galaxies with an IRX value of log LTIR /LFUV < 0. The solid black line represents the leastsquares fit to the galaxies represented with bursting SFRs (N = 33) and the dotted line
represents the least-squares fit to the galaxies represented with continuous SFRs (N = 57).
– 24 –
are still highly sensitive to the mean population age.
Figure 10 shows the U − B colors as a function of the UV colors, βGLX . When we divide
the sample into bursting and continuous star forming galaxies, the bursting (continuous)
galaxies have a Spearman correlation coefficient value of ρ = −0.64 (ρ = −0.48), significant
to the 3.9σ (3.7σ) level for our sample size of bursting (continuous) galaxies. A Kendall
correlation coefficient gives a significance level of 4.3σ (4.1σ) at ρ = −0.49 (ρ = −0.36).
While an increase in the redness of the U − B colors is accompanied with an increase in the
redness of the UV colors, we do not consider this correlation to be very significant.
The U − B color has a Spearman correlation that is significant at the 2.7σ level to the
perpendicular distance dp (Figure 11) from the starburst IRX relation, where we disregard
the three galaxies that are above the dp completeness line in Figure 1. A Kendall correlation
gives a coefficient that is significant at the 2.9σ level. This leads us to believe that the stellar
population, as traced by U − B colors, is not responsible for the perpendicular deviation
of normal star-forming galaxies from the starburst attenuation relation. Kong et al. (2004)
found a strong correlation (5σ significance) between the D(4000) break and dp . We do not
recover this relation between dp and the age of the stellar population. However, D(4000) is
largely insensitive to dust, while our estimate of the stellar population age, log (LU /LB ), is
largely sensitive to the presence of dust, despite the criterion of our galaxies being largely
dust-free.
Figure 12 shows the age of each galaxy – determined from a χ2 fitting to our models – as
a function of the UV spectral index βGLX , where the range of ages of acceptable models gives
the size of the vertical error bars. There is a trend toward increasing age with increasing
redness in bursting galaxies, however, there does not appear to be any visible trend in
continuous star-forming galaxies. The distribution of stellar age in continuous star forming
galaxies seems to show a build up at 5 Gyr, the oldest age value we consider. This occurs
because bursting star-forming galaxies pins down an age for the stellar population, while
the age for continuous star-forming galaxies gives a duration of the SFH, not revealing any
information about the mean age of the stellar population. For the bursting galaxies, we
determine the optimal fit to the data with a Levenberg-Marquandt algorithm for non-linear
least-squares optimization, where our function is of the form
log Age(βGLX ) = log[10A1
βGLX +A2
− 1] + A3 .
(18)
The statistics between the age and βGLX for the entire sample of bursting and continuous
star-forming galaxies can be found in Table 3 and Table 4. We have listed both the Spearman
rank correlation coefficient ρ and the Kendall correlation coefficient τ , where we separate
out the continuous and bursting star-forming galaxies in addition to separating into subsets
– 25 –
Fig. 10.— The U − B colors as a function of the UV colors βGLX . U − B is expressed as
the ratio of the fluxes of the two bands log (LU /LB ) and the UV colors are expressed as
βGLX . The solid squares represent galaxies that are best-fit with bursting SFRs and open
circles represent galaxies that are best-fit with continuous SFRs. The colors represent the
location of each galaxy on the IRX diagram, where red points represent galaxies with an
IRX value of 0 < log LTIR /LFUV < 0.5 and blue points represent galaxies with an IRX value
of log LTIR /LFUV < 0. The solid black line represent the least-squares fit to the galaxies best
represented with bursting SFRs and the dotted line represents the least-squares fit to the
galaxies represented with continuous SFRs. The size of each square/circle represents the age
of the best-fit model, shown in the right-hand panel. The average error bar size is shown in
the top right-hand corner.
– 26 –
Fig. 11.— The U −B colors as a function of the perpendicular distance dp . U −B is expressed
as the ratio of the fluxes of the two bands log (LU /LB ) and the UV colors are expressed as
βGLX . The solid squares represent galaxies that are best-fit with bursting SFRs and open
circles represent galaxies that are best-fit with continuous SFRs. The colors represent the
location of each galaxy on the IRX diagram, where red points represent galaxies with an
IRX value of 0 < log LTIR /LFUV < 0.5 and blue points represent galaxies with an IRX value
of log LTIR /LFUV < 0. The solid black line represent the least-squares fit to the galaxies best
represented with bursting SFRs and the dotted line represents the least-squares fit to the
galaxies represented with continuous SFRs. The size of each square/circle represents the age
of the best-fit model, shown in the right-hand panel.
– 27 –
Fig. 12.— Top panel: The age of each galaxy as a function of βGLX for galaxies that are
best-fit with continuous SFRs (open circles). Bottom panel: The age of each galaxy as
a function of βGLX for galaxies that are best-fit with bursting SFRs (solid squares). The
colors represent the location of each galaxy on the IRX diagram, where red points represent
galaxies with an IRX value of 0 < log LTIR /LFUV < 0.5 and blue points represent galaxies
with an IRX value of log LTIR /LFUV < 0. The solid black line represent the least-squares
fit to the entire sample of bursting galaxies. The vertical error bars represent the range of
acceptable model ages for each galaxy.
– 28 –
on the IRX diagram; the galaxies below log LTIR /LFUV < 0 and the galaxies in the regime
0 < log LTIR /LFUV < 0.5. We find that for the bursting galaxies, the correlation between
age and βGLX is very significant > 4σ. The results of age with perpendicular distance dp can
also be found in 3 and Table 4. For the bursting galaxies, we find that the perpendicular
distance also correlates with the age of the young stellar population (Figure 13). Overall,
we find that the relation between age and perpendicular distance, in addition to βGLX , is
very sensitive to the presence of dust attenuation, decreasing in significance for increasing
attenuation.
5.4.
Metallicity
We have gathered oxygen (12+log(0/H)) metallicity measurements for 61% of the galaxies in our sample (60/98) from the literature. Since metallicity is a parameter we allow to
vary in the Starburst99 spectra, when a galaxy has a measured metallicity value available,
we require the generated SED models to match the metallicity of the observed data. If there
is no metallicity known a prior, the metallicity of the synthetic SEDs are allowed to accept
any metallicity between Z = 0.0004 to 0.02.
Figure 14 shows the metallicity as a function of the UV spectral index. Over 93% of the
galaxies in our sample with known metallicity measurements lie below the solar metallicity
value; LVL galaxies that lie in the bottom region of the IRX diagram are sub-solar metallicity
galaxies. Figure 15 shows the metallicity as a function of the perpendicular distance. Again,
we fail to recover any correlation between the distance from a galaxy to the IRX relation
for starburst galaxies and the metallicity of the galaxy. Since our sample excluded dust-rich
galaxies, we can conclude that the correlation between metallicity and β is not a fundamental
one.
– 29 –
Fig. 13.— Top panel: The age of each galaxy as a function of perpendicular distance dp
for galaxies that are best-fit with a continuous SFR (open circles). Bottom panel: The age
of each galaxy as a function of perpendicular distance dp for galaxies that are best-fit with
a bursty SFR (solid squares). The colors represent the location of each galaxy on the IRX
diagram, where red points represent galaxies with an IRX value of 0 < log LTIR /LFUV < 0.5
and blue points represent galaxies with an IRX value of log LTIR /LFUV < 0. The vertical
error bars represent the range of acceptable model ages for each galaxy.
– 30 –
Fig. 14.— 12+log(O/H) measurements as a function of the UV spectral slope βGLX for our
sample. The solid squares represent galaxies that are best-fit with bursting SFRs and open
circles represent galaxies that are best-fit with continuous SFRs. The colors represent the
location of each galaxy on the IRX diagram, where red points represent galaxies with an IRX
value of 0 < log LTIR /LFUV < 0.5 and blue points represent galaxies with an IRX value of
log LTIR /LFUV < 0. The size of the galaxy symbols represent the best-fit age of each galaxy,
represented by the legend on the right-hand panel. The vertical error bars represent the 1σ
error for the metallicity values. The solid dotted line represents the solar metallicity value.
– 31 –
Fig. 15.— 12+log(O/H) measurements as a function of the perpendicular distance dp . The
solid squares represent galaxies that are best-fit with bursting SFRs and open circles represent galaxies that are best-fit with continuous SFRs. The colors represent the location
of each galaxy on the IRX diagram, where red points represent galaxies with an IRX
value of 0 < log LTIR /LFUV < 0.5 and blue points represent galaxies with an IRX value
of log LTIR /LFUV < 0. The size of the galaxy symbols represent the best-fit age of each
galaxy, represented by the legend on the right panel. The solid dotted line represents the
solar metallicity value.
– 32 –
6.
Discussion
Figure 12 shows the main result of our study: We find that examining galaxies by their
star formation type (continuous or bursting) gives the best insight on the parameter responsible for the deviation of normal star-forming galaxies from the IRX attenuation relation,
where the age of the stellar population increases with increasing values of βGLX for bursttype galaxies. The same age−βGLX trend is not present in continuous galaxies. For bursting
galaxies, the age-βGLX relation, either directly derived from SED fittings or approximated
with the FUV/NIR color, is the strongest correlation we find for the “second parameter”,
with a Kendall τ correlation coefficient significance at the 5.1σ level for bursting galaxies and
at the 6σ level for our entire sample of galaxies. This may help alleviate confusion in prior
scientific studies that failed to reveal any type of relation between the age of the stellar population and β; studies that are predominantly composed of continuous star-forming galaxies
will not reveal a connection between the age and the UV spectral index as determined by
SED fitting. The increase in the age of the duration of current star formation for our bursting galaxies is in agreement with Burgarella et al. (2005); Dale et al. (2009) but disagrees
with the work of Siebert et al. (2005); Boquien et al. (2009). Figure 13 shows the age as a
function of the perpendicular distance from the starburst IRX relation, where we do not see
the strong correlation for bursting galaxies as traced by βGLX , suggesting that the “second
parameter” may not be determined with the distance from the IRX relation for starburst
galaxies. Figure 16 shows the IRX diagram accounting for the type of star-formation and
the age that best represents the observed SED for each galaxy from Starburst99 SEDs.
The intrinsic ultraviolet spectral slope βGLX is sensitive to the age of the stellar population, where the evolved young stellar population (∼100 Myr) dominates the UV emission in
normal star-forming systems. As a result, the increased contamination in the UV spectral
slope from evolved stellar populations exhibits itself as a larger dispersion for normal starforming galaxies in the IRX diagram. We do not believe the U − B colors, which serve as
a proxy for the D(4000) break and an indicator of the SFH, to be a good indicator of the
mean stellar population age for the low-dust, local galaxies of our survey.
Dale et al. (2009) found that a majority of the galaxies in the LVL survey showed signs
that the outer edges were older than the inner regions of the galaxies. As a result, the
observed global total luminosity ratios of the TIR to FUV for normal star-forming galaxies
will appear to have a redder stellar population and a mixing of all stellar populations present.
They observed that this global flux issue may be why LVL galaxies tend to exhibit older
stellar populations than those seen in more active star-forming galaxies, an issue that could
be resolved with spatial resolution studies of galaxies.
We have also examined the relationship between the metallicity and βGLX (or dp ), where
– 33 –
Fig. 16.— The IRX diagram, showing the ratio log LTIR /LFUV as a function of βGLX , the
UV spectral index. All downward pointing arrows represent galaxies with upper limit values
of LTIR . The solid black squares represents galaxies that are best-fit with bursting SFRs and
the open circles represent galaxies that are best-fit with a continuous SFR. The solid black
line shows the starburst IRX attenuation relation, determined by a least-squares fit to the
starburst galaxies from Meurer et al. (1999). The perpendicular distance dp represents the
shortest distance between each galaxy to the starburst attenuation curve. The size of each
square/circle represents the age of the best-fit model, shown in the right-hand panel. The
average error bar size is shown in the bottom right-hand corner.
– 34 –
we expect metal rich galaxies to present more UV attenuation when compared with metalpoor galaxies. The work of Cortese et al. (2006) found a correlation between βGLX and the
12+log(O/H) value, where the correlation was nearly the same for both starburst and normal
star-forming galaxies, significant at the 3σ level. We do not replicate the metallicity−βGLX
relation as seen in Cortese et al. (2006); galaxies with redder UV spectral colors do not
necessarily represent galaxies with higher metallicities. The correlation found by Cortese
et al. (2006) may simply reflect a change in β due to increased dust attenuation in more
metal-rich galaxies. Because our sample excludes the dust-rich galaxies as used in the Cortese
et al. (2006) sample, it is not surprising that our results fail to find the metallicity−βGLX
relation, where we believe the correlation between metallicity and βGLX as seen by Cortese
et al. (2006) is an attenuation effect and not driven by metallicity.
The strongest correlation for our galaxies described with continuous star formation are
the EW(Hα) and FUV/NIR colors versus βGLX ; in this case the EW(Hα) of our normal
star-forming galaxies supports the hypothesis that the present to past-averaged SFR may
be the second parameter responsible for the deviation of normal star-forming galaxies from
the IRX relation as compared to the dust content of the galaxy, in agreement with previous
results (Kong et al. 2004; Dale et al. 2009). Both our continuous star-forming galaxies and
our instantaneous star-forming galaxies have linear trends of EW(Hα) that correlate with
the perpendicular distance, however this correlation is only significant to the 4σ level. The
strongest correlation for our entire sample of galaxies is the FUV/NIR colors versus βGLX ,
with a significance greater than the 6σ level. We believe the “second parameter”, as traced
by the FUV/NIR ratio, does suggest that the mean age of the stellar population depends on
the UV colors but not on the distance from the starburst IRX relation.
7.
Conclusions
We present a multiwavelength analysis of a sample of 98 dust-free normal star-forming
spiral and compact galaxies from the Spitzer LVL survey. Our work attempts to study
normal star-forming galaxies on the IRX diagram, a method used to account for dust attenuation in galaxies from observations solely in the UV (Meurer et al. 1999; Calzetti 2001),
where the relationship breaks down for non-starburst galaxies. We focused on investigating the impact that the underlying stellar population age has on the IRX, where we model
galaxy observations with a combination of UV, optical, and IR photometric data to better
understand if the stellar population mean age or other characteristics are the second parameter responsible for the failure of the starburst attenuation relation to apply to normal
star-forming galaxies.
To re-construct the full UV through near-IR SED curve for our 98 galaxies, we use
– 35 –
Starburst99 to produce synthetic model spectra (metallicities of Z = 0.0004 to 0.020, a
Kroupa IMF from 0.1 M⊙ to 100 M⊙ , a continuous or bursting SFH having an age range
of 10 Myr to 5 Gyr) that are representative of extremes in the SFH of our observations.
The comparison between the galaxy observations and our synthetic models are done with a
reduced χ2 routine in order to find the best-fit model which produces the best match between
the observed SED and the assumed, synthetic SED. In addition, all models must correctly
account for the amount of stellar light in the UV and optical that is absorbed by dust and
re-radiated away in the FIR.
We use both the ratio of the FUV to NIR luminosity and the EW(Hα) to serve as
tracers of the birthrate parameter, b-parameter. We find that for galaxies that are best
represented with a bursting star formation type, there is a correlation between both the
age of the stellar population from direct modeling and the mean stellar population age,
as traced by the FUV/NIR colors, and βGLX to the UV attenuation relation for starburst
galaxies. This correlation does not hold for galaxies that are best-fit with a continuous
SFR, as the age is indicative of the duration of current star forming activity. The strong
correlation of the FUV/NIR ratio with βGLX indicates that bursting systems with lower
birthrates are associated with redder UV colors, and less significantly, closer to the starburst
IRX relationship. This suggests that the SFH does play a role in determining the location
of a galaxy on the IRX diagram, where the longest-lasting star forming systems are located
further from the starburst IRX curve and/or in the reddest systems. This FUV/NIR color
ratio is the strongest when we consider the entire sample of galaxies, suggesting that the
mean age of the stellar populations highly depends on the amount of UV reddening of the
system, regardless of whether the SFH is bursting or continuous. We also find that for both
continuous and bursting star forming galaxies, the U − B colors – which serve as a SFH
indicator – do not correlate with the distance from the starburst attenuation curve, however,
redder U − B colors are well represented with redder βGLX colors for the entire sample of
galaxies, continuous and bursting.
We find that the EW(Hα) of galaxies that are well represented with a continuous SFR
correlate with the distance from the starburst attenuation relation on the IRX diagram. We
also find that the age of the constant SFR galaxies tend to be old (∼5 Gyr), where low
and intermediate age ranges are sparse. Galaxies that are well-defined with bursting SFHs,
as opposed to continuous SFHs, cover the full age range explored and are generally much
younger than the age of continuous star-forming galaxies. This relation between EW(Hα)
and dp for continuous galaxies is the only correlation we find with dp in our entire sample of
galaxies. The near lack of any correlation to any variable with the perpendicular distance on
the IRX diagram suggests that the second parameter responsible for the scatter of normal
star-forming galaxies and does not depend on the distance from the starburst IRX relation,
– 36 –
but instead with increasing redness, as measured with β. Lack of evidence that the scattering
parameter scales in the direction of dp indicates that the source of the scatter for normal
star-forming galaxies is mainly along the horizontal direction, as probed by β.
While the dust content is the predominant driver for the location of galaxies on the
IRX diagram (which is especially evident in galaxies with increasing values on the IRX), our
results support evidence that the location of normal star-forming galaxies is affected by both
dust content and the mean stellar population age. Therefore, the attenuation cannot be
determined for normal star-forming galaxies as a simple relation from the UV colors alone as
it can for starburst galaxies. The effect of the star formation history on quiescent galaxies is
better understood when the galaxies are binned into two groups, represented with bursting
or continuous SFRs. Once the galaxies have been separated by types of SFRs, the effect of
the stellar population age is then better understood and how that influences their location
on the IRX diagram. The large scatter between βGLX and log LTIR /LFUV for normal starforming galaxies implies that a greater care must be used to determine if βGLX is a viable
means to recover the intrinsic UV flux lost to dust attenuation, as already supported by
Hao et al. (2011). However, as the dust content of a galaxy increases, the dust distribution
also plays a role in increasing the scatter in β at a fixed IR/UV ratio, as concluded by prior
studies.
This research has made use of the NASA/IPAC Extragalactic Database (NED) which
is operated by the Jet Propulsion Laboratory, California Institute of Technology, under
contract with NASA. Funding for SDSS and SDSS-II has been provided by the Alfred P.
Sloan Foundation, the Participating Institutions, the National Science Foundation, the U.S.
Department of Energy, the National Aeronautics and Space Administration, the Japanese
Monbukagakusho, the Max Planck Society, and the Higher Education Funding Council for
England. The SDSS Web Site is http://www.sdss.org/. The SDSS is managed by the
Astrophysical Research Consortium for the Participating Institutions. The Participating
Institutions are the American Museum of Natural History, Astrophysical Institute Potsdam,
University of Basel, University of Cambridge, Case Western Reserve University, University
of Chicago, Drexel University, Fermilab, the Institute for Advanced Study, the Japan Participation Group, Johns Hopkins University, the Joint Institute for Nuclear Astrophysics,
the Kavli Institute for Particle Astrophysics and Cosmology, the Korean Scientist Group,
the Chinese Academy of Sciences (LAMOST), Los Alamos National Laboratory, the MaxPlanck-Institute for Astronomy (MPIA), the Max-Planck-Institute for Astrophysics (MPA),
New Mexico State University, Ohio State University, University of Pittsburgh, University
of Portsmouth, Princeton University, the United States Naval Observatory, and the University of Washington. This publication makes use of data products from the Two Micron
– 37 –
All Sky Survey, which is a joint project of the University of Massachusetts and the Infrared
Processing and Analysis Center/California Institute of Technology, funded by the National
Aeronautics and Space Administration and the National Science Foundation. We gratefully acknowledge NASAs support for the GALEX mission, developed in cooperation with
the Centre National dEtudes Spatiales of France and the Korean Ministry of Science and
Technology. Knock knock! Who’s there?! Cows go. Cows go who? No, silly; cows go moo!
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This preprint was prepared with the AAS LATEX macros v5.2.
– 41 –
Table 1. Galaxy Properties
Galaxy
R.A.
Dec.
D
(Mpc)
SFR
Age
(Gyr)
EW(Hα)
(Å)
log LTIR
(L⊙ )
IRX
12+log(O/H)
B
C
B
B
B
B
C
C
B
C
C
B
B
C
C
C
B
C
C
C
B
C
C
B
C
B
C
C
C
C
B
C
C
C
C
C
C
B
B
C
C
C
C
C
C
0.5+0.2
−0
5+0
−0
0.7+0
−0.2
0.5+0.2
−0
0.7+0
−0.2
0.4+0.1
−0
5+0
−0
5+0
−0
0.5+0.5
−0
+0
5−0
5+0
−0
0.7+0
−0.2
0.3+0.1
−0
5+0
−0
5+0
−0
5+0
−0
+0
0.1−0.13
5+0
−0
5+0
−0
5+0
−0
0.4+0.3
−0
5+0
−0
5+0
−4
0.2+0.5
−0.13
5+0
−0
0.5+0.2
−0
5+0
−0
5+0
−0
5+0
−0
+0
5−0
0.4+0.1
−0.1
1+0
−0
5+0
−0
5+0
−0
0.7+0.3
−0
1+0
−0.3
+0
0.3−0.1
0.4+0.1
−0.12
0.7+0
−0.2
5+0
−0
5+0
−0
2.5+2.5
−1.5
5+0
−0
5+0
−0
0.7+0.3
−0
16.2
31.2
32.8
35.2
8.65
0
9.24(5)
8.87(5)
7.50(4)
8.88(3)
<7
<6.5
0.29(2)
−0.12(2)
0.238(9)
0.122(16)
< −0.11
< −0.19
8.93(11)1
···
8.05(10)2,3
8.40(11)4
···
···
20
12.5
20.6
20.5
27.9
29.6
26.9
22.8
33.6
26.1
30.6
50.9
31.3
10.8
55.5
100
25.6
9.31(5)
7.43(6)
7.24(5)
8.50(12)
9.01(2)
7.49(6)
7.50(4)
9.40(4)
7.97(8)
9.02(3)
7.73(4)
8.34(4)
9.37(3)
8.28(4)
8.85(3)
7.84(4)
8.64(9)
0.037(2)
−0.50(3)
−0.66(2)
0.298(2)
0.416(15)
−0.45(2)
−0.68(2)
0.204(19)
−0.362(15)
0.059(9)
−0.194(19)
0.107(14)
−0.0030(15)
0.45(2)
0.0761(15)
−0.624(17)
−0.29(2)
8.73(4)3,5,6
8.20(10)7,8
7.94(5)9,10
8.36(5)11,3,12
8.10(10)13,4
7.97(3)1
7.70(10)14,15,4
···
7.7(1)1
8.41(9)3,16
···
8.2(2)17,18
8.4(2)19,17
8.56(12)1
8.20(12)20
8.21(5)21,19
···
26.5
30.7
35.7
33
24.4
9.35
122.5
17.4
15.5
43.3
31.4
8.65
5.88
4.81
25.7
43.8
54.6
46.9
42.7
35.2
8.32(4)
8.70(4)
8.70(4)
8.14(4)
8.27(7)
<7.7
6.98(4)
7.42(5)
6.78(2)
7.51(4)
<7.1
6.49(2)
<5.5
5.89(6)
6.87(5)
7.34(3)
8.96(4)
8.09(4)
6.88(6)
8.01(4)
0.045(18)
0.173(18)
0.225(17)
−0.239(13)
−0.112(14)
<0.027
−0.328(17)
−0.19(4)
−0.13(2)
−0.572(18)
< −0.73
−0.97(3)
−0.693(4)
−1.28(3)
−0.26(2)
−0.233(15)
−0.066(16)
−0.175(13)
−0.65(3)
−0.514(11)
8.4(2)19,22
···
7.71(10)23,24
8.08(19)25,26
8.39(17)26,27
···
7.52(8)28,29,27,4
8.2(2)10
···
7.83(8)30,31
7.21(3)32,33
8.7(3)56
7.30(5)34,29
7.84(5)29,35,36,37
7.98(10)38
···
···
8.3(3)22,27
7.95(4)7,8,33
8.30(10)7,8
NGC 24
NGC 45
NGC 55
NGC 59
IC 1574
UGCA 15
00
00
00
00
00
00
09
14
14
15
43
49
56.5
04.0
53.6
25.1
03.8
49.2
−24
−23
−39
−21
−22
−21
57
10
11
26
14
00
47
55
48
40
49
54
8.13
7.07
2.17
5.3
4.92
3.34
NGC 300
UGC 891
UGC 1104
NGC 598
NGC 625
UGC 1176
ESO 245−G005
NGC 672
ESO 154−G023
NGC 1313
NGC 1311
NGC 1487
NGC 1510
NGC 1512
NGC 1522
NGC 1705
NGC 1744
00
01
01
01
01
01
01
01
02
03
03
03
04
04
04
04
04
54
21
32
33
35
40
45
47
56
18
20
55
03
03
06
54
59
53.5
18.9
42.5
50.9
04.6
09.9
03.7
54.5
50.4
16.1
07.0
46.1
32.6
54.3
07.9
13.5
57.8
−37
+12
+18
+30
−41
+15
−43
+27
−54
−66
−52
−42
−43
−43
−52
−53
−26
41
24
19
39
26
54
35
25
34
29
11
22
24
20
40
21
01
04
43
02
37
10
17
53
58
17
54
08
05
00
56
06
40
20
2
10.84
7.5
0.84
4.07
9
4.43
7.2
5.76
4.15
5.45
9.08
9.84
9.64
9.32
5.1
7.65
NGC 1800
NGC 2500
NGC 2537
UGC 4278
NGC 2552
UGC 4426
UGC 4459
UGC 4787
CGCG 035−007
UGC 5272
UGC 5340
UGC 5336
UGC 5364
UGC 5373
UGC 5423
UGC 5456
NGC 3239
NGC 3274
UGC 5764
UGC 5829
05
08
08
08
08
08
08
09
09
09
09
09
09
10
10
10
10
10
10
10
06
01
13
13
19
28
34
07
34
50
56
57
59
00
05
07
25
32
36
42
25.7
53.2
14.6
58.9
20.5
28.4
07.2
34.9
44.7
22.4
45.7
32.0
26.5
00.1
30.6
19.6
04.9
17.3
43.3
41.9
−31
+50
+45
+45
+50
+41
+66
+33
+06
+31
+28
+69
+30
+05
+70
+10
+17
+27
+31
+34
57
44
59
44
00
51
10
16
25
29
49
02
44
19
21
21
09
40
32
26
15
14
23
32
35
24
54
36
32
16
35
45
47
56
52
43
49
08
48
56
8.24
7.63
6.9
7.59
7.65
10.28
3.56
6.53
5.17
7.1
5.9
3.7
0.69
1.44
5.3
3.8
8.29
6.5
7.08
7.88
– 42 –
Table 1—Continued
Galaxy
NGC 3344
UGC 5889
UGC 5923
UGC 5918
NGC 3432
NGC 3486
UGC 6457
UGC 6541
NGC 3738
NGC 3741
UGC 6782
UGC 6817
UGC 6900
NGC 4068
NGC 4144
NGC 4163
UGC 7267
CGCG 269−049
NGC 4288
UGC 7408
UGCA 281
UGC
UGC
NGC
UGC
7559
7577
4449
7599
UGC 7605
UGC 7608
NGC 4485
UGC 7690
UGC 7698
UGC 7719
UGC 7774
NGC 4618
NGC 4625
UGC 7866
NGC 4707
UGC 8024
UGC 8091
UGCA 320
UGC 8201
NGC 5023
CGCG 217−018
R.A.
Dec.
D
(Mpc)
SFR
Age
(Gyr)
EW(Hα)
(Å)
log LTIR
(L⊙ )
IRX
12+log(O/H)
B
B
B
B
C
C
C
C
C
C
B
B
B
C
C
B
C
C
C
B
B
C
C
B
C
B
B
C
C
B
C
C
B
C
C
C
C
B
B
B
C
C
C
C
B
0.7+0.3
−0.2
0.7+0
−0.2
0.5+0
−0.2
0.4+0.1
−0.1
5+0
−0
+0
5−0
5+0
−0
5+0
−0
5+0
−0
1+4
−0
0.7+0.3
−0.2
0.3+0.1
−0
0.7+0.3
−0.2
5+0
−0
5+0
−0
0.5+0.2
−0
5+0
−0
5+0
−0
5+0
−0
1+0
−0.5
0.03+0.07
−0
0.1+0.2
−0
5+0
−0
0.7+0
−0.2
5+0
−0
+0.3
0.2−0.1
0.1+0.1
−0
0.8+0.2
−0.1
+0
5−0
0.1+0
−0.13
5+0
−0
5+0
−0
0.7+0
−0.2
5+0
−0
5+0
−0
5+0
−0
5+0
−0
0.1+0.1
−0
0.7+0
−0.2
0.07+0.13
−0
1+0
−0
1+0
−0.3
5+0
−4
5+0
−0
0.5+0.2
−0
27
7.34
13
18.1
54.2
34.7
25.2
82.7
23.9
54.9
0
25
9.43
26.2
19.3
6.78
10.5
0
40.5
0
325.2
9.43(5)
<7.7
7.45(4)
<7.3
9.24(8)
9.37(4)
7.36(6)
6.72(4)
8.16(4)
6.50(2)
<7.8
6.37(6)
<7.4
7.45(4)
8.83(4)
6.59(5)
7.10(6)
<6.3
8.37(4)
<7.4
7.49(3)
0.38(2)
−0.070(4)
0.233(13)
−0.064(4)
0.357(18)
0.22(2)
−0.44(3)
−0.78(5)
−0.029(16)
−0.79(2)
<0.011
−0.83(3)
<0.26
−0.402(17)
0.044(4)
−0.56(2)
−0.47(3)
< −0.42
0.12(2)
< −0.18
−0.293(15)
8.76(2)39,40,3
···
8.3(2)10
7.84(4)56
···
···
···
7.82(6)41,26,42,43
8.23(1)44,22,27
8.1(2)45
···
7.53(2)1
8.1(3)27
···
···
7.56(14)1
···
7.43(6)46
8.5(2)10
···
7.80(3)47,42
36.2
8.57
58.5
11.4
6.97(5)
6.60(6)
9.38(3)
<7.1
−0.77(2)
−0.69(6)
0.142(18)
< −0.48
···
7.97(6)1
8.31(7)44,40,22,48
···
29.8
49.5
<7.56
7.65(4)
< −0.62
−0.501(18)
7.66(11)1
···
66.7
21.2
40
49.5
21
25.6
16.1
46.2
22.7
26
98.1
50
6.48
17.4
20
8.77(5)
8.06(4)
7.44(5)
7.44(3)
7.43(5)
9.16(4)
8.70(4)
7.01(5)
7.57(5)
<6.7
5.99(5)
7.71(4)
6.88(8)
7.96(2)
7.72(4)
0.19(2)
−0.157(16)
−0.57(3)
−0.30(2)
−0.16(2)
0.196(11)
0.33(3)
−0.80(2)
−0.46(2)
< −0.94
−0.92(2)
−0.801(18)
−1.06(6)
0.042(6)
−0.150(18)
···
···
8.0(2)27
···
···
···
8.4(2)49
···
8.4(2)27
7.67(6)7,50
7.65(6)34,35,51,37
8.1(2)52
7.80(6)55
···
···
10
10
10
10
10
11
11
11
11
11
11
11
11
12
12
12
12
12
12
12
12
43
47
49
49
52
00
27
33
35
36
48
50
55
04
09
12
15
15
20
21
26
31.2
22.3
07.6
36.5
31.1
23.9
12.2
28.9
48.8
06.2
57.4
53.0
39.7
00.8
58.6
09.2
23.7
46.6
38.1
15.0
15.9
+24
+14
+06
+65
+36
+28
−00
+49
+54
+45
+23
+38
+31
+52
+46
+36
+51
+52
+46
+45
+48
55
04
55
31
37
58
59
14
31
17
50
52
31
35
27
10
21
23
17
48
29
20
10
02
50
08
30
41
14
26
01
15
49
07
18
26
09
00
14
30
41
37
6.64
9.3
7.16
7.4
7.89
8.24
10.24
3.9
4.9
3.19
14
2.64
7.47
4.31
9.8
2.96
7.33
3.23
7.67
6.87
5.7
12
12
12
12
27
27
28
28
05.2
40.9
11.1
28.6
+37
+43
+44
+37
08
29
05
14
33
44
37
01
4.87
2.74
4.21
6.9
12 28 38.7
12 28 44.2
+35 43 03
+43 13 27
4.43
7.76
12
12
12
12
12
12
12
12
12
12
12
13
13
13
13
+41
+42
+31
+39
+40
+41
+41
+38
+51
+27
+14
−17
+67
+44
+40
7.07
7.73
6.1
9.39
7.44
7.79
8.65
4.57
7.44
4.3
2.13
7.24
4.57
5.4
8.21
30
32
32
34
36
41
41
42
48
54
58
03
06
12
12
31.1
26.9
54.4
00.5
22.7
32.8
52.7
15.1
22.9
05.2
40.4
16.7
24.9
12.6
51.8
42
42
32
01
00
09
16
30
09
08
13
25
42
02
32
04
15
28
09
19
03
26
12
53
59
03
23
25
28
35
– 43 –
Table 1—Continued
Galaxy
UGC
UGC
NGC
NGC
UGC
NGC
UGC
NGC
R.A.
Dec.
D
(Mpc)
SFR
Age
(Gyr)
EW(Hα)
(Å)
log LTIR
(L⊙ )
IRX
12+log(O/H)
C
C
C
B
C
C
B
C
B
C
B
C
B
C
5+0
−0
5+0
−0
5+0
−0
0.5+0.5
−0
5+0
−0
0.8+0.2
−0.1
0.7+0
−0.2
5+0
−0
0.3+0.2
−0
5+0
−0
+0.2
0.5−0
5+0
−0
0.7+0
−0
5+0
−0
43.4
0
49.6
7.63
11.7
54.2
3.92
24.6
8.76(4)
7.39(7)
8.37(4)
7.75(9)
<6.6
7.72(4)
<6.1
8.50(4)
−0.0101(12)
−0.406(19)
−0.108(14)
−0.09(2)
< −0.44
−0.47(2)
< −0.43
−0.11(2)
···
8.29(7)27,51
···
8.7(2)52
···
8.14(7)53
7.75(5)7,8,29
···
8.26
39.7
0
0.98
23.6
6.85(4)
8.67(4)
8.34(4)
5.93(5)
6.85(5)
−0.519(18)
0.371(12)
−0.368(9)
0.057(8)
−0.77(5)
7.95(3)9,51
···
···
7.93(14)54
7.72(3)13,15,52
8313
8320
5204
5264
8760
5477
9128
5585
13
13
13
13
13
14
14
14
13
14
29
41
50
05
15
19
53.9
28.0
36.5
36.7
50.6
33.3
56.5
48.2
+42
+45
+58
−29
+38
+54
+23
+56
12
55
25
54
01
27
03
43
31
09
07
47
09
40
19
45
8.72
4.33
4.65
4.53
3.24
7.7
2.24
5.7
UGC 9240
IC 5052
NGC 7064
UGC 12613
UGCA 442
14
20
21
23
23
24
52
29
28
43
43.4
05.6
03.0
36.3
45.6
+44
−69
−52
+14
−31
31
12
46
44
57
33
06
03
35
24
2.8
5.86
9.86
0.76
4.27
Note. — Columns list the (1) Galaxy name, (2) Right Ascension, (3) Declination in J2000 coordinates, (4) Luminosity
distance in Mpc, (5) Instantaneous (B) or continuous (C) SFR of the best-fit model to observations, (6) Age of the best-fit
model in Gyr, (7) Hα equivalent width from Kennicutt et al. (2008) in Å, (8) log of the total integrated IR luminosity per solar
luminosity L⊙ , (9) IRX values of log LTIR /LFUV , and (10) 12+log(O/H) metallicity values for each galaxy (when available)
and respective references. Numbers in parentheses indicate uncertainties in the final digit(s) of listed quantities, when available.
In some cases, a galaxy SED can be best-fit with both a continuous and bursting SFR. We have listed both the continuous and
bursting model for these select galaxies.
Metallicity references: (1) – Moustakas et al. (2010); (2) – Tüllmann et al. (2003); (3) – Zaritsky et al. (1994); (4) – Saviane
et al. (2008); (5) – Christensen et al. (1997); (6) – Vila-Costas & Edmunds (1993); (7) – van Zee et al. (1997a); (8) – van Zee
et al. (1997b); (9) – van Zee & Haynes (2006a); (10) – Kewley et al. (2005); (11) – Magrini et al. (2007); (12) – Rosolowsky &
Simon (2008); (13) – Skillman et al. (2003); (14) – Hidalgo-Gámez et al. (2001); (15) – Miller (1996); (16) – Walsh et al. (1997);
(17) – Raimann et al. (2000); (18) – Agüero & Paolantonio (1997); (19) – Storchi-Bergmann et al. (1994); (20) – Masegosa
et al. (1994); (21) – Lee & Skillman (2004); (22) – Hunter et al. (1982); (23) – Gil de Paz et al. (2000a); (24) – Gil de Paz et al.
(2000b); (25) – Kniazev et al. (2004); (26) – Izotov et al. (2006); (27) – Hunter & Hoffman (1999); (28) – Pustilnik et al. (2003);
(29) – Skillman et al. (1989); (30) – Kinman & Davidson (1981); (31) – Hopp & Schulte-Ladbeck (1991); (32) – Pustilnik et al.
(2005); (33) – Hunter & Gallagher (1985); (34) – van Zee et al. (2006b); (35) – Moles et al. (1990); (36) – Lee et al. (2005); (37)
– Stasińska et al. (1986); (38) – Miller & Hodge (1996); (39) – Moustakas & Kennicutt (2006); (40) – McCall et al. (1985); (41)
– Guseva et al. (2000); (42) – Thuan & Izotov (2005); (43) – Buckalew et al. (2005); (44) – Martin (1997); (45) – Gallagher &
Hunter (1989); (46) – Kniazev et al. (2003); (47) – Pérez-Montero & Dı́az (2003); (48) – Kobulnicky (1999); (49) – Gil de Paz
et al. (2007); (50) – Kennicutt & Skillman (2001); (51) – Hidalgo-Gámez & Olofsson (2002); (52) – Lee et al. (2003); (53) –
Izotov et al. (2007); (54) – Skillman et al. (1997); (55) – Berg et al. (2012); (56) – Croxall et al. (2009).
– 44 –
Table 2. Multi-Wavelength Data
Band
Wavelength
Instrument/Survey
FUV
NUV
U
B
V
J
H
Ks
3.6
4.5
5.8
8
MIPS 24
MIPS 70
MIPS 160
1520 Å
2310 Å
3660 Å
4410 Å
5540 Å
1.235 µm
1.662 µm
2.159 µm
3.6 µm
4.5 µm
5.8 µm
8 µm
24 µm
70 µm
160 µm
GALEX
GALEX
RC3/VATT
RC3/VATT
RC3/VATT
2MASS
2MASS
2MASS
Spitzer/IRAC
Spitzer/IRAC
Spitzer/IRAC
Spitzer/IRAC
Spitzer/MIPS
Spitzer/MIPS
Spitzer/MIPS
Note. — Columns list the (1) Photometric band,
(2) Central wavelength of each band, and (3) Instrument or survey the photometric data came
from.
– 45 –
Table 3. Probabilities for Instanteneous Star-Forming Galaxies
Total
Variables
IRX< 0
0 <IRX< 0.5
N
Spearman
Kendall
N
Spearman
Kendall
N
Spearman
Kendall
E(B − V ) vs βGLX
38
FUV/NIR vs βGLX
38
FUV/NIR vs dp
36
EW(Hα) vs βGLX
35
EW(Hα) vs dp
33
U − B vs βGLX
38
U − B vs dp
36
Age vs βGLX
38
Age vs dp
36
12+log(O/H) vs βGLX
24
12+log(O/H) vs dp
22
ρ = −0.013
0.06σ
ρ = 0.32
1.6σ
ρ = −0.80
3.9σ
ρ = 0.59
2.9σ
ρ = −0.76
3.7σ
ρ = 0.78
3.8σ
ρ = −0.68
3.3σ
ρ = 0.44
2.2σ
ρ = 0.76
3.7σ
ρ = −0.77
3.8σ
ρ=0
0σ
ρ = 0.13
0.5σ
τ = −0.03
0.18σ
τ = 0.24
1.7σ
τ = −0.65
4.5σ
τ = 0.41
2.9σ
τ = −0.56
3.9σ
τ = 0.57
3.9σ
τ = −0.51
3.6σ
τ = 0.34
2.4σ
τ = 0.65
4.6σ
τ = −0.61
4.3σ
τ =0
0σ
τ = 0.03
0.2σ
13
36
τ = 0.15
1.3σ
τ = 0.30
2.5σ
τ = −0.61
5.4σ
τ = 0.04
0.4σ
τ = −0.40
3.4σ
τ = 0.40
3.3σ
τ = −0.49
4.3σ
τ = 0.10
0.8σ
τ = 0.57
5.1σ
τ = −0.38
3.2σ
τ = 0.31
2.2σ
τ = 0.29
1.9σ
25
E(B − V ) vs dp
ρ = 0.23
1.4σ
ρ = 0.39
2.3σ
ρ = −0.78
4.7σ
ρ = 0.11
0.6σ
ρ = −0.55
3.2σ
ρ = 0.59
3.3σ
ρ = −0.64
3.9σ
ρ = 0.12
0.7σ
ρ = 0.69
4.2σ
ρ = −0.49
2.9σ
ρ = 0.41
2.0σ
ρ = 0.39
1.8σ
ρ = 0.34
1.1σ
ρ=0
0σ
ρ = −0.66
2.3σ
ρ = −0.05
0.14σ
ρ = −0.11
0.3σ
ρ = −0.37
1.0σ
ρ = −0.28
1.0σ
ρ=0
0σ
ρ = 0.58
2.0σ
ρ = −0.67
2.1σ
ρ = −0.58
1.8σ
ρ = 0.73
1.9σ
τ = 0.25
1.2σ
τ =0
0σ
τ = −0.51
2.4σ
τ = −0.09
0.4σ
τ = −0.018
0.08σ
τ = −0.39
1.5σ
τ = −0.27
1.2σ
τ = 0.02
0σ
τ = 0.48
2.3σ
τ = −0.52
2.3σ
τ = −0.36
1.4σ
τ = 0.55
1.9σ
25
25
25
24
24
25
25
25
25
14
14
11
13
11
11
9
13
11
13
11
10
8
Note. — The Spearman (ρ) and Kendall (τ ) rank correlation coefficients for (1) the entire sample of bursting galaxies, (2)
the bursting galaxies in the range log LTIR /LFUV < 0, and (3) the galaxies in the range 0 < log LTIR /LFUV < 0.5, where
N represents the number of galaxies in each sample for Figure 4 (E(B − V ) vs βGLX ), Figure 5 (E(B − V ) vs dp ), Figure 6
(FUV/NIR vs βGLX ; proxy for the mean age of the stellar population), Figure 7 (FUV/NIR vs dp ), Figure 8 (EW(Hα) vs
βGLX ), Figure 9 (EW(Hα) vs dp ), Figure 10 (U − B vs βGLX ), Figure 11 (U − B vs dp ), Figure 12 (Age vs βGLX ), Figure 13
(Age vs dp ), Figure 14 (12+log(O/H) vs βGLX ), and Figure 15 (12+log(O/H) vs dp ). Below the correlation coefficients are
the significance of the correlation.
– 46 –
Table 4. Probabilities for Continuous Star-Forming Galaxies
Total
Variables
IRX< 0
0 <IRX< 0.5
N
Spearman
Kendall
N
Spearman
Kendall
N
Spearman
Kendall
E(B − V ) vs βGLX
60
FUV/NIR vs βGLX
60
FUV/NIR vs dp
59
EW(Hα) vs βGLX
58
EW(Hα) vs dp
57
U − B vs βGLX
60
U − B vs dp
59
Age vs βGLX
60
Age vs dp
59
12+log(O/H) vs βGLX
36
12+log(O/H) vs dp
35
ρ = 0.26
1.7σ
ρ = 0.13
0.8σ
ρ = −0.65
4.2σ
ρ = 0.38
2.5σ
ρ = −0.58
3.6σ
ρ = 0.61
3.8σ
ρ = −0.56
3.6σ
ρ = 0.38
2.5σ
ρ = 0.46
3.0σ
ρ = −0.35
2.3σ
ρ = 0.46
2.4σ
ρ = −0.14
0.7σ
τ = 0.19
1.8σ
τ = 0.09
0.9σ
τ = −0.47
4.5σ
τ = 0.28
2.7σ
τ = −0.41
3.7σ
τ = 0.44
4.0σ
τ = −0.41
3.9σ
τ = 0.28
2.6σ
τ = 0.37
3.5σ
τ = −0.28
2.7σ
τ = 0.33
2.4σ
τ = −0.12
0.9σ
17
59
τ = 0.15
1.7σ
τ = 0.32
3.5σ
τ = −0.37
4.2σ
τ = −0.02
0.2σ
τ = −0.36
4.0σ
τ = 0.37
4.1σ
τ = −0.36
4.1σ
τ = 0.19
2.1σ
τ = 0.33
3.7σ
τ = −0.13
1.4σ
τ = 0.31
2.7σ
τ = 0.07
0.6σ
43
E(B − V ) vs dp
ρ = 0.21
1.6σ
ρ = 0.42
3.2σ
ρ = −0.52
4.0σ
ρ = −0.05
0.3σ
ρ = −0.51
3.8σ
ρ = 0.52
3.9σ
ρ = −0.49
3.7σ
ρ = 0.25
2.0σ
ρ = 0.41
3.1σ
ρ = −0.15
1.2σ
ρ = 0.44
2.6σ
ρ = 0.11
0.6σ
ρ = −0.010
0.04σ
ρ = 0.06
0.2σ
ρ = −0.52
2.1σ
ρ = 0.40
1.5σ
ρ = −0.33
1.3σ
ρ = 0.48
1.9σ
ρ = −0.25
1.0σ
ρ = 0.20
0.8σ
ρ=0
0σ
ρ=0
0σ
ρ = 0.20
0.6σ
ρ = −0.19
0.5σ
τ =0
0σ
τ = 0.04
0.2σ
τ = −0.38
2.1σ
τ = 0.27
1.4σ
τ = −0.25
1.5σ
τ = 0.34
1.9σ
τ = −0.26
1.4σ
τ = 0.18
0.9σ
τ =0
0σ
τ =0
0σ
τ = 0.06
0.2σ
τ = −0.14
0.5σ
43
43
43
40
40
43
43
43
43
27
27
16
17
16
18
17
17
16
17
16
9
8
Note. — The Spearman (ρ) and Kendall (τ ) rank correlation coefficients for (1) the entire sample of continuous star-forming
galaxies, (2) the continuous galaxies in the range log LTIR /LFUV < 0, and (3) the galaxies in the range 0 < log LTIR /LFUV <
0.5, where N represents the number of galaxies in each sample for Figure 4 (E(B − V ) vs βGLX ), Figure 5 (E(B − V ) vs dp ),
Figure 6 (FUV/NIR vs βGLX ), Figure 7 (FUV/NIR vs dp ), Figure 8 (EW(Hα) vs βGLX ), Figure 9 (EW(Hα) vs dp ), Figure
10 (U − B vs βGLX ), Figure 11 (U − B vs dp ), Figure 12 (Age vs βGLX ), Figure 13 (Age vs dp ), Figure 14 (12+log(O/H)
vs βGLX ), and Figure 15 (12+log(O/H) vs dp ). Below the correlation coefficients are the significance of the correlation.
– 47 –
Table 5. Probabilities for Entire Sample of Star-Forming Galaxies
Total
Variables
IRX< 0
0 <IRX< 0.5
N
Spearman
Kendall
N
Spearman
Kendall
N
Spearman
Kendall
E(B − V ) vs βGLX
98
FUV/NIR vs βGLX
98
FUV/NIR vs dp
95
EW(Hα) vs βGLX
93
EW(Hα) vs dp
90
U − B vs βGLX
98
U − B vs dp
95
Age vs βGLX
98
Age vs dp
95
12+log(O/H) vs βGLX
60
12+log(O/H) vs dp
57
ρ = 0.15
1.3σ
ρ = 0.19
1.6σ
ρ = −0.69
5.6σ
ρ = 0.47
3.8σ
ρ = −0.59
4.7σ
ρ = 0.65
5.1σ
ρ = −0.55
4.5σ
ρ = 0.41
3.4σ
ρ = 0.27
2.2σ
ρ = −0.18
1.5σ
ρ = 0.30
1.9σ
ρ = −0.07
0.4σ
τ = 0.10
1.2σ
τ = 0.15
2.1σ
τ = −0.51
6.1σ
τ = 0.33
4.0σ
τ = −0.41
4.8σ
τ = 0.46
5.3σ
τ = −0.40
4.8σ
τ = 0.29
3.5σ
τ = 0.21
2.5σ
τ = −0.13
1.5σ
τ = 0.22
2.0σ
τ = −0.05
0.5σ
30
95
τ = 0.15
2.2σ
τ = 0.46
4.6σ
τ = −0.45
6.6σ
τ = 0.016
0.2σ
τ = −0.36
5.1σ
τ = 0.37
5.1σ
τ = −0.39
5.7σ
τ = 0.16
2.3σ
τ = 0.12
1.7σ
τ = −0.05
0.7σ
τ = 0.21
2.4σ
τ = 0.15
1.6σ
68
E(B − V ) vs dp
ρ = 0.22
2.1σ
ρ = 0.42
4.0σ
ρ = −0.62
6.1σ
ρ = 0.02
0.2σ
ρ = −0.51
4.9σ
ρ = 0.52
4.9σ
ρ = −0.53
5.3σ
ρ = 0.22
2.2σ
ρ = 0.15
1.5σ
ρ = −0.07
0.6σ
ρ = 0.31
2.3σ
ρ = 0.20
1.5σ
ρ = −0.05
0.2σ
ρ = 0.11
0.6σ
ρ = −0.64
3.4σ
ρ = 0.28
1.5σ
ρ = −0.35
1.8σ
ρ = 0.31
1.6σ
ρ = −0.41
2.2σ
ρ = 0.24
1.2σ
ρ = −0.13
0.7σ
ρ = 0.15
0.7σ
ρ = −0.20
0.9σ
ρ = 0.18
0.7σ
τ = −0.03
0.2σ
τ = 0.09
0.6σ
τ = −0.46
3.6σ
τ = 0.19
1.4σ
τ = −0.25
1.9σ
τ = 0.22
1.6σ
τ = −0.33
2.5σ
τ = 0.19
1.4σ
τ = −0.09
0.7σ
τ = 0.10
0.7σ
τ = −0.12
0.7σ
τ = 0.12
0.6σ
68
68
68
64
64
68
68
68
68
41
41
27
30
27
29
26
30
27
30
27
19
16
Note. — The Spearman (ρ) and Kendall (τ ) rank correlation coefficients for (1) the entire sample of star-forming
galaxies (continuous and bursting), (2) the galaxies in the range log LTIR /LFUV < 0, and (3) the galaxies in the range
0 < log LTIR /LFUV < 0.5, where N represents the number of galaxies in each sample for Figure 4 (E(B − V ) vs βGLX ),
Figure 5 (E(B − V ) vs dp ), Figure 6 (FUV/NIR vs βGLX ), Figure 7 (FUV/NIR vs dp ), Figure 8 (EW(Hα) vs βGLX ),
Figure 9 (EW(Hα) vs dp ), Figure 10 (U − B vs βGLX ), Figure 11 (U − B vs dp ), Figure 12 (Age vs βGLX ), Figure 13 (Age
vs dp ), Figure 14 (12+log(O/H) vs βGLX ), and Figure 15 (12+log(O/H) vs dp ). Below the correlation coefficients are the
significance of the correlation.