Download Constraining Star-Formation History in SN Ia Host Galaxies Using

Constraining Star-Formation History in SN Ia Host Galaxies
Using Multi-wavelength Photometry
Ravi Gupta1, M. Sako1, C. Conroy2, M. Bernardi1, L. Greggio3, M. Morris1, B. Dilday4, J. A. Frieman5, R. C. Nichol6, M. Smith6, 7
1University of Pennsylvania; 2Princeton University; 3INAF, Osservatorio
Astronomico di Padova, Italy; 4Rutgers University;
5Kavli Institute for Cosmological Physics, University of Chicago; 6Institute of Cosmology and Gravitation, University of Portsmouth, United Kingdom; 7University of Cape Town, South Africa.
INTRODUCTION
Observations of Type Ia supernovae (SNe Ia) are a key component of the standard cosmological model.
Their luminosity-distance calibration provides evidence for the existence of dark energy, but the nature of their
progenitor system remains unknown. Studying physical properties of the host galaxies can provide insight into
the environment and stellar populations in which SNe Ia occur. Using host galaxy photometry spanning the
ultraviolet, optical, and infrared bands allows us to constrain stellar masses and star-formation history of SNe
Ia host galaxies by comparing the observed photometry to synthetic photometry generated from stellar
population synthesis codes. Future work with spectroscopy can be used to better estimate the metallicity of
these galaxies as well. Knowledge of both star formation and metallicity of host galaxies will improve our
understanding of SNe Ia progenitors and the diversity of their light curves.
RESULTS
ƒ ~ 30% of our galaxies have SFHs best fit by τ = 10 Gyr (very slow exponential decline) and/or C = 1 (all stars
formed at a constant rate) which implies that they are actively star-forming
ƒ ~ 30% best fit by dust2 = 0.0, with the distribution of higher values decreasing exponentially
ƒ ⅓ best fit by the lowest metallicity allowed (Z = 0.001), and the fraction steadily decreases with increasing Z until
a gentle turn-around point is reached at Z☼ (~5% of galaxies), while a Z of 0.03 best fits ~11% of galaxies
ƒ From the best-fit parameters we derive two galaxy properties:
• stellar mass – calculated by multiplying the luminosity in the r band by the mass-to-light ratio in the same
band as provided by the best-fit model
• current average star-formation rate (SFR) – calculated by averaging the best-fit normalized SFH, Ψ(t),
over the past 108 years and converting into an absolute SFR (in M☼/yr) using the stellar mass
DATA SAMPLE
ƒ Galaxy Evolution Explorer (GALEX) FUV &
NUV + Sloan Digital Sky Survey (SDSS) ugriz +
UKIRT Infrared Deep Sky Survey (UKIDSS) YJHK
photometry
Plot comparing the data with the best­fit model for a particular galaxy. Synthetic photometry is redshifted to the observed frame and the model spectrum is overplotted.
ƒ 459 SDSS galaxies identified as hosts for the
spectroscopically confirmed SNe Ia discovered in
the SDSS-II Supernova Survey (0.01 < z < 0.48)
ƒ We attempt to correlate our galaxy properties with SN Ia properties obtained
from the multicolor light-curve shape method fitter of Jha et al. 2007
(MLCS2k2). [Of the galaxies in our sample, 313 passed the S/N and temporal
coverage cuts necessary for MLCS]
ƒ GALEX magnitudes and UKIDSS Petrosian
magnitudes are matched to these SDSS galaxies
with a search radius of 5″. Of the 459 SDSS
galaxies, 192 GALEX matches and 289 UKIDSS
matches were found
ƒ In particular, we hoped to confirm the result that, on average, intrinsically
luminous SNe Ia tend to occur in blue spiral galaxies with ongoing star
formation while dimmer SNe Ia tend to occur in passive galaxies (Hamuy et al.
2000, Sullivan et al. 2006, and others). We do find some evidence that
supports this trend that galaxies with higher SFRs host more luminous SNe Ia
(i.e., SNe Ia with lower values of the MLCS parameter Δ, which is a particular
SN’s under- or overluminosity)
ƒ 3 surveys together provide complete coverage
from the ultraviolet through the near infrared
METHOD
ƒ Generate grid of models using the flexible stellar population synthesis (FSPS) code of Conroy et al.
2009a. The code is able to flexibly handle elements such as the initial mass function (IMF), spectral libraries,
dust attenuation, convective overshooting, and advanced stages of stellar evolution
The data shows that a large fraction of our SNe are intrinsically brighter (low Δ), and that many of these have SFR > 0. In addition, galaxies with Δ > 0.5 (dimmer SNe Ia) all have SFR < 0.
ƒ More work is needed to discover further correlations between galaxy
properties such as metallicity, age, dust attenuation, stellar mass, and SFR and
SN Ia properties such as AV and Δ. A thorough investigation of errors is also
necessary to quantify the certainty of any correlations found
The SFR­Mass plane. These results roughly reproduce the results of Sullivan et al. 2006 (Fig. 5). “Passive” galaxies are grouped toward the lower right of the plot.
ƒ We use Padova isochrones, the BaSeL3.1 spectral library, and the Chabrier 2003 IMF
ƒ We assume a flat ΛCDM cosmology of (Ωm, ΩΛ, h) = (0.26, 0.74, 72). We also assume the age of each
galaxy is equal to the age of the Universe at the redshift of the galaxy and star formation begins at t=0 (Big
Bang)
ƒ Model photometry is redshifted to the observed frame of the galaxy for comparison
ƒ We choose to parametrize the star formation history (SFH), Ψ, by a 2-component τ + C model, where τ is
the e-folding timescale of the exponentially decreasing component and C is the fraction of star formation that
occurs at a constant rate:
Ψ (t ) =
(1 − C )
τ
e −t /τ
C
+
1 − e −Tuniv /τ Tuniv
ƒ We find some evidence that galaxies with higher SFRs on average host more luminous SNe Ia
where Ψ is normalized such that 1 M☼ of stars is created over the age of the Universe, Tuniv
ƒ We also employ the 2-component dust model of Charlot & Fall 2000, in which the attenuation of starlight
is described by the optical depth, τλ(t):
−0.7
t ≤ 107 yr
⎪⎧τ (λ / 5500 Å)
τ λ (t ) ≡ ⎨ 1
−0.7
t > 107 yr
⎪⎩τ 2 (λ / 5500 Å)
ƒ The models are composite stellar populations (CSPs) generated by looping over 4 fit parameters:
metallicity (Z), the 2 SFH parameters (τ and C), and the τ2 describing attenuation of old stellar light (denoted
by dust2 in the FSPS code). We fix τ1 = 3τ2 which, on average, has been shown to be a good approximation
(Charlot & Fall 2000, Kong et al. 2004). Our fit parameter ranges and values are
Z
0.001, 0.0025, 0.0049, 0.0077, 0.012, 0.019(☼), 0.03
τ (Gyr)
0.1, 0.5, 1, 2, 3, 4, 6, 8, 10
C
0.0, 0.2, 0.4, 0.6, 0.8, 1
dust2
0.0, 0.1, 0.3, 0.5, 1.0, 1.5
CONCLUSIONS & FUTURE WORK
ƒ We fit for metallicity, extinction, and SFH & derive stellar masses and SFRs for a sample of 459 SNe Ia host
galaxies and find that ~ 30% of them are actively star-forming
ƒ Further analysis is needed to investigate sources of error and estimate uncertainties on stellar masses and SFRs
ƒ Even with just 4 fit parameters, degeneracies in the models exist and can affect best-fit determinations and
derived parameters. For example, it is very likely that two or more completely different sets of the parameters (Z, τ,
C, dust2) can produce very similar colors but result in different calculations of the SFR
ƒ For increased accuracy and efficiency, we intend to refine the parameter grid and switch from a grid search to a
MCMC method. The MCMC method will also make it easier to quantify errors and determine confidence limits on
parameters
ƒ We will also cull a larger, more complete data set and hope to include available galaxy spectra in order to better
constrain parameters such as metallicity
ƒ In addition, we would like to compute SN Ia rates and see how they are related to the host galaxy properties we
derive. Investigating SN Ia rates as a function of known morphological type would also provide insight into the
environment of Ia’s.
This produces a grid of 2268 models
REFERENCES
ƒ Observed photometry is corrected for Milky Way extinction and converted from magnitudes to fluxes
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