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The handle http://hdl.handle.net/1887/21950 holds various files of this Leiden University
dissertation.
Author: Sadatshirazi, Maryamosadat
Title: Nearby and distant star-forming galaxies as seen through emission lines
Issue Date: 2013-10-15
Chapter
1
Introduction
Galaxies with all their varieties, have been home to billions of stars during their
life. It is because of the presence of these shining stars that we are able to observe
them through the cosmic time. Although we observe galaxies mostly through the
light emitted by their stars, we cannot resolve these stars individually unless they
are very close by. Because of this, the cumulative light from billions of stars in
every galaxy is analyzed using stellar population models to extract information
about the evolution of galaxies. Stellar light does not reach us without passing
through the interstellar medium (ISM) which contains clouds of gas and dust
particles. Gas and dust can absorb and re-emit the light from stars, or scatter it
towards us and make interpreting what we observe in galaxies very complicated.
Despite all these difficulties, just by analyzing the total light from galaxies, we
can constrain the global physical properties of galaxies such as stellar mass, star
formation rate and age, based on the stellar population models. By combining
stellar population models and photoionization models we can further analyze the
emission line spectrum of star-forming galaxies coming from ionized gas around
young stars which provide us with a wealth of information about the small-scale
properties of galaxies e.g., the ISM. This thesis is an attempt in understanding
the relation between these small-scale properties and global properties of starforming galaxies over cosmic time using stellar population synthesis models and
photoionization models.
Introduction
1.1
Nebular physics
Star formation in galaxies can be traced not only by stars directly, but also through
studying their impacts on their surrounding gas. The radiation from stars ionizes
the gas around them and produces nebular emission lines as a result of ionized gas
recombination. We can measure the star formation rate, along with other galaxy
properties, from emission lines coming from both nearby and distant galaxies.
In this thesis I extensively use nebular emission lines for measuring the intrinsic
properties of galaxies in the nearby and the distant Universe. In the following,
I briefly review the physics of ionized gas and mechanisms that produce strong
optical emission lines. I also discuss how we can use emission line ratios to constrain
various galaxy properties. I continue by discussing that how we can use emission
lines to indirectly trace massive stars and also to probe small-scale properties of
the ISM.
1.1.1
Nebular emission lines in star-forming galaxies
The optical emission line spectrum contains the Balmer series of Hydrogen (e.g.,
Hδ, Hγ, Hβ, Hα), together with lines of Helium (e.g., He ii λ4686) and numerous
other emission lines such as Oxygen (e.g., [O i] λ6300, [O ii] λ3726, [O iii] λ5007),
Nitrogen (e.g., [N ii]λ6584), Sulfur (e.g., [S ii]λ6716). Figure 3.7 shows an example
of the emission line spectrum of a nearby actively star-forming galaxy. The source
of radiation for producing these emissions in star-forming galaxies is mostly hot,
luminous stars with spectral types O and B. OB stars are very massive and shortlived stars with effective temperatures, T e , > 20000 K which enable them to emit
ultraviolet radiation at λ <912 Å. These ultraviolet photons can ionize neutral Hydrogen atoms (H i) around OB stars and produce ionized Hydrogen (H ii) regions.
The photoionization (the removal of a bound electron from the atom by a photon,
e.g., H i+hν → H ii + e− ) and recombination processes (combining free electron by
the ionized atom, e.g., H ii + e− → H i + hν) are two basic processes that derive
the physics of nebulae. The Balmer lines are produced by cascades of electrons in
Hydrogen atoms from energy levels above n = 2, following recombination of the
highly ionized gas (see Table 1.1 for Hydrogen Balmer recombination lines). The
energy of the photons created through the recombination process depends on the
kinetic energy of the free electrons and the binding energy of the bound-level into
which the recombination occurs (e.g., n = 2 for Balmer lines).
The other lines of importance are the collisionally-excited lines (e.g.,
[O iii] λ5007 Å). These lines are known also as forbidden lines because they are
forbidden by quantum selection rules. Excitation potential from sub-levels or fine
structure splitting of the ground level in elements that are heavier than Hydrogen
and Helium which are called metals (e.g., see fine structure splitting of [O iii] in
Figure 2.2) to upper energy level is ∼ 1 eV. This is approximately equal to the
typical thermal energy of the electrons (KT e ∼ 1 eV at T e = 104 K). Collisions with
free electrons excite bound electrons in the lower level of atoms to higher level
and take the energy of free electrons. Therefore, the formation process of these
forbidden lines by removing kinetic energy from the gas and transforming it to
2
Nebular physics
line
Hα
Hβ
Hγ
Hδ
λ (Å)
6563
4861
4340
4102
transition
n=3→n=2
n=4→n=2
n=5→n=2
n=6→n=2
Table 1.1 Balmer transitions in Hydrgen
photon energy which escapes from the gas can cool the nebula. The Differences
in the wavelengths of the lines lead to differences in their velocities and therefore
differences in their collisionally-excitation rates.
The low energy states of singly ionized Oxygen ([O ii] ) and doubly ionized
Oxygen ([O iii] ) are illustrated in Figure 2.2. The forbidden transitions amongst
these levels include the lines at 3726, 3729, 7319, 7320, 7329 and 7330 Å for [O ii]
and 5007, 4959 and 4363 Å for [O iii] .
1.1.2
Measuring galaxy properties using emission line ratios
Two fundamental measures of the physical properties in the ISM of galaxies are
the temperature and the density. Emission line intensities of forbidden lines can be
used for measuring the temperature of H ii regions. For instance, the line ratios of
([O iii] λ4959 + [O iii] λ5007)/[O iii] λ4363 can be used to measure the electron temperature. In a hot nebula, the [O iii]λ4363/[O iii]λ5007 or [O iii]λ4363/[O iii]λ4959
ratios are higher than a cooler nebula. This is because to produce [O iii] λ4363
an electron needs to be excited to 5.3 eV, which requires more energy than for
4959/5007 which result from decay from 1 D2 level, 2.5 eV above the ground-state
(see Figure 2.2). Therefore, these ratios can be used to constrain the electron
temperature in a nebula.
For measuring the electron densities, the [O ii] λ3726, 3729 doublet or the
[S ii] λ6716, 6731 doublet can be used (see Figure 5.8 in Osterbrock & Ferland
2006 for the density dependence of these line ratios for a given nebula temperature). For example, [O ii] λ3726 and [O ii] λ3729 are two lines of the same ion which
are emitted from different levels with nearly the same excitation energy. However,
because they have different transition probabilities and different collisional excitation rates, the relative population of the two levels or the ratio of their intensities
depend on electron densities.
Depending on the production rate of hydrogen ionizing photons, Q, produced
by stars and density of H i cloud surrounding stars, a specific radius is ionized by
them which is known as Strömgren radius (3Q/4πnH 2 αB ) where is the volume
filling factor of the ionized gas, which is defined as the ratio between the volumeweighted and mass-weighted average hydrogen densities (Charlot & Longhetti,
2001) and αB is the case-B Hydrogen recombination coefficient (Osterbrock & Ferland, 2006). The Strömgren sphere (H ii region) grows with time until equilibrium
between ionization and recombination is reached. After assuming that most of
ionizing photons are absorbed locally, the volume averaged ionization parameter
3
Introduction
Figure 1.1 A spectrum of a nearby strongly star-forming galaxy with strong emission lines indicated. The galaxy spectra (PlateID-MJD-FiberID: 752-52251-340) is
taken from SDSS DR7 (Abazajian et al., 2009) which covers a wavelength range of
3800-9200 Å. Bruzual & Charlot (2003) stellar population model is used to fit the
continuum which is shown by black solid line. The spectra were analyzed using
the methodology discussed in Tremonti et al. (2004, see also Brinchmann et al.
2004) to provide accurate continuum subtraction and were additionally analyzed
using the platefit pipeline discussed in Brinchmann et al. (2008b) to measure a
wider gamut of emission lines. The blue line shows the measured nebular emission
lines.
in a typical ionized region is:
< U >3 ≈
α2B 3Q(t) nH 2
(
),
4π
c3
(1.1)
The ionization parameter which is a measure of intensity of ionizing sources
and also Hydrogen number density can be estimated using emission line ratios of
high ionization lines to low ionization lines (e.g., [O iii] λ5007/[O ii] λ3727 ratio).
Emission line ratios are also used for classifying galaxies in terms of their
sources of ionization/excitation (see Figure 2.3). For instance, [O iii] λ5007/Hβ
is strongly correlated with hardness of source of ionization/excitation and temperature of ionized gas. Therefore, galaxies with different main ionizing sources
are distributed differently in diagnostic diagrams (e.g. the BPT diagram, Bald4
Nebular physics
[OII]
2
2
[OIII]
1
1/2
P3/2
7330
7329
7320
7319
3/2
D5/2
3729
4
S3/2
S0
4363
1
2321
D2
3726
5007
3
P
4959
2
1
0
Figure 1.2 Energy level diagram of the 2p3 ground configuration of singly ionized Oxygen ([O ii] ) and doubly ionized Oxygen ([O iii] ) are illustrated based on
Osterbrock & Ferland (2006). Splitting of the ground 3 p is exaggerated for the
[O iii] and energy levels in the two diagrams are not comparable. All emissions
are in the optical except lines 2321 Å which is in the ultraviolet. All wavelengths
shown are in Å.
win, Phillips & Terlevich, 1981) that are based on the line ratios that correlated with the metallicity (e.g., [N ii] λ6584/Hα) and ionization properties (e.g.,
[O iii] λ5007/Hβ) of galaxies. In star-forming galaxies where the main source of
ionization/excitation is star formation, electrons lose their energy more efficiently
through optical transitions (strong [O iii] λ5007 or high [O iii] /Hβ) at low metallicity. When the metallicity increases, metal line cooling gets stronger and the
electron temperature drops. Thus, electrons evacuate their energies through low
ionization lines in infrared (e.g., [O iii] λ88 µm) which makes high ionization lines
weaker (weak [O iii] λ5007 or low [O iii] /Hβ). Other sources of ionization such as
shock and active galactic nuclei (AGN) can be the source of strong [O iii] λ5007
or high [O iii] /Hβ, at high metallicities. Using these line ratio diagrams, galaxies
are generally classified as star-forming, AGN or composite galaxies (Kewley et al.,
2001; Kauffmann et al., 2003; Kewley et al., 2013), where the source of ionizing radiation for composite galaxies could be a combination of star formation and AGN
or shock.
5
Introduction
Figure 1.3 This plot shows the BPT diagnostic diagram that is used for classifying
galaxies in terms of their main source of ionization/excitation. The distribution
of emission line galaxies in the SDSS is shown by the colored 2D distribution
where the color-scale shows the logarithm of the number of galaxies in each bin.
The classification line presented by Kauffmann et al. (2003) is shown as a dashed
line, galaxies below this line are star-forming (SF) galaxies. Kewley et al. (2001)
classification line is shown as a solid line, galaxies between this line and dashed
line are composite galaxies (comp) and galaxies above this line are classified as
AGN.
1.1.3
Emission lines as indirect tracers of massive stars
As was mentioned earlier, the main source of ionization for producing strong emission lines in star-forming galaxies is very massive stars. However, our knowledge
about these massive stars is limited because direct observations of them often
cannot be carried out as these stars are often heavily enshrouded and at low
metallicities, they are only found outside the Milky Way. Despite the small fraction of these stars among billions of stars in star-forming galaxies, and their very
short life times (e.g., a few million years), they have a significant impact on the
galaxy evolution through their hard radiation, strong winds and their explosions
as supernovae.
At very high energies (e.g. λ < 228 Å) normal OB stars, emit a negligible
number of photons. In standard models of massive stars, stars in the Wolf-Rayet
(WR) phases have sufficiently hard spectra at these wavelengths. This is, however,
6
Nebular physics
a poorly tested assumption in general, and particularly at low metallicities. The
ionizing spectrum of WR stars is still subject to significant uncertainty and softer
spectra are being predicted in more recent models (Schaerer, 1996; Smith et al.,
2002). WR stars are very short lived (e.g., a few Myr) and typically have masses
of 10 − 25 M and they are descended from O-type stars (Meynet & Maeder, 2005;
Crowther, 2007).
In order to produce the observed number ratio of WR to O stars, rotation
and binary evolution should be considered in the models of massive stars (see
e.g, Brinchmann et al., 2008a). These two effects help removing the outermost
atmospheres of stars, thus encouraging the formation of hot WR stars. However,
despite an extensive effort in modeling these massive stars, the impact of rotation
on the models is still uncertain (Meynet & Maeder, 2005; Heger et al., 2005). The
role of binary evolution or single star evolution, especially at low metallicities is
also still open for discussion (Han et al., 2007; Eldridge et al., 2009).
Although we cannot observe the high energy part of the continuum coming
from massive stars due to interstellar absorption we can observe emission lines
that are produced by hard ionization from these stars. We can use these high
ionization emission lines such as the nebular He ii λ4686 emission line to indirectly
trace massive stars. This is a recombination line with ionization potentials of 54.4
eV, coming from ionized gas around very massive stars. Using this, we can probe
the high energy part of the spectral energy distribution of very massive stars.
1.1.4
Emission lines as indirect tracers of the ISM
Recent studies have shown that star formation conditions at high redshift (high-z)
are different from what we observe in the local Universe. For instance, it has been
shown that high-z emission line galaxies are systematically offset from low redshift
(low-z) trends in emission line ratio diagrams. This is seen particularly well in the
BPT diagram, log [N ii]/Hα vs. [O iii]/Hβ diagram (Brinchmann et al., 2008b;
Liu et al., 2008). In this diagram, high-z star-forming galaxies unlike low-z ones
are distributed in the regions that need other sources of ionization/excitation than
star formation (e.g., Shapley et al., 2005; Erb et al., 2006; Newman et al., 2013).
Kewley et al. (2013) recently studied the cosmic evolution of the BPT diagram and
showed that the extreme ISM conditions at high-z cause this offset between distant
and nearby galaxies in the BPT diagram. Therefore, we can use the emission line
intensities of distant galaxies to indirectly trace the ISM at high-z and to study
the evolution of intrinsic physical properties of star-forming galaxies from distant
to nearby Universe. However, studying the evolution of the physical conditions at
which stars are forming has proven very challenging and is hidden in the strong
evolution of global mean properties of galaxies such as stellar mass and SFR. In
the next section, I briefly summarize these evolutions in the global properties of
star-forming galaxies from distant to nearby Universe.
7
Introduction
1.2
Evolution in the properties of star-forming
galaxies
The average integrated properties of star-forming galaxies have evolved significantly during the last ∼ 12 Gyr. When the Universe was only 2-3 Gyr old (redshift,
z ∼ 3 − 2), star formation in typical galaxies was happening at a rate that today is
only found in the most extreme star-forming galaxies (Brinchmann et al., 2004).
The star formation rate of the Universe within a comoving volume element as a
function of redshift was first presented by the Madau plot (Madau et al., 1996) and
confirmed by further observations (see Figure 1.4 that shows the evolution of the
SFR density of Universe with redshift, data are taken from Hopkins & Beacom,
2006).
These very high star formation rates have been measured based on rest-frame
UV emission from young stars in high-z galaxies (e.g., Noeske et al., 2007; Daddi et
al., 2007) or infrared observations that determine the contribution of obscured light
to the SFR of high-z galaxies (e.g., Elbaz et al., 2007, see also Shapley, 2011 for
other methods used for estimating SFR of distant galaxies). A strong correlation
between SFR and stellar mass is observed at high-z known as the star-forming
main sequence (Noeske et al., 2007; Daddi et al., 2007). This tight main sequence
locus evolves smoothly with redshift showing that galaxies with the same stellar
mass at low-z and high-z have higher SFR at high-z (see Bouché et al., 2010). The
high SFR of these galaxies implies they are much more gas rich than local starforming galaxies. High gas fractions (several times higher than what we observe in
the local galaxies) also were observed for some of these high-z galaxies (e.g., Daddi
et al., 2008; Tacconi et al., 2010; Genzel et al., 2010; Tacconi et al., 2013). A more
clumpy morphology has been observed for many of high-z star-forming galaxies
(e.g., Elmegreen & Elmegreen, 2006; Genzel et al., 2011; Wuyts et al., 2012).
These clumpy structures can be caused from gravitational instability within these
very gas rich disks at high-z.
1.2.1
Stellar mass evolution
Stellar population synthesis models can be used to estimate the global physical
properties of galaxies such as stellar mass. Based on stellar population synthesis
models (e.g., Bruzual & Charlot, 2003), a combination of optical broadband photometry and spectral indices (the 4000Å spectral break and the strength of Balmer
absorption lines) can be modeled to measure stellar masses in nearby Universe
(e.g., Kauffmann et al., 2003). At higher redshifts, however, due to lower signalto-noise, stellar absorption features are difficult to measure and only broadband
photometry tends to be used for measuring stellar masses. The rest-frame near-IR
luminosity is more closely tied to stellar mass than the rest-frame optical luminosity (Bell & de Jong, 2001). The rest-frame UV emission from distant galaxies
can only probe their massive stars light. Therefore, these data should be combined with longer wavelengths observations to probe older stellar populations of
high-z galaxies. With availability of these observations for many of high-z galaxies
in recent years, the global evolution of the stellar content in galaxies from the
8
Evolution in the properties of star-forming galaxies
Figure 1.4 The SFR density of the Universe as a function of redshift. The data
have been taken from Hopkins & Beacom (2006).
distant to the nearby Universe has become possible to estimate with good accuracy. The evolution in stellar mass can be described by constructing the galaxy
stellar mass function at a range of redshifts. Based on recent observations in the
COSMOS/ULTRAVISTA survey which confirms previous measurments, a strong
evolution in stellar mass has been observed from z = 4 to z = 0.2 (e.g., Ilbert et al.,
2013; Muzzin et al., 2013). These studies show the mass density of star-forming
galaxies grows by a factor of 1.59 since z = 3.5 and the typical mass of a galaxy of
Log(M∗ /M ) = 10.5 at z = 0.3 would be Log(M∗ /M ) . 9.5 at z = 2.
1.2.2
Mass-metallicity evolution
As galaxies evolve, they form more stars and their gas content is converted into
stars and their metal content increases. Thus, stars made out of material that has
been enriched for many generations will be more metal-rich. Some of the metals
that are produced will be ejected out of the galaxy through outflows into the
intergalactic medium (IGM). Galaxies also accrete some gas from the IGM. Gas
metallicities are derived from emission line properties and stellar metallicities are
derived from Lick indices (Faber, 1973). Based on these metallicity measurements,
we can study the evolution in the metallicity of galaxies from distant to nearby
Universe.
There is an evolution in the relation between stellar mass and metallicity known
as mass-metallicity relation as we look back in time. This evolution shows that
galaxies with the same stellar mass have lower metallicities at high-z compared to
9
Introduction
similar galaxies in the local Universe. However, Mannucci et al. (2010); Lara-López
et al. (2010) found that when including the SFR, mass-metallicity-SFR relation
holds up to z ∼ 3 which means galaxies with the same stellar mass have higher
SFR when they show lower metallicity. There should be also a more fundamental
relation between atomic gas mass, SFR and metallicity as it is observed for local
galaxies (Bothwell et al., 2013). This suggests that the reason for having high
SFR at low metallicity is because of having more gas. However, because of the
lack of enough atomic gas data available for high-z galaxies (e.g., H i gas cannot
be observed at z > 0.4 with current instrumentation, and the molecular, i.e. H2 ,
contents of high-z galaxies are estimated from CO observations), this has not been
studied yet.
1.3
Star and galaxy formation over cosmic times
After the Big Bang, the Universe had no stars but was filled with only gas and dark
matter. Dark matter perturbations in the early universe grew gravitationally and
ended up as galaxy dark matter haloes. The gas which is bound to dark matter
haloes radiates its energy away and cools down. Because of the conservation of
angular momentum, this collapsing gas forms a rotating disk within which smallscale instabilities could grow to form molecular clouds. These molecular clouds
have been the birth place for most stars. Gravitational instability with the critical
density that is set by turbulence from stellar feedback (both negative and positive
through heating of gas, and compressing it) is believed to determine star formation
processes. Although we know the formation of stars to the first order, we do not
know well the physical processes that control the interplay between gas and stars.
Therefore, understanding the star formation history over cosmic time remains a
major theoretical and observational challenge.
Understanding how galaxies were assembled across the cosmic time also remains
a challenging question. For instance, how today’s Hubble Sequence with different
galaxy morphologies is shaped and which physical processes can constrain and
control the galaxy evolution, are important questions to be addressed. In the
standard scenario, galaxies are believed to form as disk galaxies, which can then
be transformed into ellipticals mainly due to major mergers. If new gas from the
merger remnants is able to cool then new disks can form and this process can make
disk-bulge systems (e.g., Kauffmann et al., 1993; Baugh et al., 1996). However,
galaxies at the peak of star formation in the Universe show very distinct features,
such as clumpy morphologies. The nature of these clumps and their evolution
determines whether host galaxies have inside-out growth and form bulges from
migration of these clumps towards the center.
Another important question in this regard is what fuels star formation. Hydrodynamical cosmological simulations predict that at high-z, gas accretion plays
a significant role for fueling star formation (Kereš et al., 2005). The existence
of disk-like kinematics in star-forming galaxies during the peak of star formation
suggests that gas accretion is the dominant process for growth of galaxies. In the
local Universe, however, mergers are believed to play a more dominant role.
10
Towards probing the small-scale properties of distant galaxies
1.3.1
From clumps to bulges
Small-scale instabilities in a rotationally supported gaseous disk are unstable
against gravitational collapse and can grow if the Toomre stability parameter
(Toomre 1964), Qgas < 1 where the Q parameter for stability of a disk is:
Qgas =
κσ
πGΣgas
(1.2)
where σ is velocity dispersion and Σgas is the gas mass density. κ = a v/R is the
epicyclic frequency where a is a dimensionless factor 1 < a < 2 depending on the
rotational structure of the disk, v is the circular velocity and R is the radius.
A clump of gas that is large enough, i.e. larger than the Jeans length
L J ' σ2 /GΣgas , can collapse under its self-gravity despite its velocity dispersion.
Because of its rotation within the disk this clump experiences an outward centrifugal acceleration ' L J κ2 ; if this acceleration is larger than the gravitational
acceleration, GΣgas , then the disk is stable.
It is widely accepted that the majority of gas clumps form from gravitational
instability with Jeans scale of ≈1 kpc. Clumpy galaxies at high-z usually show disklike kinematics with high turbulent Hα and CO velocity dispersions (e.g., Genzel
et al., 2006; Epinat et al., 2012; Tacconi et al., 2013). From a theoretical point of
view, violent disk instabilities and high velocity dispersions are required to regulate
disks with a Toomre parameter Qgas ≈ 1 (Dekel et al., 2009). Observationally, the
Qgas ≈ 1 instability limit has been estimated for gas within high-z galaxies (see
Genzel et al., 2011). However, local spiral disks tend to have Qgas ∼ 2 (van der
Kruit & Freeman, 1986) and they cannot form gas clumps.
The formation and evolution of clumps has an important impact on the formation of the central bulges in galaxies. However, it is not known yet whether they
can survive stellar feedback long enough and migrate inward to build the central
bulge, or whether these clumps disrupt like the molecular clouds.
Recent high-resolution optical and near-infrared images and spatially-resolved
observations allow us to study the stellar populations of these kpc-size clumps
within high-z galaxies.
1.4
Towards probing the small-scale properties of
distant galaxies
The last decade has seen a dramatic increase in our knowledge about galaxy population at z ∼ 1–3 (e.g., Shapley , 2011). This has been achieved mostly by studying
the integrated properties of galaxies. However, recently using near-IR integral field
unit (IFU) observations with adaptive optics (AO), many high-z galaxies have
been resolved spatially. The steadily growing effort to obtain resolved near-IR
spectra of high-z galaxies in a systematic manner, such as the SINS, MASSIV and
LSD/AMAZE surveys (Förster Schreiber et al., 2006; Contini et al., 2012; Maiolino
et al., 2010), is leading to large samples of spatially-resolved emission line maps
of distant star-forming galaxies. These maps provide us with spatially-resolved
11
Introduction
galaxy properties such as metallicity and SFR at high-z. Based on these studies, we know a large fraction (30% or larger) of these galaxies are dominated by
rotating disk kinematics with an increased fraction towards higher stellar masses
(e..g., Förster Schreiber et al., 2009, see their Figure 17 for kinematics of some
SINS galaxies) and they show high turbulent Hα velocity dispersions (e.g., Genzel et al., 2006). Metallicity gradients were measured within these galaxies based
on spatially-resolved emission line maps and it was discovered that some of these
galaxies show a lower metallicity in their centers (Cresci et al., 2010) as opposed to
what we observe in the local Universe for rotating disks systems, see Glazebrook
(2013) for a review on kinematic studies of star-forming galaxies across cosmic
time.
However, obtaining sub-kpc resolution, even with AO observations, is very
difficult because of the intrinsic faintness and the small sizes of high-z galaxies.
Our limitation to reach sub-kpc scale resolution for high-z galaxies using current
instruments can be resolved by observing high-z galaxies which are significantly
magnified due to gravitational lensing. This allows us to study the properties of
those high-z galaxies at a level similar to what is achieved at lower redshifts (e.g.,
Yee et al., 1996; Pettini et al., 2000, 2002; Teplitz et al., 2000; Savaglio et al., 2002;
Siana et al., 2008). However, even the ∼100 pc resolution achieved for some lensed
galaxies at high-z (e.g., Swinbank et al., 2009; Jones et al., 2010) is not enough
to study the small-scale properties of the ISM in high-z galaxies at the same level
that we can study those in the local Universe (e.g., Kennicutt & Evans, 2012).
The Atacama Large Millimeter/submillimeter Array (ALMA) observations will
soon provide us with insightful information about sub-kpc scale kinematics and
distribution of gas and star formation within distant galaxies. Afterwards, integral
field spectroscopic capability with the James Webb Space Telescope (JWST) will
allow us to accurately map distant galaxies in emission and absorption. This will
significantly change our view about the resolved small-scale properties of high-z
galaxies in the coming decade.
1.5
This thesis
In this thesis I analyze emission lines from gas ionized by very massive stars in
nearby and distant star-forming galaxies. In the local Universe, based on these
emission lines we can determine the source of ionization for producing them at
different environments. At high-z, these emission lines provide us with information
about the small-scale properties of the ISM. A brief summary of the contents of
this thesis is given below.
Chapter 2: Strongly star-forming galaxies in the local universe with
nebular He ii λ4686 emission
The evolution of massive stars is a complex and not fully understood process.
While we are limited by interstellar absorption in observing the stellar continuum
at λ < 228 Å, using the nebular He ii λ4686 emission line gives us valuable information about this high energy part of the stellar spectral energy distributions.
Only the most extreme star-forming galaxies show nebular He ii emission and it
12
This thesis
is generally believed that Wolf-Rayet (WR) stars provide the required ionizing radiation for them. In Chapter 2, we study the physical properties of emission line
galaxies in the SDSS showing He ii emission. Based on these data, we find that
the He ii is not associated with WR features in a large number of star-forming
galaxies with this emission at low metallicities. This lack of WR stars has important implications for the evolution of the most massive stars at low metallicity.
Non-homogenous stellar evolution models (e.g., Yoon et al., 2006) and spatial offset between the location of WR stars and He ii regions (e.g., Kehrig et. al, 2008)
might be two possible explanations for this discrepancy. We also show the current
stellar population models cannot produce observed He ii /Hβ ratios in low metallicity environments. This result has implications for interpreting observations of
high-z galaxies where the metallicity is expected to be typically lower. Another
key result from this study is to define a new diagnostic diagram using the He ii /Hβ
ratio, which can be used to constrain AGN contribution in star-forming galaxies
showing He ii emission.
Chapter 3: The physical nature of the 8 o’clock arc based on near-IR
IFU spectroscopy with SINFONI
The detailed analysis of distant galaxies is limited by their small angular sizes
and faint apparent magnitudes. Both of these limitations can be overcome by
observing gravitationally lensed galaxies. In Chapter 3, we analyze spatiallyresolved data of the 8 o’clock arc, a lensed Lyman break galaxy, in conjunction
with HST imaging of this galaxy, from which the lens model for the galaxy was
reconstructed. Based on this lens modeling, the de-lensed Hβ map, velocity and
velocity dispersion maps are reconstructed. We show a simple rotating disk model
is unable to fit the observed velocity field of the galaxy, and a more complex
velocity field is needed. The Hβ profile of the galaxy shows a broad blueshifted
wing, suggesting an outflow of 200 km/s. The estimated gas surface density and
gas mass of the 8 o’clock arc shows a factor of 2.5-7 higher gas content compared
to similar galaxies in the SDSS.
Chapter 4: Stars were born in significantly denser regions in the early
Universe
Most of stars that surround us today were formed several billion years ago,
around the peak of star formation activity in the Universe. The conditions under which these stars were born is of great interest but very difficult to study
due to limited observational resolution for distant objects. In Chapter 4, we
present a novel approach to directly compare the density in the star-forming regions of galaxies that are near the peak of star formation activity in the Universe
to those of nearby galaxies. To indirectly trace the ISM at high-z, we use the emission line intensities of distant galaxies. We calibrate a new relation between the
[O iii] λ5007/[O ii] λ3727 emission line ratio and ionization parameter to estimate
the difference between the ionization parameters in the high and low-z samples.
We analyze the ionization properties of a sample of high-z galaxies at redshift
2.6–3.4, including the 8 o’clock arc, and compare them with that of galaxies with
similar physical properties in the local Universe. We show that after accounting
13
Introduction
for all differences in large-scale properties, such as mass and specific star formation
rate, the density in the star-forming regions was eight times higher in the past.
This implies that the majority of stars in the Universe were formed in gas that
obeyed very different scaling relations than what we see in the present day Universe. This is a striking result that provides strong constraints on the conditions
of star formation in normal galaxies in the early Universe.
Chapter 5: On the spatial distribution of star formation in distant and
nearby galaxies
In Chapter 5, we study the differences between the spatial distribution of
star formation in the distant and the nearby Universe for galaxies with the similar
global properties (e.g., stellar mass and specific star formation rate). We use the
multi-band imaging data available in the HUDF and compare this quantitatively
with the low-z data from the SDSS. Based on this, we study the physical processes
that cause clumpy star formation distribution for galaxies with similar star formation activity at low-z and high-z. We compare the resolved stellar populations
of these galaxies by measuring the structural parameters of distant galaxies and
their nearby counterparts. We show galaxies at high-z have more concentrated
stellar content but their star formation is more extended compared to galaxies
with the same global properties at z∼0. We show high-z galaxies are more clumpy
in their star formation distributions than their local analogs. This clumpy morphology suggests that distant galaxies need to have more surface density of the
disk compared to their local analogs.
1.5.1
Observations
In this thesis I use emission line intensities of a sample of distant galaxies from
the literature (Maiolino et al., 2008; Mannucci et al., 2009; Richard et al., 2011;
Dessauges-Zavadsky et al., 2011). Analyzing near-IR IFU spectroscopy of the 8
o’clock arc, a lensed Lyman break galaxy at redshift 2.735 is also added to my
high-z studies. These data were taken with SINFONI on VLT covering λ = 2900
Å to 6500 Å in the rest-frame. The SINFONI data are analyzed in conjunction
with the HST images of the galaxy.
The low-z studies in this thesis are based on the Sloan Digital Sky Survey
(SDSS) (York et al., 2000) data. The MPA-JHU1 value added catalogues (Brinchmann et al., 2004; Tremonti et al., 2004) for SDSS DR7 (Abazajian et al., 2009)
are used and star-forming galaxies following Brinchmann et al. (2004) are selected.
Furthermore, SDSS DR8 (Aihara et al., 2011) photometry are used to estimate
stellar masses in Chapter 4.
In this thesis I also use the multi-band imaging data available in the Hubble
Ultra Deep Field (HUDF) to study the spatial distribution of star formation in
some HUDF galaxies that have confirmed spectroscopic redshifts2 (Coe et al.,
2006) .
1 http://www.mpa-garching.mpg.de/SDSS/DR7
2 http://adcam.pha.jhu.edu/~coe/UDF/paper/zspec.cat
14
Summary
1.6
Summary
This thesis aims at studying the physical properties of galaxies from the distant
to the nearby Universe based on their emission line observations. We study these
properties using Charlot & Longhetti (2001, CL01) models which combine Bruzual
& Charlot (2003) stellar population models with CLOUDY photoionization models
(Ferland et al., 1998). We use high ionization emission lines to probe the high
energy part of the stellar SEDs at low metallicities and constrain current stellar
populations. We show that stellar population models need to consider harder
spectra for O type stars in order to explain observations of high ionization emission
lines such as He ii λ4686. These results can be further confirmed by studying the
spatially-resolved analysis of He ii galaxies.
Recent observations have shown that high-z star-forming galaxies form a different population compared to star-forming galaxies in the local Universe. These
studies, however, cannot tell if the main difference between low-z and high-z starforming galaxies is related to their strongly evolving global properties (e.g., mass,
SFR) or their different intrinsic properties (e.g., the ISM). Based on the CL01
models and emission line ratios, we probe the intrinsic properties of high-z galaxies
that show no evolution in their global properties compared to a sample of nearby
star-forming galaxies. Using this, we show that star-forming regions are denser at
high-z and also there is an evolution in the relation between the surface density
of gas and the surface density of SFR known as the star-formation law towards
less efficient relation at high-z. Emission line observations for a larger sample of
star-forming galaxies at high-z and also follow up observations of emission line
ratios that are density tracers can confirm this higher density at high-z.
We compare also the distribution of star formation between distant and nearby
galaxies with similar physical properties based on their deep imaging. We show
that stellar content of distant star-forming galaxies is more compact than their
local analogs. The same result has been shown before for elliptical galaxies in the
local and high-z Universe. To do a more statistical analysis our study should be
done for a larger sample of star-forming galaxies at z > 1.5.
15
REFERENCES
References
Abazajian, K. N., Adelman-McCarthy, J. K., Agüeros, M. A., et al. 2009, ApJS,
182, 543
Aihara, H., Allende Prieto, C., An, D., et al. 2011, ApJS, 193, 29
Baldwin, J. A., Phillips, M. M., & Terlevich, R. 1981, PASP, 93, 5
Baugh, C. M., Cole, S., & Frenk, C. S. 1996, MNRAS, 283, 1361
Bell, E. F., & de Jong, R. S. 2001, ApJ, 550, 212
Bothwell, M. S., Maiolino, R., Kennicutt, R., et al. 2013, MNRAS, 433, 1425
Bouché, N., Dekel, A., Genzel, R., et al. 2010, ApJ, 718, 1001
Brinchmann, J., Charlot, S., White, S. D. M., et al. 2004, MNRAS, 351, 1151
Brinchmann, J. ,Kunth, D., Durret, F., 2008, A&A, 485, 657
Brinchmann, J., Pettini, M., & Charlot, S. 2008b, MNRAS, 385, 769
Bruzual, G. ,Charlot, S., 2003, MNRAS, 344, 1000
Charlot, S., & Longhetti, M. 2001, MNRAS, 323, 887
Coe, D., Benı́tez, N., Sánchez, S. F., et al. 2006, AJ, 132, 926
Contini T., et al., 2012, A&A, 539, 91
Cowie, L. L., Hu, E. M., & Songaila, A. 1995, AJ, 110, 1576
Cresci, G., Mannucci, F., Maiolino, R., et al. 2010, Nature, 467, 811
Crowther, P. A., 2007, ARA&A, 45, 177
Daddi, E., Dickinson, M., Morrison, G., et al. 2007, ApJ, 670, 156
Daddi, E., Dannerbauer, H., Elbaz, D., et al. 2008, ApJL, 673, L21
Dekel, A., Birnboim, Y., Engel, G., et al. 2009, Nature, 457, 451
Dessauges-Zavadsky, M., Christensen, L., D’Odorico, S., et al. 2011, A&A, 533,
A15
Elbaz, D., Daddi, E., Le Borgne, D., et al. 2007, A&A, 468, 33
Elbaz, D., Dickinson, M., Hwang, H. S., et al. 2011, A&A, 533, A119
Eldridge, J. J., Stanway, E. R., 2009, MNRAS, 400, 1019
Elmegreen, B. G., & Elmegreen, D. M. 2006, ApJ, 650, 644
Epinat, B., Tasca, L., Amram, P., et al. 2012, A&A, 539, A92
Erb, D. K., Shapley, A. E., Pettini, M., Steidel, C. C., et al. 2006, ApJ, 644, 813
Faber, 1973, ApJ, 179, 731
Ferland, G. J., Korista, K. T., et al., 1998, PASP, 110, 761
Förster Schreiber N., et al., 2006, Msngr, 125, 11
Förster Schreiber, N. M., Genzel, R., Bouché, N., et al. 2009, ApJ, 706, 1364
Förster Schreiber, N. M., Shapley, A. E., Genzel, R., et al. 2011, ApJ, 739, 45
Genzel, R., Newman, S., Jones, T., et al., 2011, ApJ, 733, 101
Genzel, R., Tacconi, L. J., Gracia-Carpio, J., et al. 2010, MNRAS, 407, 2091
Genzel, R., Tacconi, L. J., Eisenhauer, F., et al. 2006, Nature, 442, 786
Jones, T. A., Swinbank, A. M., Ellis, R. S., et al. 2010, MNRAS, 404, 1247
Han, Z., Podsiadlowski, P., & Lynas-Gray, A. E. 2007, MNRAS, 380, 1098
Heger, A., Woosley, S. E., & Spruit, H. C. 2005, ApJ, 626, 350
Hopkins, A. M., & Beacom, J. F. 2006, ApJ, 651, 142
Ilbert, O., McCracken, H. J., Le Fevre, O., et al. 2013, arXiv:1301.3157
Kauffmann, G., Heckman, T.M., et al. 2003, MNRAS, 346, 1055
Kauffmann, G., White, S. D. M., & Guiderdoni, B. 1993, MNRAS, 264, 201
16
REFERENCES
Kehrig, C., Vı́lchez, J. M., et al., D., 2008, A&A, 477, 813
Kennicutt, Jr., R. C. 1998, ApJ, 498, 541
Kennicutt, R. C., & Evans, N. J. 2012, ARA&A, 50, 531
Kewley, L. J., Maier, C., Yabe, K., et al. 2013, arXiv:1307.0514
Kewley, L. J., & Dopita, M. A. 2002, ApJS, 142, 35
Kewley, L. J., Dopita, M. A., et al., 2001, ApJ, 556, 121
Kereš, D., Katz, N., Weinberg, D. H., & Davé, R. 2005, MNRAS, 363, 2
Glazebrook, K. 2013, arXiv:1305.2469
Lara-López, M. A., Cepa, J., Bongiovanni, A., et al. 2010, A&A, 521, L53
Liu, X., Shapley, A. E., Coil, A. L., et al. 2008, ApJ, 678, 758
Madau et al. 1996, MNRAS, 283, 1388
Maiolino R., et al., 2010, Msngr, 142, 36
Maiolino, R., Nagao, T., Grazian, A., et al. 2008, A&A, 488, 463
Mannucci, F., Cresci, G., Maiolino, R., Marconi, A., & Gnerucci, A. 2010,
MNRAS, 408, 2115
Mannucci, F., Cresci, G., Maiolino, R., et al. 2009, MNRAS, 398, 1915
Meynet, G., & Maeder, A. 2005, A&A, 429, 581
Muzzin, A., Marchesini, D., Stefanon, M., et al. 2013, arXiv:1303.4409
Noeske, K. G., Weiner, B. J., Faber, S. M., et al. 2007, ApJL, 660, L43
Newman, S. F., Buschkamp, P., Genzel, R., et al. 2013, arXiv:1306.6676
Osterbrock, D. E., & Ferland, G. J. 2006, Astrophysics of gaseous nebulae and
active galactic nuclei, 2nd. ed. by D.E. Osterbrock and G.J. Ferland. Sausalito,
CA: University Science Books, 2006
Penston, M. V., Robinson, A., Alloin, D., et al. 1990, A&A, 236, 53
Pettini, M., Rix, S. A., Steidel, C. C., et al. 2002, ApJ, 569, 742
Pettini, M., Steidel, C. C., Adelberger, K. L., et al. 2000, ApJ, 528, 96
Richard, J., Jones, T., Ellis, R., et al. 2011, MNRAS, 413, 643
Savaglio, S., Panagia, N., & Padovani, P. 2002, ApJ, 567, 702
Schaerer, D., Vacca, W. D., 1998, ApJ, 497, 618
Schaerer, D. 1996, ApJ, 467, 17
Siana, B., Teplitz, H. I., Chary, R.-R., et al. 2008, APJ, 689, 59
Shapley A., 2011, ARA&A, 49, 525
Shapley, A. E., Steidel, C. C., Erb, D. K., et al. 2005, ApJ, 626, 698
Shirazi, M., Brinchmann, J., & Rahmati, A. 2013, arXiv:1307.4758
Smith, L. J., Norris, R. P. F., Crowther, P. A., 2002, MNRAS, 337, 1309
Swinbank, A. M., Webb, T. M., Richard, J., et al. 2009, MNRAS, 400, 1121
Tacconi, L. J., Genzel, R., Neri, R., et al. 2010, Nature, 463, 781
Tacconi, L. J., Neri, R., Genzel, R., et al. 2013, ApJ, 768, 74
Teplitz, H. I., McLean, I. S., Becklin, E. E., et al. 2000, ApJ, 533, L65
Tremonti, C. A., Heckman, T. M., Kauffmann, G., et al. 2004, ApJ, 613, 898
van der Kruit, P. C., & Freeman, K. C. 1986
Wuyts, S., Förster Schreiber, N. M., Genzel, R., et al. 2012, ApJ, 753, 114
Yee, H. K. C., Ellingson, E., Bechtold, J., et al. 1996, AJ, 111, 1783
Yoon, S.-C., Langer, N., Norman, C., 2006, A&A, 460, 199
York, D. G., Adelman, J., Anderson, Jr., J. E., et al. 2000, AJ, 120, 1579
17