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High redshift star-forming galaxies in absorption and emission
Galaxies in the redshift range 1 < z < 3 existed during the most vigorous period of star formation in the history of the Universe. In the past 15 years, large rest-frame UV spectroscopic samples of z ∼ 3 star-forming galaxies have been assembled. However, this particular redshift range, the so-called Redshift Desert, has only begun to be characterized. Most studies involve low resolution, low signal-to-noise spectra because the small angular size (δ ≤ 1′′) and faintness (RAB = 24 − 25.5) of high redshift galaxies limit what can be accomplished with a reasonable investment of observing time, even using the world’s largest optical telescopes.
One way to circumvent these two issues is to study gravitationally lensed galaxies. The magnification boost (up to a factor of 30×) and morphological distortion of a high redshift galaxy by an intervening mass concentration allow for the study of the high redshift Universe in unprecedented detail. I present a detailed analysis of the rest-UV spectrum of two gravitationally lensed galaxies: the ‘Cosmic Horseshoe’ (zsys = 2.38115) and the ‘Cosmic Eye’ (zsys = 3.07331). The characterization of the stellar populations and the interstellar gas geometry, kinematics, and composition which I achieve is a preview of the type of information that will be available for unlensed high redshift galaxies with the next generation of optical telescopes.
I probe the lower redshift end of the Redshift Desert with a study of Fe ii and Mg ii features in the rest-frame near-UV spectrum of 96 star-forming galaxies in the redshift range 1 < z < 2. Stacked spectra are used to explore average outflow and line profile trends with stellar mass and reddening. I also investigate the phenomenon of emission filling of absorption lines which has implications for the line strength and velocity offset of interstellar absorption lines. Individual galaxies are used to assess the range of outflow velocities as well as the prevalence of emission filling in galaxies from this epoch. This is the first large scale study of fine-structure emission from Feii in high redshift galaxies, both in stacked and individual galaxy spectra.
An alternative to investigating galaxies by collecting their light is to study them as seen in absorption against a cosmic backlight, such as a quasar. The Sloan Digital Sky Survey, an imaging and spectroscopic survey which covers about one-quarter of the night sky, has collected many thousands of quasar spectra. I search ∼ 44 600 of these spectra, up through Data Release 4, for Mg ii λλ2796,2803 absorption doublets. The final catalog includes ∼ 16700 Mgii absorption line systems in the redshift range 0.36 ≤ z ≤ 2.28. Measurements of the absorption redshift and rest equivalent widths of the Mg ii doublet as well as select metal lines are available in the catalog. This is the largest publicly available catalog of its kind and its combination of large size and well understood statistics make it ideal for precision studies of the low-ionization and neutral gas regions of galaxies.
I conclude this thesis by suggesting several avenues for extending the studies of high redshift star-forming galaxies presented herein.This work was supposed by the Marshall Aid Commemoration Commission, National Science Foundation Graduate Research Fellowship Program, and Cambridge Overseas Trusts
The Evolution of Neutral Gas in the Universe
We give references to some of our work on the properties and evolution of the neutral gas component of the Universe (see reference list). The bulk of the observed neutral gas has been detected by identifying intervening damped Lyα (DLA) quasar absorption-line systems with N(H) \u3e 2 × 1020 atoms cm −2. We also present some initial results from a program to identify DLA absorbers near redshift z = 0.5 using Hubble Space Telescope ACS prism spectr
Testing metallicity indicators at z~1.4 with the gravitationally lensed galaxy CASSOWARY 20
We present X-shooter observations of CASSOWARY 20 (CSWA 20), a star-forming
(SFR ~6 Msol/yr) galaxy at z=1.433, magnified by a factor of 11.5 by the
gravitational lensing produced by a massive foreground galaxy at z=0.741. We
analysed the integrated physical properties of the HII regions of CSWA 20 using
temperature- and density-sensitive emission lines. We find the abundance of
oxygen to be ~1/7 of solar, while carbon is ~50 times less abundant than in the
Sun. The unusually low C/O ratio may be an indication of a particularly rapid
timescale of chemical enrichment. The wide wavelength coverage of X-shooter
gives us access to five different methods for determining the metallicity of
CSWA 20, three based on emission lines from HII regions and two on absorption
features formed in the atmospheres of massive stars. All five estimates are in
agreement, within the factor of ~2 uncertainty of each method. The interstellar
medium of CSWA 20 only partially covers the star-forming region as viewed from
our direction; in particular, absorption lines from neutrals and first ions are
exceptionally weak. We find evidence for large-scale outflows of the
interstellar medium (ISM) with speeds of up 750 km/s, similar to the values
measured in other high-z galaxies sustaining much higher rates of star
formation.Comment: 18 pages, 11 figures, accepted for publication in MNRA
Probing star formation across cosmic time with absorption line systems
We present an empirical connection between cold gas in galactic halos and
star formation. Using a sample of more than 8,500 MgII absorbers from SDSS
quasar spectra, we report the detection of a 15 sigma correlation between the
rest equivalent width W0 of MgII absorbers and the associated OII luminosity,
an estimator of star formation rate.
This correlation has interesting implications: using only observable
quantities we show that MgII absorbers trace a substantial fraction of the
global OII luminosity density and recover the overall star formation history of
the Universe derived from classical emission estimators up to z~2. We then show
that the distribution function of MgII rest equivalent widths, dN/dW0 inherits
both its shape and amplitude from the OII luminosity function Phi(L). These
distributions can be naturally connected, without any free parameter.
Our results imply a high covering factor of cold gas around star forming
galaxies: C>0.5, favoring outflows as the mechanism responsible for MgII
absorption. We then argue that intervening MgII absorbers and blue-shifted MgII
absorption seen in the spectra of star forming galaxies are essentially the
same systems. These results not only shed light on the nature of MgII absorbers
but also provide us with a new probe of star formation, in absorption, i.e. in
a way which does not suffer from dust extinction and with a
redshift-independent sensitivity. As shown in this analysis, such a tool can be
applied in a noise-dominated regime, i.e. using a dataset for which emission
lines are not detected in individual objects. This is of particular interest
for high redshift studies.Comment: 13 pages, 7 figures, submitted to MNRA
Cosmic dust in MgII absorbers
MgII absorbers induce reddening on background quasars. We measure this effect
and infer the cosmic density of dust residing in these systems to be \Omega\ ~
2e-6, in units of the critical density of the Universe, which is comparable to
the amount of dust found in galactic disks or about half the amount inferred to
exist outside galaxies. We also estimate the neutral hydrogen abundance in MgII
clouds to be \Omega\ ~ 1.5e-4, which is approximately 5% of hydrogen in stars
in galaxies. This implies a dust-to-gas mass ratio for MgII clouds of about
1/100, which is similar to the value for normal galaxies. This would support
the hypothesis of the outflow origin of MgII clouds, which are intrinsically
devoid of stars and hence have no sources of dust. Considerations of the dust
abundance imply that the presence of MgII absorbers around galaxies lasts
effectively for a few Gyr. High redshift absorbers allow us to measure the
rest-frame extinction curve to 900 Angstroms at which the absorption by the
Lyman edge dominates over scattering by dust in the extinction opacity.Comment: 8 pages, 7 figures. Matches version accepted for publication in ApJ.
Minor change
Optical properties and spatial distribution of MgII absorbers from SDSS image stacking
We present a statistical analysis of the photometric properties and spatial
distribution of more than 2,800 MgII absorbers with 0.37<z<1 and rest
equivalent width W_0(\lambda2796)>0.8\AA detected in SDSS quasar spectra. Using
an improved image stacking technique, we measure the cross-correlation between
MgII gas and light (in the g, r, i and z-bands) from 10 to 200 kpc and infer
the light-weighted impact parameter distribution of MgII absorbers. Such a
quantity is well described by a power-law with an index that strongly depends
on W_0, ranging from ~-1 for W_0~ 1.5\AA. At redshift
0.37<z<0.55, we find the average luminosity enclosed within 100 kpc around MgII
absorbers to be M_g=-20.65+-0.11 mag, which is ~0.5 L_g*. The global
luminosity-weighted colors are typical of present-day intermediate type
galaxies. However, while the light of weaker absorbers originates mostly from
red passive galaxies, stronger systems display the colors of blue star-forming
galaxies. Based on these observations, we argue that the origin of strong MgII
absorber systems might be better explained by models of metal-enriched gas
outflows from star-forming/bursting galaxies. Our analysis does not show any
redshift dependence for both impact parameter and rest-frame colors up to z=1.
However, we do observe a brightening of the absorbers related light at high
redshift (~50% from z~0.4 to 1). We argue that MgII absorbers are a phenomenon
typical of a given evolutionary phase that more massive galaxies experience
earlier than less massive ones, in a downsizing fashion. (abridged)Comment: ApJ in press, 28 pages, 16 figures, using emulateapj. Only typo
corrections wrt the original submission (v1
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