Galaxies in the Early Universe: Characterized in Absorption and Emission

Publikation: Bog/antologi/afhandling/rapportPh.d.-afhandlingForskning

Jens-Kristian Krogager

Understanding how galaxies evolved from the early Universe through cosmic time is a fundamental
part of modern astrophysics. In order to study this evolution it is important to sample the galaxies at
various times in a consistent way through time. In regular luminosity selected samples, our analyses
are biased towards the brightest galaxies at all times (as these are easier to observe and identify). A
complementary method relies on the absorption imprint from neutral gas in galaxies, the so-called
damped Ly absorbers (DLAs) seen towards distant bright objects. This thesis seeks to understand
how the absorption selected galaxies relate to the emission selected galaxies by identifying the faint
glow from the absorbing galaxies at redshift z 2.
In Chapters 2 and 3, the emission properties of DLAs are studied in detail using state-of-the-art
instrumentation. The specific DLA studied in Chapter 3 is found to be a young, star-forming galaxy with
evidence for strong outflows of gas. This suggests that the more evolved and metal-rich DLAs overlap
with the faint end of the luminosity selected galaxies in terms of mass, metallicity, star formation rate,
and age. DLAs are generally observed to have low dust content, however, indications of significant
reddening caused by foreground absorbers have been observed. Since most quasar samples, from which
the samples of DLAs are composed, are selected through optical criteria in large all-sky surveys, e.g.,
Sloan Digital Sky Survey (SDSS), there might exist a bias against dusty foreground absorbers due to the
reddening causing the background quasars to appear star-like in their optical colours. In Chapters 4 and
5, these hypothesized dusty absorbers are sought for through a combination of optical and near-infrared
colour criteria. While a large number of previously unknown quasars are identified, only a handful
of absorbers are identified in the two surveys (a total of 217 targets were observed, 137 are previously
unknown). One of these targets, quasar J2225+0527, is followed up in detail with spectroscopy from the
X-shooter intrument at the Very Large Telescope. The analysis of J2225+0527 is presented in Chapter 6.
The dust reddening along the line of sight is found to be dominated by dust in the metal-rich foreground
DLA. Moreover, the absorbing gas has a high content of dense, cold and molecular gas with a projected
area smaller than the background emitting region of the broad emission lines.
In the last Chapter, a study of the more evolved, massive galaxies is presented. These galaxies are
observed to be a factor of 2􀀀6 times smaller than local galaxies of similar masses. A new spectroscopically
selected sample is presented and the increased precision of the redshifts allows a more detailed
measurement of the scatter in the mass–size relation. The size evolution of massive, quiescent galaxies
is modelled by a “dilution” scenario, in which progressively larger galaxies at later times are added to
the population of denser galaxies, causing an increase of the mean size of the population. This model
describes the evolution of both sizes and number densities very well, however, the scatter in the model
increases with time, contrary to the data. It is thus concluded that a combination of “dilution” and
individual growth, e.g., through mergers, is needed.
ForlagThe Niels Bohr Institute, Faculty of Science, University of Copenhagen
Antal sider242
StatusUdgivet - 2015

ID: 147930025