Galaxies in the Early Universe characterized in absorption and emission

TL;DR

This paper investigates galaxy evolution from the early universe to later times using absorption and emission data, focusing on size evolution, galaxy relations, and the need for combined models involving dilution and mergers.

Contribution

It introduces a new spectroscopic sample for massive galaxies at z~2 and models their size evolution, highlighting the limitations of the dilution scenario alone.

Findings

Massive galaxies at z~2 are 2-6 times smaller than local counterparts.

The size evolution is well described by a dilution model, but scatter increases over time.

A combination of dilution and mergers is necessary to explain galaxy growth.

Abstract

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-alpha 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 the last Chapter, a study of the more evolved, massive galaxies is presented. These galaxies are observed to be a factor of 2 to 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. For brevity, the individual chapters based on published peer-reviewed articles are omitted and a link to the given article is given instead. Principal supervisor: Johan P. U. Fynbo