PhD defense by Jens-Kristian Krogager

PhD Student Jens-Kristian Krogager will defend his PhD thesis Friday, Nov. 6. at 14:00 hrs. at HCØ Aud. 10

Galaxies in the Early Universe – characterized in absorption and emission


Johan Fynbo
Cédric Ledoux 

Assessment committee:

Darach Watson

Valentina D’Odorico (Trieste Observatory)
Stephen Warren (Imperial College)

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.

Read thesis here >>