TY - BOOK

T1 - Galaxies in the Early Universe

T2 - Characterized in Absorption and Emission

AU - Krogager, Jens-Kristian

PY - 2015

Y1 - 2015

N2 - Understanding how galaxies evolved from the early Universe through cosmic time is a fundamentalpart of modern astrophysics. In order to study this evolution it is important to sample the galaxies atvarious times in a consistent way through time. In regular luminosity selected samples, our analysesare biased towards the brightest galaxies at all times (as these are easier to observe and identify). Acomplementary method relies on the absorption imprint from neutral gas in galaxies, the so-calleddamped Ly absorbers (DLAs) seen towards distant bright objects. This thesis seeks to understandhow the absorption selected galaxies relate to the emission selected galaxies by identifying the faintglow 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-artinstrumentation. The specific DLA studied in Chapter 3 is found to be a young, star-forming galaxy withevidence for strong outflows of gas. This suggests that the more evolved and metal-rich DLAs overlapwith 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 significantreddening caused by foreground absorbers have been observed. Since most quasar samples, from whichthe 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 thereddening causing the background quasars to appear star-like in their optical colours. In Chapters 4 and5, these hypothesized dusty absorbers are sought for through a combination of optical and near-infraredcolour criteria. While a large number of previously unknown quasars are identified, only a handfulof absorbers are identified in the two surveys (a total of 217 targets were observed, 137 are previouslyunknown). One of these targets, quasar J2225+0527, is followed up in detail with spectroscopy from theX-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 foregroundDLA. Moreover, the absorbing gas has a high content of dense, cold and molecular gas with a projectedarea 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 areobserved to be a factor of 2􀀀6 times smaller than local galaxies of similar masses. A new spectroscopicallyselected sample is presented and the increased precision of the redshifts allows a more detailedmeasurement of the scatter in the mass–size relation. The size evolution of massive, quiescent galaxiesis modelled by a “dilution” scenario, in which progressively larger galaxies at later times are added tothe population of denser galaxies, causing an increase of the mean size of the population. This modeldescribes the evolution of both sizes and number densities very well, however, the scatter in the modelincreases with time, contrary to the data. It is thus concluded that a combination of “dilution” andindividual growth, e.g., through mergers, is needed.iii

AB - Understanding how galaxies evolved from the early Universe through cosmic time is a fundamentalpart of modern astrophysics. In order to study this evolution it is important to sample the galaxies atvarious times in a consistent way through time. In regular luminosity selected samples, our analysesare biased towards the brightest galaxies at all times (as these are easier to observe and identify). Acomplementary method relies on the absorption imprint from neutral gas in galaxies, the so-calleddamped Ly absorbers (DLAs) seen towards distant bright objects. This thesis seeks to understandhow the absorption selected galaxies relate to the emission selected galaxies by identifying the faintglow 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-artinstrumentation. The specific DLA studied in Chapter 3 is found to be a young, star-forming galaxy withevidence for strong outflows of gas. This suggests that the more evolved and metal-rich DLAs overlapwith 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 significantreddening caused by foreground absorbers have been observed. Since most quasar samples, from whichthe 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 thereddening causing the background quasars to appear star-like in their optical colours. In Chapters 4 and5, these hypothesized dusty absorbers are sought for through a combination of optical and near-infraredcolour criteria. While a large number of previously unknown quasars are identified, only a handfulof absorbers are identified in the two surveys (a total of 217 targets were observed, 137 are previouslyunknown). One of these targets, quasar J2225+0527, is followed up in detail with spectroscopy from theX-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 foregroundDLA. Moreover, the absorbing gas has a high content of dense, cold and molecular gas with a projectedarea 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 areobserved to be a factor of 2􀀀6 times smaller than local galaxies of similar masses. A new spectroscopicallyselected sample is presented and the increased precision of the redshifts allows a more detailedmeasurement of the scatter in the mass–size relation. The size evolution of massive, quiescent galaxiesis modelled by a “dilution” scenario, in which progressively larger galaxies at later times are added tothe population of denser galaxies, causing an increase of the mean size of the population. This modeldescribes the evolution of both sizes and number densities very well, however, the scatter in the modelincreases with time, contrary to the data. It is thus concluded that a combination of “dilution” andindividual growth, e.g., through mergers, is needed.iii

UR - https://soeg.kb.dk/permalink/45KBDK_KGL/fbp0ps/alma99122445317505763

M3 - Ph.D. thesis

BT - Galaxies in the Early Universe

PB - The Niels Bohr Institute, Faculty of Science, University of Copenhagen

ER -