Nils Henrik Pehlivan Rhodin
A thesis for the degree of Doctor of Philosophy defended august 2019.
The PhD School of Science, Faculty of Science, DARK, Niels Bohr Institute, University of Copenhagen
Assoc. Prof. Lise Bech Christensen
Deep emission studies of high redshift absorption selected galaxies Probing the Galaxy Population With Beacons From Afar
In its heart, astronomy is an empirical science: what we see is what we get. .is detection-driven approach gives us remarkable insights on the nature and contents of the Universe that we inhabit, including galaxies. However, these results o.en rely on observations that select on luminosity, and conclusions are therefore biased to the tip of the iceberg. .is e.ect becomes increasingly severe when we a.empt to understand the earliest stages of galaxy formation in the young, high redshi. Universe. is thesis a.empts to mitigate these shortcomings by studying galaxies with a complementary technique, utilising a gas cross-section selection. .is is made possible by favourable chance alignments in nature which occur when galaxies intervene the line of sight towards distant beacons of light; quasars.
For gaseous galaxies, these con€gurations imprint characteristic neutral hydrogen (H i) Lyman Alpha absorption features in quasar continua, and thereby provide a tell-tale for galaxy position and redshi. With targeted follow-up studies, the galaxies responsible for the absorption can be studied in great detail, utilising the complementary powers of imaging and absorption- and emission spectroscopy. Part I provides context to the research. Chapter 1 presents a brief overview of galaxy formation and evolution, mechanisms regulating galaxy growth, and how such objects are studied to large redshift Chapter 2 substantiates the notion that strong Hi absorbers probe galaxy environments, before delving into a discussion on how the €eld has progressed in recent years, the original research contribution of this thesis, and future outlooks. Part II presents the three €rst author publications which form the base to this thesis, a.ached in journal format.
In Paper I, I used ground-based long-slit spectroscopy to follow-up and con€rm ten candidate absorbing galaxies at redshi. z 0:7. By combining pre-existing photometry of the quasar €elds with my spectroscopic redshi.s, I derived stellar masses and show that absorbing galaxies fall on predicted mass-metallicity relations, but systematically display sub-main-sequence star formation. In Paper II, I confronted state-of-the-art cosmological zoom-in simulations of a Milky Way mass galaxy with a recent compilation of spectroscopically con€rmed galaxies harbouring strong Hi absorbers. .e simulation captures observed distributions of projected separations between absorbers and their galaxy counterparts. I show that both extended Hi discs and haloes play a role at all redshi.s. .e recovery of absorbers at high impact parameters is only made possible by ecient feedback and high numerical resolution, which reveals that such line-of-sights are associated with dwarf satellites in the main halo, stripped metal-rich gas and out.ows. In Paper III, I present the results of a photometric follow-up of ten spectroscopically con€rmed absorbing galaxies at a complementary redshi. z 2 3. Utilising
the exquisite spatial resolution reached with the Hubble Space Telescope, this paper presents the first systematic morphological characterisation of absorbing galaxies at high redshi.s. Combined with stellar masses and a heterogenous set of star formation rate indicators, I show that the absorbing galaxies are consistent with the low-mass end of late-type star-forming galaxy populations at high-redshift In summary, the results from observations and simulations presented in this thesis support the idea that strong Hi absorbers probe neutral gas across galaxy environments. By contributing to the detection and characterisation of these objects in emission, I have helped the €eld mature and to undergo a transition to a stage where the properties of absorbing galaxies can directly be compared with standard luminosity selected galaxy samples.