Johannes ZablJohannes Zabl

A thesis submitted April 20, 2015 for the degree of Doctor of Philosophy and defended July 2, 2015.

The PhD School of Science
Faculty of Science
Niels Bohr Institute, Dark Cosmology Centre, University of Copenhagen

Supervisor:
Johan Peter Uldall Fynbo

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Abstract

Emission line imaging and spectroscopy of distant galaxies

One of the main quests of contemporary astrophysics is to understand how and when galaxies form and how they evolve over cosmic times. Until star formation in galaxies is quenched through a still debated processes, they form stars more or less steadily out of gas that has cooled down in their interiors. The necessary gas supply is either provided through accretion from the surrounding medium or through mergers. Feedback mechanisms, e.g. provided by active galactic nuclei (AGN) or supernovae, regulate the star-formation and create outflows, which enrich the surrounding medium with energy, momentum, and metals both on small and large scales.

While galaxies are star-forming, the telltale signature of their spectra are strong emission lines. Emission lines are an extremely valuable "tool". The strength and shape of and ratios between these lines allow to constrain physical properties like star-formation rate, dust-extinction, energetics of the ionizing source, gas-phase metallicity, and kinematics of the gas. Further, they provide a unique signature which allows to safely secure redshifts even for low mass galaxies, whose stellar continuum emission is out of reach of spectrographs on the current generation of telescopes. Finally, emission lines can also be used in the first place to select large samples of galaxies at well defined redshifts through narrowband (NB) surveys. The idea here is that an emission line in a NB filter raises the filter averaged flux density compared to that expected from the continuum. Di erent emission lines can cause such an excess, which are at the wavelength of the NB filter at di erent redshifts. Contemporary NB surveys have a strong focus on searches for Ly emitters at the highest redshifts.

Ly is so well suited for high redshift studies because it is the intrinsically brightest emission line and has a low rest-frame wavelength. However, as it is a resonant line, it can be diluted through scattering at neutral hydrogen, and has due to the resulting increased optical path length an enhanced probability of being destroyed by dust. As part of this thesis I will present XSHOOTER rest-frame far-UV spectroscopy for Himiko, a NB selected giant Ly emitter at redshift z = 6:6. This redshift corresponds to a time when the universe was only 6 per cent of its present age. While this is not the most distant object known, it is due to its strong Ly emission arguably one of the most remarkable ones.

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