Abstract
The star formation history of the Universe is one of the most complex and interesting chapters in our quest to understand galaxy formation and evolution. Gamma Ray Bursts (GRBs) are beacons of actively star forming galaxies from redshifts near zero back to the cosmic dawn. In addition, they provide a unique method for selecting galaxies without a luminosity bias as the GRB detectability is unrelated to the brightness of the host galaxy. Even at the highest redshifts, where the hosts are often too faint to be detected in emission, their properties can be inferred from the absorption features that their interstellar media imprint on the GRBs’ spectra. Hence they are invaluable tools to probe the star formation history of the Universe back to the earliest cosmic epochs.
To this end, it is essential to achieve a comprehensive picture of the interplay between star formation and its fuel, neutral gas, in GRB selected galaxies. Moreover, it is crucial to investigate whether this galaxy population differs from the general population of star forming galaxies (if GRB hosts are a distinct galaxy population), before applying the findings from this selected population to the general population of galaxies.
In this thesis I focus on the gas kinematics in GRB host galaxies. This niche area of study provides interesting insights into the GRB host population as well as galaxy formation and evolution.
For a large sample of GRB hosts, I use the ‘spatially averaged’ velocity of gas which is measured from an integrated spectrum. This is done using both absorption and emission lines, tracing the gas velocity in neutral and ionised phases respectively. I also map the HI 21 cm emission line for a GRB host galaxy (first study of its kind). This allows studying the spatial distribution and the kinematics of the atomic gas simultaneously with a high velocity resolution.
For the large GRB sample, I find the spatially averaged velocity to correlate with metallicity in both gas phases. This is an indicator of a mass-metallicity relation. Moreover, the velocity widths in the two gas phases correlate with each other which too points towards a relation between gas kinematics and mass. This also provides information on how the metallicities measured from absorption and emission methods differ from each other. Finally, in a direct study I show that gas velocity widths in both phases can be used as a proxy of stellar mass in these galaxies.
I compare the large GRB host sample to other populations of galaxies using scaling relations. I find GRB hosts to follow the same velocity-metallicity correlation as Damped Lyman-α galaxies which are a population of high redshift galaxies detected in the sightlines of quasars. I also show that GRB hosts obey the same mass-metallicity relation as the general population of star-forming galaxies. This is contradictory to several previous studies that claimed GRB hosts to be below the general mass-metallicity relation.
For the sole GRB host with spatially resolved velocity field from HI 21 cm observations, the kinematically resolved data reveal disturbed gas with more than 20% of the gas mass being offset in velocity from the main gas disk. Such significantly massive offset gas could only be the remnant of a minor merger. In addition, I detect an HI knot about 12 kpc away from the galaxy, rotating aligned with the main gas disk which is possibly related to the merging event. This corroborates the fact that using scaling relations of the large GRB sample I find indication of merging systems being responsible for the kinematic characteristics of gas in GRB hosts galaxies.
To this end, it is essential to achieve a comprehensive picture of the interplay between star formation and its fuel, neutral gas, in GRB selected galaxies. Moreover, it is crucial to investigate whether this galaxy population differs from the general population of star forming galaxies (if GRB hosts are a distinct galaxy population), before applying the findings from this selected population to the general population of galaxies.
In this thesis I focus on the gas kinematics in GRB host galaxies. This niche area of study provides interesting insights into the GRB host population as well as galaxy formation and evolution.
For a large sample of GRB hosts, I use the ‘spatially averaged’ velocity of gas which is measured from an integrated spectrum. This is done using both absorption and emission lines, tracing the gas velocity in neutral and ionised phases respectively. I also map the HI 21 cm emission line for a GRB host galaxy (first study of its kind). This allows studying the spatial distribution and the kinematics of the atomic gas simultaneously with a high velocity resolution.
For the large GRB sample, I find the spatially averaged velocity to correlate with metallicity in both gas phases. This is an indicator of a mass-metallicity relation. Moreover, the velocity widths in the two gas phases correlate with each other which too points towards a relation between gas kinematics and mass. This also provides information on how the metallicities measured from absorption and emission methods differ from each other. Finally, in a direct study I show that gas velocity widths in both phases can be used as a proxy of stellar mass in these galaxies.
I compare the large GRB host sample to other populations of galaxies using scaling relations. I find GRB hosts to follow the same velocity-metallicity correlation as Damped Lyman-α galaxies which are a population of high redshift galaxies detected in the sightlines of quasars. I also show that GRB hosts obey the same mass-metallicity relation as the general population of star-forming galaxies. This is contradictory to several previous studies that claimed GRB hosts to be below the general mass-metallicity relation.
For the sole GRB host with spatially resolved velocity field from HI 21 cm observations, the kinematically resolved data reveal disturbed gas with more than 20% of the gas mass being offset in velocity from the main gas disk. Such significantly massive offset gas could only be the remnant of a minor merger. In addition, I detect an HI knot about 12 kpc away from the galaxy, rotating aligned with the main gas disk which is possibly related to the merging event. This corroborates the fact that using scaling relations of the large GRB sample I find indication of merging systems being responsible for the kinematic characteristics of gas in GRB hosts galaxies.
| Original language | English |
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| Publisher | The Niels Bohr Institute, Faculty of Science, University of Copenhagen |
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| Publication status | Published - 2016 |