CHRISTINA CARINA THÖNE

Title: GAMMA-RAY BURST HOST GALAXIES AT HIGH RESOLUTION

Dissertation Submitted for the Degree PHILOSOPHIÆ DOCTOR

Dark Cosmology Centre,
Niels Bohr Institut, Det Naturvidenskabelige Fakultet - Københavns Universitet

Submission:
July 4th, 2008

Defence:
October 3rd, 2008

Opponents:
Prof. Max Pettini
Dr. Chryssa Kouveliotou

Abstract

GAMMA-RAY BURST HOST GALAXIES AT HIGH RESOLUTION CHRISTINA CARINA THÖNE 

GRBs are the most powerful explosions in the universe and a very interesting phenomenon in themselves. Even though they are the brightest objects detected, they are nevertheless only stellar explosions or mergers of compact objects. Since their discovery in 1967, a lot of progress has been made in identifying them as high redshift events, investigating their high energy and afterglow properties, dividing them into subgroups of long and short duration GRBs with likely
different origins and studying their host galaxies. As more and better sampled data become available, notably since the launch of the Swift satellite (Nov. 2004), explanations for what actually happens in the explosion start to be more complicated and more questions are raised than answered.

The following thesis does not deal with the GRB phenomenon itself but it is studying their environment and host galaxies through optical spectroscopy and using them as lightsources in
the distant universe. The work is divided in three parts.

The first Chapter gives in introduction to the history of the GRB discovery and research, explains the processes involved in producing a GRB and its afterglow radiation and the different progenitor models for GRBs. It also summarizes the properties of its host galaxies and the differences between short and long GRB hosts. Finally, an overview is given over the fast follow-up observing strategies for GRB afterglows, the special type of proposals needed, and some examples are mentionied in which the author of this thesis was involved in.

Part II presents three examples of using GRBs as powerful lightsources that allow to study the high redshift universe with low and high resolution spectrosocpy. An introduction to the methods of how to observe the high redshift universe is given, what information can be obtained from high resolution spectra and what they tell us about the environment and host galaxy of a GRB. For the first case, GRB 060526, follow-up observations from a large range of telescopes were put together in order to study its afterglow properties. This luminous GRB also allowed us to obtain a dataset of low to medium resolution spectra with FORS 1 at the VLT. A range of UV restframe metal absorption lines are detected of which we determine the column densitiy with the CoG method and subsequently the metallicity.

The next Chapter presents high resolution spectra of the afterglow of GRB 030329 that has both resolved absorption and emission lines, which was possible due to the low redshift of the GRB of z=0.1686. The absorption lines split up in  different components with a range of 230 km/s blueshifted compared to the host. We argue that they likely originate from a starburst wind outflow as observed in some nearby starburst galaxies, e.g. M 82.

The last Chapter finally presents a large range of resonant and finestructure transitions detected in the medium resolution spectra of z=4.048 GRB 060206. Those also split up in different components spanning a range of  400 km/s by both the resonant and finestructure absorption lines where the latter trace the resonant lines. This has never been observed so far in a GRB afterglow spectrum. The ratio between finestructure and resonant transitions allow us to determine the properties in the different absorbing system and to get a rough estimate on their distance from the GRB. Finally, we reinvestigate the claim of variability in an intervening system
at z=1.45 in another series of spectra of this GRB by Hao et al. (2007) with our and new data from the Subaru archive which disprove this claim.

Part III deals with the low redshift universe where we can study the host galaxies of GRBs in detail with spatially resolved emission line spectroscopy. Both events studied here, GRB 060505 and SN 2008D, caused some debate in the GRB community. GRB 060505 was the first presumably long GRB not connected to a SN and its nature is therefore highly controversial. Type Ib SN 2008D was connected to an X-ray flash which had never been observed so far for a SN. The introduction to this part therefore gives an overview over the GRB-SN connection, the SN nondetection and the properties of SN 2008D. Furthermore, the different techniques on how to study low redshift galaxies and HII regions with emission line diagnostics are explained and an example for a study using an integral field unit investigating the host of GRB 980425, the
first SN-associated GRB, is given.

The first chapter then presents a rough long-slit spatially resolved study of the host of GRB 060505 seen nearly face on. The slit covers the major axis of the galaxy and the GRB region and our study reveals that the properties of the HII region where the GRB took place is different from the rest of the galaxy, esp. in terms of metallicity and SFR. These results argue in favour of the theory that GRB 060505 came from the collapse of a massive star. We also investigate part of the galaxy with high resolution spectroscopy, fit a rotation curve and study the kinematics.

The second chapter does a similar study of the host of SN 2008D, NGC 2770, covering the SN site, the major axis and the site of two other Ib SNe in this galaxy with four longslit spectra. While the SFR at the SN Ib sites is not especially high, the metallicity lies inbetween the one of broadline Ic SNe and GRB connected Ic SNe. We further compare the properties of all galaxies with at least 3 SNe detected to those of NGC 2770. While some galaxies seem to favour a specific type of SN, also reflected in their global properties, this is not the case for NGC 2770. From this and statistical considerations we conclude that the occurence of three Ib SNe (and no other SN
type) was likely just a coincidence.

With the observational progress made in the last years with faster follow-up and better datasets, GRBs have shown their capability as probes of the high redshift universe in great detail. Some fortunate low redshift GRBs allow us to have a closer look to their places of origin in order to get more insights in the progenitor properties and to compare it with progenitors e.g. SNe. The sample is still small and the generally high redhift prohibits such studies for a larger sample, but surprises and a very low redshift case might come every day.