Gamma-ray Burst Selected High Redshift Galaxies: Comparison to Field

We study the internal structural properties of 37 long duration gamma-ray burst (GRB) host galaxies imaged with the Hubble Space Telescope. Our goal is to gain insights in the type of galaxies that gives rise to GRBs, and the relationship of GRB hosts to high-redshift galaxies selected through more traditional photometric methods. We measure structural properties of our sample from Hubble Space Telescope observations obtained after the GRB afterglow faded. Fitting exponential disk (typical for spirals) and r (typical for ellipticals) models to the surface brightness profiles of eight z < 1.2 bright host galaxies, we find that the disk model is slightly preferred for most hosts, although two galaxies are fit best with an r profile. We furthermore measure the central concentrations and asymmetries of all 37 host galaxies using the CAS (concentration, asymmetry, clumpiness) system, and compare with values for galaxies in the Hubble Deep Field, and systems present on the gamma-ray burst host images. Our first main conclusion is that GRB hosts exhibit a surprisingly high diversity of galaxy types. A significant fraction (68%) of host galaxies are situated in a region of the concentration-asymmetry diagram occupied by spirals or peculiar/merging galaxies. Twelve hosts (32%) are situated in the region occupied by elliptical galaxies, having high concentration indices indicative of early-types or early types in formation. These results show that GRB host galaxies are not a single morphological type, but span the available range of galaxy types seen at high redshift. We also find some evidence for evolution in GRB host galaxy morphology, such that hosts at z > 1 have a relatively high light concentration, indicating that these systems are perhaps progenitors of massive galaxies, or are compact blue star forming galaxies. We find that GRB hosts at z > 1 are different from the general field population at z > 1 in terms of light concentration at >99.5% confidence, yet have sizes similar to the general z > 1 galaxy population. This is the opposite of the effect at z < 1 where GRB hosts are smaller than average. We argue that GRB hosts trace the starburst population at high redshift, as similarly concentrated galaxies at z > 1 are undergoing a disproportionate amount of star formation for their luminosities. Furthermore, our results show that GRBs are not only an effective tracer of star formation, but are perhaps ideal tracers of typical galaxies undergoing star formation at any epoch, making them perhaps our best hope of locating the earliest galaxies at z > 7. Subject headings: gamma rays: observations — galaxies: ISM — quasars: absorption lines — early universe