A Morphological Study of Gamma-Ray Burst Host Galaxies

We present a comprehensive study of the morphological properties of 42 γ-ray burst (GRB) host galaxies imaged with the Hubble Space Telescope in the optical band. The purpose of this study is to understand the relation of GRBs to their macroenvironments and to compare the GRB-selected galaxies to other high-redshift samples. We perform both qualitative and quantitative analyses by categorizing the galaxies according to their visual properties and by examining their surface brightness profiles. We find that the majority of the galaxies have approximately exponential profiles, indicative of galactic disks, and have a median effective radius of about 1.7 kpc; ~20% of the hosts are better fit with a bulge-dominated profile. Inspection of the visual morphologies reveals a high fraction of merging and interacting systems, with ~30% showing clear signs of interaction and an additional 30% exhibiting irregular and asymmetric structure, which may be the result of recent mergers; these fractions are independent of redshift and galaxy luminosity. The fraction of mergers appears to be elevated compared to other high-redshift samples (i.e., the HDF), particularly for the low luminosities of GRB hosts (M_B~-16 to -21 mag). Finally, we show that GRB hosts clearly follow the size-luminosity relation present in other galaxy samples, but thanks to spectroscopic absorption redshifts they help to extend this relation to fainter luminosities.