  We present a comprehensive study of the morphological properties of 42
gamma-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 macro-environments, 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 all of the
galaxies have approximately exponential profiles, indicative of galactic disks,
and have a median scale length of about 1.7 kpc. 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. On the other
hand, the three GRB host galaxies for which submillimeter and radio emission
has been detected are isolated and compact, unlike the luminous
submillimeter-selected galaxies. The fraction of mergers appears to be elevated
compared to other high redshift samples, particularly for the low luminosities
of GRB hosts (M_B ~ -16 to -21 mag). This suggests that merging and interacting
galaxies undergoing a burst of star formation may be an efficient site for the
production of GRB progenitors. Finally, we show that GRB hosts clearly follow
the size-luminosity relation present in other galaxy samples, but thanks to
absorption redshifts they help extend this relation to lower luminosities.

Astrophysics