A deep search for the host galaxies of GRBs with no detected optical
  afterglow
  Gamma-Ray Bursts can provide information about star formation at high
redshifts. Even in the absence of a optical/near-infrared/radio afterglow, the
high detection rate of X-ray afterglows by swift/XRT and its localization
precision of 2-3 arcsec facilitates the identification and study of GRB host
galaxies. We focus on the search for the host galaxies of a sample of 17 bursts
with XRT error circles but no detected long-wavelength afterglow. Three of
these events can also be classified as truly dark bursts: the observed upper
limit on the optical flux of the afterglow was less than expected based on the
X-ray flux. Our study is based on deep R and K-band observations performed with
ESO/VLT instruments, supported by GROND and NEWFIRM. To be conservative, we
searched for host galaxies in an area with a radius twice the 90% swift/XRT
error circle. For 15 of the 17 bursts we find at least one galaxy inside the
doubled XRT error circle. In seven cases we discover extremely red objects in
the error circles. The most remarkable case is the host of GRB 080207 which as
a colour of R-K~4.7 mag (AB), one of the reddest galaxies ever associated with
a GRB. As a by-product of our study we identify the optical afterglow of GRB
070517A. Optically dim afterglows result from cosmological Lyman drop out and
dust extinction, but the former process is only equired for a minority of cases
(<1/3). Extinction by dust in the host galaxies might explain all other events.
Thereby, a seemingly non-negligible fraction of these hosts are globally
dust-enshrouded, extremely red galaxies. This suggests that bursts with
optically dim afterglows trace a subpopulation of massive starburst galaxies,
which are markedly different from the main body of the GRB host galaxy
population, namely the blue, subluminous, compact galaxies.
