The faint afterglow and host galaxy of the short-hard GRB 060121

We present optical and X-ray observations of the afterglow and host galaxy of the short-hard GRB 060121. The faint R-band afterglow is seen to decline as t^-0.66±0.09 while the X-ray falls as t ^-1.18±0.04, indicating the presence of the cooling break between the two frequencies. However, the R-band afterglow is very faint compared to the predicted extrapolation of the X-ray afterglow to the optical regime (specifically, β_OX ∼ 0.2), while the K-band is consistent with this extrapolation (β_KX ∼ 0.6), demonstrating suppression of the optical flux. Late-time HST observations place stringent limits on the afterglow R-band flux, implying a break in the R-band light curve. They also show that the burst occurred at the edge of a faint red galaxy, presumably the host, which most likely lies at a significantly higher redshift than the previous optically identified short-duration bursts. Several neighboring galaxies also have very red colors that are similarly suggestive of higher redshift. The least extreme explanation for the faintness and color of the burst is that it occurred at moderately high redshift and was significantly obscured; however, it is also possible that it lies at z > 4.5, in which case the faintness of the R-band afterglow could be attributed to the Lyman break. We discuss the implications that either scenario would have for the nature of the progenitors of short bursts.