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We present the discovery of the optical afterglow and early-type host galaxyof the short-duration GRB 100117A. The faint afterglow is detected 8.3 hr afterthe burst with r_AB = 25.46 +/- 0.20 mag. Follow-up optical and near-IRobservations uncover a coincident compact red galaxy, identified as anearly-type galaxy at a photometric redshift of z~0.6-0.9 (2-sigma) with a massof 3x10^10 M_Sun, an age of ~1 Gyr, and a luminosity of L_B~0.5L_star.Spectroscopic observations of the host reveal a notable break corresponding tothe Balmer 4000-Angstrom break at z~0.9, and stellar population spectralevolution template fits indicate z~0.915, which we adopt as the redshift of thehost, with stellar population ages of ~1-3 Gyr. From a possible weak detectionof [OII]-3727 emission at z=0.915 we infer an upper bound on the star formationrate of ~0.1 M_Sun per yr, leading to a specific star formation rate of <0.004per Gyr. Thus, GRB 100117A is only the second short burst to date with a secureearly-type host (the other being GRB 050724 at z=0.257) and it has one of thehighest short GRB redshifts. The offset between the host center and the burstposition, 470 +/- 310 pc, is the smallest to date. Combined with the oldstellar population age, this indicates that the burst likely originated from aprogenitor with no significant kick velocity. However, from the brightness ofthe optical afterglow we infer a relatively low density of n~3x10^-4 cm^-3 whenepsilon_e and epsilon_B = 0.1. The combination of an optically faint afterglowand host suggest that previous such events may have been missed, therebypotentially biasing the known short GRB host population against z>1 early-typehosts.

The Optical Afterglow and z=0.92 Early-type Host Galaxy of the Short GRB 100117A