GRB 070125: The first long-duration gamma-ray burst in a halo environment

We present the discovery and high signal-to-noise spectroscopic observations of the optical afterglow of the long-duration gamma-ray burst GRB 070125. Unlike all previously observed long-duration afterglows in the redshift range 0.5 less than or similar to z less than or similar to 2.0, we find no strong (rest-frame equivalent width W-r greater than or similar to 1.0 angstrom) absorption features in the wavelength range 4000-10000 angstrom. The sole significant feature is a weak doublet that we identify as Mg II lambda lambda 2796 (W-r = 0.18 +/- 0.02 angstrom), 2803 (W-r = 0.08 +/- 0.01 angstrom) at z = 1.5477 +/- 0.0001. The low observed Mg II and inferred H I column densities are typically observed in galactic halos, far away from the bulk of massive star formation. Deep ground-based imaging reveals no host directly underneath the afterglow to a limit of R > 25.4 mag. Either of the two nearest blue galaxies could host GRB 070125; the large offset (d >= 27 kpc) would naturally explain the low column densities. To remain consistent with the large local (i.e. parsec scale) circum-burst density inferred from broadband afterglow observations, we speculate GRB 070125 may have occurred far away from the disk of its host in a compact star-forming cluster. Such distant stellar clusters, typically formed by dynamical galaxy interactions, have been observed in the nearby universe, and should be more prevalent at z > 1 where galaxy mergers occur more frequently.