New search strategy for high z intervening absorbers : GRB 021004 , a pilot study ⋆

We present near-infrared narrow-and broad-band imaging of the field of GRB 021004, performed with ISAAC on the UT1 of the ESO Very Large Telescope. The narrow-band filters were chosen to match prominent emission lines at the redshift of the absorption-line systems found against the early-time afterglow of GRB 021004: [O iii] at z=1.38 and Hα at z=1.60, respectively. For the z=1.38 system we find an emission-line source at an impact parameter of 16 ′′ , which is somewhat larger than the typical impact parameters of a sample of Mg ii absorbers at redshifts around unity. Assuming that this tentative redshift-identification is correct, the star formation rate of the galaxy is 13±2 M⊙ yr −1. Our study reaches star-formation rate limits (5σ) of 5.7 M⊙ yr −1 at z=1.38, and 7.7 M⊙ yr −1 at z=1.60. These limits correspond to a depth of roughly 0.13 L *. Any galaxy counterpart of the absorbers nearer to the line of sight either has to be fainter than this limit or not be an emission-line source.