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 H (alpha) 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 IIabsorbers at redshifts around unity. Assuming that this tentative redshift-identification is correct, the star formation rate of the galaxy is 13 +- 2 M(circled dot operator) yr^ -1. Our study reaches star-formation rate limits (5 (sigma)) of 5.7  M (circled dot operator) yr^-1 at z=1.38, and 7.7  M (circled dot operator)  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.