VLT Spectroscopy of GRB 990510 and GRB 990712: Probing the Faint and Bright Ends of the Gamma‐Ray Burst Host Galaxy Population

We present time-resolved optical spectroscopy of the afterglows of thegamma-ray bursts GRB 990510 and GRB 990712. Through the identification ofseveral absorption lines in the first epoch GRB 990510 spectrum, we determinethe redshift for this burst at z >= 1.619. No clear emission lines aredetected. The strength of the MgI feature is indicative of a dense environment,most likely the host galaxy of GRB 990510. Although the host is extremely faint(V >~ 28), the GRB afterglow allows us to probe its interstellar medium and -in principle - to measure its metallicity. The optical spectrum of GRB 990712(whose host galaxy is the brightest of the known GRB hosts at cosmologicalredshifts), shows clear features both in emission and absorption, at a redshiftof z = 0.4331 +/- 0.0004. On the basis of several line emission diagnosticdiagrams, we conclude that the host galaxy of GRB 990712 is most likely an HIIgalaxy. We derive a unreddened [OII] star formation rate of 2.7 +/- 0.8Msun/yr. Correcting for the measured extinction intrinsic to the host galaxy(A(V) = \gpm{3.4}{2.4}{1.7}), this value increases to \gpm{35}{178}{25}Msun/yr. The [OII] equivalent width, compared to that of field galaxies at z <=1, also suggests that the host of GRB 990712 is vigorously forming stars. Weemploy the oxygen and Hbeta emission-line intensities to estimate the globaloxygen abundance for the host of GRB 990712: log(O/H) = -3.7 +- 0.4, which isslightly below the lowest metallicity one finds in nearby spiral galaxies. Forboth GRBs we study the time evolution of the absorption lines, whose equivalentwidth might be expected to change with time if the burst resides in a densecompact medium. We find no evidence for a significant change in the MgII width.Comment: 9 pages; accepted for publication in the Astrophysical Journa