Hubble Space Telescope and Spitzer Observations of the Afterglow and Host Galaxy of GRB 050904 at z = 6.295

August 2007 • 2007ApJ...665..102B

Authors • Berger, E. • Chary, R. • Cowie, L. L. • Price, P. A. • Schmidt, B. P. • Fox, D. B. • Cenko, S. B. • Djorgovski, S. G. • Soderberg, A. M. • Kulkarni, S. R. • McCarthy, P. J. • Gladders, M. D. • Peterson, B. A. • Barger, A. J.

Abstract • We present deep Hubble Space Telescope (HST) and Spitzer Space Telescope observations of GRB 050904 at z=6.295. We detect the afterglow in the H band more than 3 weeks after the burst and confirm the presence of a jet break at t~2.1 days. This leads to an estimated opening angle of about 4° and a beaming-corrected energy of about 10⁵¹ ergs, similar to those of lower redshift gamma-ray bursts (GRBs). We do not detect an underlying host galaxy with either HST or Spitzer. From the upper limits we infer an extinction-corrected absolute magnitude M_UV>~-20.3 mag, or L<~L^*, a star formation rate of <~5.7 M_solar yr^-1, and a stellar mass of <~few×10⁹ M_solar. A comparison to spectroscopically confirmed galaxies at z>5.5 reveals that the host of GRB 050904 is fainter and has a lower star formation rate than at least 80% of these objects. Finally, using our luminosity limits, and the metallicity of about 0.05 Z_solar inferred from the afterglow absorption spectrum, we place the first limit on the luminosity-metallicity relation at z>6. Future afterglow and host galaxy observations of z>~4 GRBs should elucidate whether the mass- and luminosity-metallicity relations continue to evolve beyond the present limits of z<~2.

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Ranga-Ram Chary

Senior Scientist