Very large telescope and Hubble Space Telescope observations of the host galaxy of GRB 990705

We present Very Large Telescope spectroscopic observations of the GRB 990705 host galaxy and highlight the benefits provided by the prompt phase features of gamma-ray bursts (GRBs) to derive the redshifts of the latter. In the host spectrum, we indeed detect an emission feature that we attribute to the [O II] lambdalambda3726, 3729 doublet and derive an unambiguous redshift z = 0.8424 +/- 0.0002 for this galaxy. This is in full agreement with the value z similar to 0.86 +/- 0.17 previously derived using a transient absorption edge discovered in the X-ray spectrum of GRB 990705. This burst is therefore the first GRB for which a reliable redshift was derived from the prompt phase emission itself, as opposed to redshift determinations performed using putative host galaxy emission lines or interstellar absorption lines in the GRB afterglows. Deep and high-resolution images of the host of GRB 990705 with the Space Telescope Imaging Spectrograph camera on board the Hubble Space Telescope reveal that the burst occurred in a nearly face-on Sc spiral galaxy typical of disk-dominated systems at. 0.75 less than or equal to z less than or equal to 1. Assuming a cosmology with H-0 = 65 km s(-1) Mpc(-1), Omega(m) = 0.3, and Omega(lambda) = 0.7, we derive an absolute B magnitude M-B = -21.75 for this galaxy and a star formation rate SFR approximate to 5-8 M yr(-1). Finally, we discuss the implications. of using X-ray transient features to derive GRB redshifts with larger burst samples and especially examine the case of short and dark long GRBs.