The dark nature of GRB 051022 and its host galaxy

Aims. We present multiwavelength (X-ray/optical/near-infrared/millimetre) observations of GRB 051022 between 2.5 h and similar to .15 yr after the event. It is the most intense gamma- ray burst (similar to 10-4 erg cm(-2)) detected by HETE-2, with the exception of the nearby GRB 030329. Methods. Optical and near infrared observations did not detect the afterglow despite a strong afterglow at X-ray wavelengths. Millimetre observations at Plateau de Bure (PdB) detected a source and a flare, confirming the association of this event with a moderately bright (R = 21.5) galaxy. Results. Spectroscopic observations of this galaxy show strong [O II], H beta and [O III] emission lines at a redshift of 0.809. The spectral energy distribution (SED) of the galaxy implies AV (rest frame) = 1.0 and a starburst occuring similar to 25 Myr ago, during which the star-forming-rate reached similar to 50 M-circle dot yr. In conjunction with the spatial extent (similar to 1") it suggests a very luminous (M-v = -21.8) blue compact galaxy, for which we also find Z similar to Z(circle dot). The X-ray spectrum shows evidence of considerable absorption by neutral gas with N-H,N-X-ray = 3.47(-0.47)(+0.48) x 10(22) cm(-2) ( rest frame). Absorption by dust in the host galaxy at z = 0.809 certainly cannot account for the non- detection of the optical afterglow, unless the dust- to- gas ratio is quite di. erent than that seen in our Galaxy (i. e. large dust grains). Conclusions. It is likely that the afterglow of the dark GRB 051022 was extinguished along the line of sight by an obscured, dense star forming region in a molecular cloud within the parent host galaxy. This galaxy is di. erent from most GRB hosts being brighter than L* by a factor of 3. We have also derived a SFR similar to 50 M-circle dot/yr and predict that this host galaxy will be detected at sub- mm wavelengths.