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 ∼1.15 yr after the event. It is the most intense gamma-ray burst (∼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], Hβ 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 ∼25 Myr ago, during which the star-forming-rate reached ∼50 M� /yr. In conjunction with the spatial extent (∼1 �� ) it suggests a very luminous (MV = −21.8) blue compact galaxy, for which we also find Z ∼ Z� . The X-ray spectrum shows evidence of considerable absorption by neutral gas with NH,X−ray = 3.47 +0.48 −0.47 × 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 different 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 different from most GRB hosts being brighter than L ∗ by a factor of 3. We have also derived a SFR ∼ 50 M� /yr and predict that this host galaxy will be detected at sub-mm wavelengths.