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GRB 970828 was the first well-localized gamma-ray burst (GRB) X-ray afterglow for which no optical afterglow was found despite a prompt, deep search down to R_lim ~ 24.5 mag. We report the discovery of a short-lived radio flare within the X-ray localization error circle of this burst. Such radio flares are seen in about 25% of GRB afterglows and their origin is not well understood. The precise radio position enabled us to identify the likely host galaxy of this burst, and to measure its redshift, z = 0.9578. The host appears to be an interacting/merging system. Under the assumption that the X-ray afterglow is mainly due to synchrotron mechanism we infer the optical afterglow flux. The observed upper limits to the optical flux are easily explained by invoking an intervening dusty cloud within the host galaxy. These observations support the idea that GRBs with no detectable optical afterglows or "dark GRBs" can be due to dust extinction within the host galaxies. The census of dark GRBs can then be used to constrain the fraction of the obscured star formation in the universe. We argue that the existing data already indicate that the obscured star-formation rate is no more than 1/2 of that seen at UV and optical wavelengths.Comment: Submitted to The Astrophysical Journal. LaTeX file, 27 pages, including 7 postscript figures; AASTeX style files include

Context. The AMIGA project aims to build a well defined and statistically significant reference sample of isolated galaxies in order to estimate the environmental effects on the formation and evolution of galaxies. Aims. The goal of this paper is to provide a measure of the environment of the isolated galaxies in the AMIGA sample, quantifying the influence of the candidate neighbours identified in our previous work and their potential effects on the evolution of the primary galaxies. Here we provide a quantification of the isolation degree of the galaxies in this sample. Methods. Our starting sample is the Catalogue of Isolated Galaxies (CIG). We used two parameters to estimate the influence exerted by the neighbour galaxies on the CIG galaxy: the local number density of neighbour galaxies and the tidal strength affecting the CIG galaxy. We show that both parameters together provide a comprehensive picture of the environment. For comparison, those parameters have also been derived for galaxies in denser environments such as triplets, groups and clusters. Results. The CIG galaxies show a continuous spectrum of isolation, as quantified by the two parameters, from very isolated to interacting. The fraction of CIG galaxies whose properties are expected to be influenced by the environment is however low (159 out of 950 galaxies). The isolated parameters derived for the comparison samples gave higher values than for the CIG and we found clear differences for the average values of the 4 samples considered, proving the sensitivity of these parameters. Conclusions. The environment of the galaxies in the CIG has been characterised, using two complementary parameters quantifying the isolation degree, the local number density of the neighbour galaxies and the tidal forces affecting the isolated galaxies. A final catalogue of galaxies has been produced and the most isolated of these galaxies are consequently appropriate to serve as a reference sample for the AMIGA project.