Host galaxies of gamma-ray bursts: Spectral energy distributions and internal extinction

We present BVRcIc broad-band flux spectra for the host galaxies of GRB 970508, GRB 980613, GRB 980703, GRB 990123 and GRB 991208 obtained with the 6-m telescope of SAO RAS. The comparison of the broad-band flux spectra of these host galaxies with the template spectral energy distributions (SEDs) of local starburst galaxies of different morphological types shows that the BVRcIc of the hosts are best fitted by the spectral properties of template SEDs of starburst galaxies and that there is a significant internal extinction in these host galaxies. We derived the absolute magnitudes of the GRB host galaxies making use of SEDs for the starburst galaxies. To create theoretical templates we performed the population synthesis modeling of the continuum spectral energy distribution of the host galaxies of GRB 970508 and GRB 980703 using different extinction laws (Cardelli et al. 1989 and Calzetti et al. 2000) and assuming burst and exponential scenarios of star formation. The comparison of BVRcIc broad-band flux spectra with the local starburst galaxies templates and theoretical templates as well as direct estimates (using Balmer emission lines) of the internal extinction shows that it is likely to be of great importance to take into account effects of the internal extinction in the host galaxies. From the energy distribution in the spectrum of the host galaxy of GRB 991208 and from the intensity of their spectral lines (with allowance for the effects of internal extinction) it follows that this is a GRB galaxy with the highest massive star-formation rate of all known GRB galaxies - up to hundreds of solar masses per year. The reduced luminosity of these dusty galaxies (e.g. for the host of GRB 970508 A(V) similar to 2 mag, for the host of GRB 980703 A(V) similar to 0.6 mag and for the host of GRB 991208 A(V) similar to 2 mag) could explain the observational fact (it results independently from our BVRcIc photometry and from calculated spectral distribution for the subset of galaxies having been observed with the 6-m telescope): none of the observed GRB host galaxies with known distances is brighter than the local galaxies with the luminosity L-* (where L-* is the 'knee' of the local luminosity function).