The host galaxy of GRB 011121: morphology and spectral energy distribution ^*,**

¹ Max-Planck-Institut für extraterrestrische Physik, Giessenbachstrasse 1, 85748 Garching, Germany e-mail: ayoldas@mpe.mpg.de
² Department of Astronomy, California Institute of Technology, 1200 E California Blvd, Pasadena, CA, 91125, USA
³ INAF - Osservatorio Astronomico di Trieste, via G.B. Tiepolo 11, 34131 Trieste, Italy

Received: 27 July 2005
Accepted: 14 November 2006

Abstract

Aims.We present a detailed study of the host galaxy of GRB 011121 (at ) based on high-resolution imaging in 5 broad-band, optical and near-infrared filters with HST and VLT/ISAAC.

Methods.The surface brightness profile of this galaxy is best fitted by a Sersic law with index –2.5 and a rather large effective radius (~7.5 kpc). Both the morphological analysis and the F450W–F702W colour image suggest that the host galaxy of GRB 011121 is either a disk-system with a rather small bulge, or one hosting a central, dust-enshrouded starburst. Hence, we modeled the integrated spectral energy distribution of this galaxy by combining stellar population and radiative transfer models, assuming properties representative of nearby starburst or normal star-forming, Sbc-like galaxies.

Results.A range of plausible fitting solutions indicates that the host galaxy of GRB 011121 has a stellar mass of 3.1–, stellar populations with a maximum age ranging from 0.4 to 2 Gyr, and a metallicity ranging from 1 to 29 per cent of the solar value, as a function of the time elapsed since star formation started. As for the opacity, starburst models suggest this galaxy to be nearly as opaque as local starbursts (with an A_V = 0.27–0.76 mag). On the other hand, normal star-forming Sbc-like models suggest a central opacity larger than that of local disks by up to a factor of 8, whereas the attenuation along the line of sight is only A_V = 0.12–0.57 mag owing to the galaxy's low inclination. For this subluminous galaxy (with ), we determine a model-dependent star formation rate (SFR) of 2.4– yr^-1, which gives a SFR per unit luminosity of 9.2– yr^-1  and a SFR per unit stellar mass of 0.4– yr^-1. The former specific SFR is high compared to those of most GRB host galaxies, but consistent with those of most of the hosts at similar low redshift.

Conclusions. Our results suggest that the host galaxy of GRB 011121 is a rather large disk-system in a relatively early phase of its star formation history.