We present a detailed study of the host galaxy of GRB 011121 ( at z = 0.36 ) based on high-resolution imaging in 5 broad-band , optical and near-infrared filters with HST and VLT/ISAAC .
The surface brightness profile of this galaxy is best fitted by a Sersic law with index n \sim 2 – 2.5 and a rather large effective radius ( \sim 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 .
A range of plausible fitting solutions indicates that the host galaxy of GRB 011121 has a stellar mass of 3.1 – 6.9 \times 10 ^ { 9 } ~ { } M _ { \sun } , 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 L _ { B } / L ^ { \star } _ { B } = 0.26 ) , we determine a model-dependent star formation rate ( SFR ) of 2.4 – 9.4 ~ { } M _ { \sun } ~ { } yr ^ { -1 } , which gives a SFR per unit luminosity of 9.2 – 36.1 ~ { } M _ { \sun } ~ { } yr ^ { -1 } ~ { } ( L _ { B } / L ^ { \star } _ { B } ) ^ { -1 } and a SFR per unit stellar mass of 0.4 – 2.9 \times 10 ^ { -9 } ~ { } 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 .
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 .