THE OPTICALLY UNBIASED GRB HOST (TOUGH) SURVEY. IV. Lyα EMITTERS*

摘要

We report the results of a spectroscopic search for Lyα emission from gamma-ray burst (GRB) host galaxies. Based on a well-defined parent sample (the TOUGH sample) of 69 x-ray-selected Swift GRBs, we have targeted the hosts of a subsample of 20 GRBs known from afterglow spectroscopy to be in the redshift range z = 1.8-4.5. We have obtained spectroscopy using the FORS1 instrument at the ESO Very Large Telescope to search for the presence of Lyα emission from the host galaxies. We detect Lyα emission from 7 out of the 20 hosts, with the typical limiting 3σ line flux being 8 × 10–18 erg cm–2 s–1, corresponding to an Lyα luminosity of 6 × 1041 erg s–1 at z = 3. The Lyα luminosities for the seven hosts in which we detect Lyα emission are in the range (0.6-2.3) × 1042 erg s–1, corresponding to star formation rates of 0.6-2.1 M ☉ yr–1 (not corrected for extinction). The rest-frame Lyα equivalent widths (EWs) for the seven hosts are in the range 9-40 ?. For 6 of the 13 hosts for which Lyα is not detected, we place fairly strong 3σ upper limits on the EW (20 ?), while for others the EW is either unconstrained or has a less constraining upper limit. We find that the distribution of Lyα EWs is inconsistent with being drawn from the Lyα EW distribution of bright Lyman break galaxies (LBGs) at the 98.3% level, in the sense that the TOUGH hosts on average have larger EWs than bright LBGs. We can exclude an early indication, based on a smaller, heterogeneous sample of pre-Swift GRB hosts, that all GRB hosts are Lyα emitters. We find that the TOUGH hosts on average have lower EWs than the pre-Swift GRB hosts, but the two samples are only inconsistent at the 92% level. The velocity centroid of the Lyα line (where detected) is redshifted by 200-700 km s–1 with respect to the systemic velocity (taken to be the afterglow redshift), similar to what is seen for LBGs, possibly indicating star-formation-driven outflows from the host galaxies. There seems to be a trend between the Lyα EW and the optical to x-ray spectral index of the afterglow (βOX), hinting that dust plays a role in the observed strength and even presence of Lyα emission.