We report the results of a spectroscopic search for Lyman- \alpha ( Ly \alpha ) 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 Ly \alpha emission from the host galaxies .
We detect Ly \alpha emission from 7 out of the 20 hosts , with the typical limiting 3 \sigma line flux being 8 \times 10 ^ { -18 } \mathrm { erg } \mathrm { cm } ^ { -2 } \mathrm { s } ^ { -1 } , corresponding to a Ly \alpha luminosity of 6 \times 10 ^ { 41 } \mathrm { erg } \mathrm { s } ^ { -1 } at z = 3 .
The Ly \alpha luminosities for the 7 hosts in which we detect Ly \alpha emission are in the range ( 0.6–2.3 ) \times 10 ^ { 42 } \mathrm { erg } \mathrm { s } ^ { -1 } , corresponding to star-formation rates of 0.6–2.1 M _ { \sun } \mathrm { yr } ^ { -1 } ( not corrected for extinction ) .
The rest-frame Ly \alpha equivalent widths ( EWs ) for the 7 hosts are in the range 9–40 Å .
For 6 of the 13 hosts for which Ly \alpha is not detected we place fairly strong 3 \sigma upper limits on the EW ( < 20 Å ) , while for others the EW is either unconstrained or has a less constraining upper limit .
We find that the distribution of Ly \alpha EWs is inconsistent with being drawn from the Ly \alpha 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 Ly \alpha 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 Ly \alpha line ( where detected ) is redshifted by 200–700 \mathrm { km } \mathrm { 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 Ly \alpha EW and the optical to X–ray spectral index of the afterglow ( \beta _ { \mathrm { OX } } ) , hinting that dust plays a role in the observed strength and even presence of Ly \alpha emission .