Deep Ly alpha imaging of two z=2.04 GRB host galaxy fields

We report on the results of deep narrow-band Lyalpha and broad-band U and I imaging of the fields of two Gamma-Ray bursts at redshift z = 2.04 (GRB 000301C and GRB 000926). We find that the host galaxy of GRB 000926 is an extended (more than 2 arcsec), strong Lyalpha emitter with a rest-frame equivalent width of 71(-15)(+20) Angstrom.

The galaxy consists of two main components and several fainter knots. GRB 000926 occurred in the western component, whereas most of the Lyalpha luminosity (about 65%) originates in the eastern component. Using archival HST images of the host galaxy we measure the spectral slopes (f(lambda) proportional to lambda(beta)) of the two components to beta = 2.4 +/- 0.3 (east)and -1.4 +/- 0.2 (west).

This implies that both components contain at most small amounts of dust, consistent with the observed strong Lyalpha emission. The western component has a slightly redder V - I colour than the eastern component, suggesting the presence of at least some dust. We do not detect the host galaxy of GRB 000301C in neither Lyalpha emission nor in U and I broad-band images.

The strongest limit comes from combining the narrow and U-band imaging where we infer a limit of U(AB) > 27.7 (2sigma limit per arcsec(2)). The upper limits on the Lyalpha flux implies a Lyalpha equivalent width upper limit of similar to150 Angstrom. We find eleven and eight other galaxies with excess emission in the narrow filter in the fields of GRB 000301C and GRB 000926 respectively.

These galaxies are candidate Lyalpha emitting galaxies in the environment of the host galaxies. Based on these detections we conclude that GRB 000926 occurred in one of the strongest centres of star formation within several Mpc, whereas GRB 000301C occurred in an intrinsically very faint galaxy far from being the strongest centre of star formation in its galactic environment.

Under the hypothesis that GRBs trace star formation, the wide range of GRB host galaxy luminosities implies a very steep faint end slope of the high redshift galaxy luminosity function.