Lyman − α Imaging of a Very Luminous z = 2 . 3 Starburst

We investigate the Lyα and UV continuum morphology of one of the most luminous known Lyman α emitting galaxies (the ‘Coup Fourré Galaxy’), associated with a z = 2.3 damped Lyα absorption system in the spectrum of the QSO PHL 957 (Lowenthal et al. 1991). The galaxy was observed with the HST WFPC2, through a narrow filter (F410M) corresponding to rest-frame Lyα for a total exposure time of 41.2 ksec, plus shorter exposures in F555W and F814W. In all three passbands, the galaxy is resolved into a close (∼ 0.35 arcsec) pair of two components, CFgA and CFgB, both of which are extended and elongated. The profile of CFgA is consistent with an exponential disk, with a similar scale-length in Lyα (rexp = 0.23 arcsec) and continuum (rexp = 0.20 arcsec), and no evidence of a central point source. In contrast, CFgB is closer to a bulge profile. We find that CFgA has by far the higher ratio of Lyα to continuum flux, and from the observed colours estimate rest-frame equivalent widths of W (Lyα) = 151±16Å for CFgA and 33±13Å for CFgB. From the F814W and F555W magnitudes we estimate rest-frame blue-band absolute magnitudes of -23.12 for CFgA and -23.24 for CFgB, significantly brighter than local galaxies of the same size. CFgA shows a remarkable 3.9 magnitudes of surface brightness enhancement relative to local spirals. This object appears to be at the upper limit of both the range of surface brightness evolution observed in z > 2 galaxies and the range of W (Lyα) in any star-forming galaxy. We speculate that its extreme surface brightness results from a very luminous starburst (> 200M⊙yr ), triggered by the merger of the two components, and the high W (Lyα) from a brief phase of the starburst in which most Lyα photons can escape, as predicted in the models of Tenorio-Tagle et al. (1999). We also investigate the F410M image of the QSO PHL 957. Subtraction of a normalized point-spead function leaves no significant residuals – the QSO is consistent with a pure point source and we do not detect either the host galaxy or the damped Lyα absorbing galaxy. We search for other galaxies with strong Lyα emission at z ∼ 2.3–2.4, selecting these by a colour m410−V555 < −0.2. Eight candidate Lyα sources, all fainter than the Coup Fourré galaxy, are identified in our field but it is remains uncertain whether these form a cluster. One is a point-source and may be an AGN; the others are of similar size to the Coup Fourré Galaxy but lower surface brightness, knotty and asymmetric. They appear typical of Lyman break galaxies but with colours indicating W (Lyα) ∼ 100Å.