Lyα imaging of a very luminous z = 2.3 starburst galaxy with WFPC2

We investigate the Lyα and UV continuum morphology of one of the most luminous known Lymanα emitting galaxies (the ‘Coup Fourre Galaxy’), associated with a z=2.3 damped Lyα absorption system in the spectrum of the QSO PHL 957. The galaxy is observed with the Hubble Space Telescope Wide-Field Planetary Camera 2 (HST WFPC2), through a narrow filter (F410M) corresponding to rest-frame Lyα for a total exposure time of 41.2 ks, 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 of similar scalelength 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±16 A for CFgA and 33±13 A for CFgB. From the F814W and F555W magnitudes we estimate the rest-frame blue-band absolute magnitudes (for H0=50 km s−1 Mpc−1 and q0=0.05) 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 (∼200 M⊙ yr−1), 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 investigated 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 colour (m410−V555)AB<−0.2. Eight candidate Lyα sources, all fainter than the Coup Fourre Galaxy, are identified in our field. One is a point-source and may be an AGN; the others are of similar size to the Coup Fourre Galaxy but of lower surface brightness, knotty and asymmetric. They appear typical of Lyman break galaxies but with colours indicating W(Lyα)∼100 A.