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

We investigate the Ly alpha and UV continuum morphology of one of the most luminous known Lyman alpha emitting galaxies (the 'Coup Fourre Galaxy'), as sociated with a z = 2.3 damped Ly alpha absorption system in the spectrum o f the QSO PHL 957. The galaxy is observed with the Hubble Space Telescope W ide-Field Planetary Camera 2 (HST WFPC2), through a narrow filter (F410M) c orresponding to rest-frame Ly alpha for a total exposure time of 41.2 ks, p lus shorter exposures in F555W and F814W. In all three passbands, the galaxy is resolved into a close (similar to 0.3 5 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 Ly alpha (r(exp) = 0.23 arcsec) and continuum (r( exp) = 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 highe r ratio of Ly alpha to continuum flux, and from the observed colours estima te rest-frame equivalent widths of W(Ly alpha) = 151 +/- 16 Angstrom for CF gA and 33 +/- 13 Angstrom for CFgB. From the F814W and F555W magnitudes we estimate the rest-frame blue-band ab solute magnitudes (for H-0 = 50 km s(-1) Mpc(-1) and q(0) = 0.05) of -23.12 for CFgA and -23.24 for CFgB, significantly brighter than local galaxies o f the same size. CFgA shows a remarkable 3.9 magnitudes of surface brightne ss 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(Ly alpha) in any star-forming galaxy. We s peculate that its extreme surface brightness results from a very luminous s tarburst (similar to 200 M. yr(-1)), triggered by the merger of the two com ponents, and the high W(Ly alpha) from a brief phase of the starburst in wh ich most Ly alpha 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 n ormalized point-speed function leaves no significant residuals - the QSO is consistent with a pure point source and we do not detect either the host g alaxy or the damped Ly alpha absorbing galaxy. We search for other galaxies with strong Ly alpha emission at z similar to 2.3-2.4, selecting these by colour (m(410) - V-555)(AB) < -0.2. Eight candi date Ly alpha sources, all fainter than the Coup Fourre Galaxy, are identif ied in our field. One is a point-source and may be an AGN; the others are o f similar size to the Coup Fourre Galaxy but of lower surface brightness, k notty and asymmetric. They appear typical of Lyman break galaxies but with colours indicating W(Ly alpha) similar to 100 Angstrom.