  We investigate a very luminous Lyman-alpha emitting galaxy, associated with a
z=2.3 damped absorption system in the spectrum of the QSO PHL 957. The galaxy
was observed with the HST WFPC2, through a narrow filter (F410M) corresponding
to rest-frame Lyman -alpha, for 41.2 ksec, with shorter exposures in F555W and
F814W. The galaxy is resolved into a close (0.35 arcsec) pair of two
components, CFgA and CFgB. The profile of CFgA is consistent with an
exponential disk, but CFgB is closer to a bulge galaxy. Fr om the observed
colours we estimate rest-frame Lyman-alpha equivalent widths of 151 Angstroms
for CFgA and 33 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, signi ficantly brighter than local galaxies of the same size. CFgA shows
a remarkable 3.9 magnitudes of surface brightness enhancement relative to local
spirals, and we speculate that this may be the result of a very luminous
starburst, triggered by the merger of the two components. The Lyman-alpha
emission could be from a brief phase early in the starburst in which most of
these photons can escape.
  We search for other z=2.3 galaxies with strong Lyman-alpha emission,
selecting these by a colour F410M-F555W<-0.2. Eight candidate sources, all
fainter than CFgA, are identified. One is a point-source and may be an AGN; the
others are irregular a nd of low surface brightness, appearing typical of Lyman
break galaxies, but with Lyman-alpha equivalent widths near 100 Angstroms.
