Detection of Lyman-α emission from a DLA galaxy: Possible implications for a luminosity-metallicity relation at z = 2-3

In an ongoing programme to identify a sample of high z DLA galaxies we have found the long sought for case of a Ly α emitter seen in the centre of a broad DLA trough. This is the predicted “textbook case” of an intervening DLA galaxy if DLA galaxies are small, but would not be expected if intervening high redshift DLA galaxies have large gaseous disks. The Ly α flux is $\rm 5.4^{+2}_{-0.8}\times 10^{-17}~erg~ s^{-1}~cm^{-2}$ similar to what has been found in previously known high z  DLA galaxies. The impact parameter is found to be $0\farcs3\pm 0\farcs3$. This is smaller than what was found in previous cases but still consistent with random sight-lines through absorbers with mean impact parameter $\approx $1´´. Of the 24 DLAs targeted in the NICMOS imaging survey five have now been identified as Ly α emitters. The DLA galaxies with detected Ly α emission tend to have higher interstellar metallicities than those with undetected Ly α emission. This is plausibly explained as a consequence of a positive correlation between the Ly α line luminosities of the galaxies and their metallicities, although the present sample is too small for a definitive conclusion. The available observations of high-redshift DLA galaxies are also consistent with a negative correlation between Ly α  equivalent widths and metallicities, as seen in nearby star-forming galaxies and usually attributed to the preferential absorption of Ly α  photons by dust grains.