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

Abstract

In an ongoing programme to identify a sample of high z DLA galaxies we have found the long sought for of aL yα 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 Lyα flux is 5.4 +2 −0.8 × 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. �� 3 ± 0. �� 3. This is smaller than what was found in previous cases but still consistent with random sight- lines through absorbers with mean impact parameter ≈1 �� . Of the 24 DLAs targeted in the NICMOS imaging survey five have now been identified as Lyα emitters. The DLA galaxies with detected Lyα emission tend to have higher interstellar metallicities than those with undetected Lyα emission. This is plausibly explained as a consequence of a positive correlation between the Lyα 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 Lyα equiv- alent widths and metallicities, as seen in nearby star-forming galaxies and usually attributed to the preferential absorption of Lyα photons by dust grains.