The CORALS survey II. Clues to galaxy clustering around QSOs from z_(abs) ~ z_(em) damped Lyman alpha systems

Abstract

We present a list of z_(abs) ~ z_(em) (Δv 〈 3000km s^(-1)) DLAs discovered during the CORALS survey for absorbers in a radio-selected QSO sample. On the assumption that z_(abs) ~ z_(em) DLAs are neither ejecta from the QSO, nor part of the host galaxy itself, we use the number density statistics of these DLAs to investigate galaxy clustering near the QSO redshift. We find that n(z) of z_(abs) ~ z_(em) DLAs in our radio-selected QSO sample is ∼4 times larger than the number density of intervening DLAs, implying an excess of galaxies near the QSO. This result is further supported with the inclusion of the radio-loud QSOs in the FIRST survey, although the total number of DLAs is still small (4) and the result is currently only significant at approximately the 2σ level. Since all of the z_(abs) ~ z_(em) DLAs we identify in CORALS are found towards optically bright (B〈 20) QSOs, there is no strong evidence (based on these limited statistics) that this population suffers from a severe dust bias. We compare our results with those from an optically-selected, radio-quiet QSO sample in order to determine whether there is evidence for an excess of galaxies around radio-loud versus radio-quiet QSOs. We find that the n(z) of z_(abs) ~ z_(em) DLAs towards radio-quiet QSOs is in agreement with the number density of intervening absorbers. This result, although currently limited by the small number statistics of our survey, supports the conclusion that radio-loud quasars are found preferentially in rich galaxy environments at high redshift. Finally, we report that one of the new z_(abs) ~ z_(em) DLAs discovered by CORALS has some residual flux in the base of the Lyman α trough which may be due to Lyman α emission, either from star formation in the DLA galaxy or from gas photoionised by the QSO.

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