An excess of damped Lyman α galaxies near quasi-stellar objects

ABSTRACT We present a sample of 33 damped Lyman α systems (DLAs) discovered in the Sloan Digital Sky Survey (SDSS) whose absorption redshifts (z_abs) are within 6000 km s^-1 of the quasi-stellar object's (QSO) systemic redshift (z_sys). Our sample is based on 731 2.5 < z_sys < 4.5 non-broad absorption line (non-BAL) QSOs from Data Release 3 (DR3) of the SDSS. We estimate that our search is ≈100 per cent complete for absorbers with N(H i) ≥ 2 × 10²⁰ cm ^-2. The derived number density of DLAs per unit redshift, n(z), within Δv < 6000 km s^-1 is higher (3.5σ significance) by almost a factor of 2 than that of intervening absorbers observed in the SDSS DR3, i.e. there is evidence for an overdensity of galaxies near the QSOs. This provides a physical motivation for excluding DLAs at small velocity separations in surveys of intervening 'field' DLAs. In addition, we find that the overdensity of proximate DLAs is independent of the radio-loudness of the QSO, consistent with the environments of radio-loud and radio-quiet QSOs being similar.