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

We present a sample of 33 damped Lyman alpha 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 approximate to 100 per cent complete for absorbers with N(H I) >= 2 x 10(20) cm(-2). The derived number density of DLAs per unit redshift, n(z), within Delta v < 6000 km s(-1) is higher (3.5 sigma 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.