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David M. Russell Sara L. Ellison Chris R. Benn 《Monthly notices of the Royal Astronomical Society》2006,367(1):412-422
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 Δ v < 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. 相似文献
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