Environment or Outflows? New Insight into the Origin of NALs

Abstract

Recent detailed studies of narrow absorption line (NAL) systems in QSO spectra have revealed that at least 50% of QSOs have NALs associated with the central engine, and in most cases they are found to be outflowing. Will studies of NALs provide the much sought-after evidence for ubiquitous QSO feedback that can halt the formation of stars in galaxies? I present new results on the distribution of line-of-sight velocity offsets between Mg ii absorbers and their background QSOs, based on a large catalogue of absorbers from SDSS DR6 and greatly improved QSO redshift estimates. The analysis reveals high velocity Mg ii NALs out to at least 6000 km s−1 from the QSO, which cannot be ascribed to the clustering of local galaxies, similar to that observed for C iv absorbers. The existence of such low-ionization, high-velocity gas clouds in the intense ionixing field of the QSO suggests that we may indeed be witnessing the mechanical expulsion of gas, alongside the heating previously observed. We also find an excess of low-velocity Mg ii NALs in radio-loud QSOs compared to radio-quiet QSOs, consistent with their different clustering amplitudes.