On the origin of intrinsic narrow absorption lines in z less than or similar to 1 QSOs

We present an exhaustive statistical analysis of the associated (Deltav(abs ) < 5000 km s(-1)), high-ionization (C IV, N V, O VI) narrow absorption lin e (NAL) systems in a sample of 59 QSOs defined from the Hubble Space Telesc ope (HST) QSO Absorption Line Key Project. The goals of the research were t wofold: (1) to determine the frequency of associated NALs at low redshift a nd in low luminosity QSOs, and (2) to address the question of what QSO prop erties either encourage or inhibit the presence of associated NAL gas. To t hat end, we have compiled the QSO rest-frame luminosities at 2500 <Angstrom >, 5 GHz, and 2 keV, spectral indices at 2500 Angstrom and 5 GHz, the H bet a emission-line FWHM, and the radio core fraction at observed 5 GHz. In add ition, we have measured the C IV emission-line FWHM. We find 17 associated NALs (16 selected by C IV and one selected by O VI) toward 15 QSOs, of whic h similar to 10 are statistically expected to be intrinsic. From a multivar iate clustering analysis, we find that the QSOs group together (in paramete r space) based primarily on radio luminosity, followed (in order of importa nce) by radio spectral index, C IV emission-line FWHM, and soft X-ray lumin osity. We find that radio-loud QSOs that have compact radio morphologies, f lat radio spectra [alpha (5 GHz)> -[0.5], and mediocre C IV FWHM (less than or similar to 6000 km s(-1)) do not have detectable associated NALs, down to W-r(C IV) = 0.35 Angstrom. We also find that broad absorption line (BAL) QSOs have an enhanced probability of hosting detectable NAL gas. In additi on, we find that the velocity distribution of associated NALs is peaked aro und the emission redshifts rather than the systemic redshifts of the QSOs. Finally, we find only one strong NAL [W-r(C IV) greater than or similar to 1.5 Angstrom] in our low-redshift sample. A comparison with previous higher redshift surveys reveals evolution in the number of strong NAL systems wit h redshift. We interpret these results in the context of an accretion disk model. We propose that NAL gas hugs the streamlines of the faster, denser, low-latitude wind, which has been associated with BALs. In the framework of this scenario, we can explain the observational clues as resulting from di fferences in orientation and wind properties, the latter presumably associa ted with the QSO radio properties.