A strong redshift dependence of the broad absorption line quasar fraction

Abstract

We describe the application of non-negative matrix factorization to generate compact reconstructions of quasar spectra from the Sloan Digital Sky Survey (SDSS), with particular reference to broad absorption line quasars (BALQSOs). BAL properties are measured for Si IV λ1400, C IV λ1550, Al III λ1860 and Mg II λ2800, resulting in a catalogue of 3547 BALQSOs. Two corrections, based on extensive testing of synthetic BALQSO spectra, are applied in order to estimate the intrinsic fraction of C IV BALQSOs. First, the probability of an observed BALQSO spectrum being identified as such by our algorithm is calculated as a function of redshift, signal-to-noise ratio and BAL properties. Secondly, the different completenesses of the SDSS target selection algorithm for BALQSOs and non-BAL quasars are quantified. Combining the detection probabilities with an intrinsic E(B − V) distribution capable of reproducing the observed increase in mean E(B − V) with increasing redshift, the intrinsic C IV BALQSO fraction is 41 ± 5 per cent. Our analysis of the selection effects allows us to measure the dependence of the intrinsic C IV BALQSO fraction on luminosity and redshift. We find a factor of 3.5 ± 0.4 decrease in the intrinsic fraction from the highest redshifts, z 4.0, down to z 2.0. The redshift dependence implies that an orientation effect alone is not sufficient to explain the presence of BAL troughs in some but not all quasar spectra. Our results are consistent with the intrinsic BALQSO fraction having no strong luminosity dependence, although with 3σ limits on the rate of change of the intrinsic fraction with luminosity of −6.9 and 7.0 per cent dex −1 we are unable to rule out such a dependence.

On the origins of C iv absorption profile diversity in broad absorption line quasars

“…The increase in the CF of C iv BAL with decreasing He ii EW and redder αUV (Fig. 5) implies that the C iv absorption in LoBALQs should be typically stronger than in HiBALQs, as indeed observed (Reichard et al 2003;Allen et al 2011;Filiz Ak et al 2014). Allen et al (2011) report that LoBALQs have a broader C iv BAL than HiBALQs.…”

“…3). Allen et al (2011) explore the distribution of the mean absorption depth of C iv BAL for BALQs from the SDSS DR6. They find that the distribution peaks at ≃0.6, which is roughly consistent with the maximum absorption of ≃ 0.7 − 0.8, implied by the composite profile in the absorber rest-frame (Fig.…”

“…Observationally, BALQs are reported to have a weaker He ii EW (Richards et al 2002;Reichard et al 2003) and a redder αUV compared to non-BALQs (Reichard et al 2003;Maddox et al 2008;Gibson et al 2009;Allen et al 2011). Theoretically, the EW of the He ii λ1640 recombination line is a measure of the continuum strength above 54 eV, compared to the near-UV continuum, and thus is indicative of the hardness of the ionizing spectral energy distribution (SED).…”

A SCUBA-2 survey of FeLoBAL QSOs. Are FeLoBALs in a ‘transition phase’ between ULIRGs and QSOs?

“…We use as a first choice the modified BI by Gibson et al (2009) where the selected species is the Mg II and the values of v 0 and v 1 are, respectively, set to 0 and 25 000 km s −1 and the minimum velocity width to 2000 km s −1 . For those objects in our sample which are not present in Gibson et al (2009), we used the BI of Allen et al (2011) where the sole difference is the value of v 0 which is set to 3000 km s −1 . Two of our FeLoBALs have quoted BI = 0, this is due to the Mg II absorption trough which is less than 10 per cent below the continuum emission.…”

“…FeLoBALs comprise ∼0.3 per cent of optically selected QSOs; this fraction however increases by a factor of ∼10 when Near Infra-Red (NIR) and radio surveys are taken into account (e.g. Dai, Shankar & Sivakoff 2012) due to the high level of dust obscuration which affects this class of quasars (Boroson & Meyers 1992;Allen et al 2011). A scenario where BAL QSOs are young and are still surrounded by gas and dust from which the absorption features emanate was initially favoured, although others believed that the origin of the BAL features seen is more likely an orientation effect (where a BAL is a normal QSO seen along a line of sight which coincides with the outflowing gas; e.g.…”