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

Abstract

There is a large diversity in the C iv broad absorption line (BAL) profile among BAL quasars (BALQs). We quantify this diversity by exploring the distribution of the C iv BAL properties, FWHM, maximum depth of absorption and its velocity shift (v md ), using the SDSS DR7 quasar catalogue. We find the following: (i) Although the median C iv BAL profile in the quasar rest-frame becomes broader and shallower as the UV continuum slope (α UV at 1700-3000Å) gets bluer, the median individual profile in the absorber rest-frame remains identical, and is narrow (FWHM = 3500 km s −1 ) and deep. Only 4 per cent of BALs have FWHM > 10, 000 km s −1 . (ii) As the He ii emission equivalent-width (EW) decreases, the distributions of FWHM and v md extend to larger values, and the median maximum depth increases. These trends are consistent with theoretical models in which softer ionizing continua reduce overionization, and allow radiative acceleration of faster BAL outflows. (iii) As α UV becomes bluer, the distribution of v md extends to larger values. This trend may imply faster outflows at higher latitudes above the accretion disc plane. (iv) For non-BALQs, the C iv emission line decreases with decreasing He ii EW, and becomes more asymmetric and blueshifted. This suggests an increasing relative contribution of emission from the BAL outflow to the C iv emission line as the ionizing spectral energy distribution (SED) gets softer, which is consistent with the increasing fraction of BALQs as the ionizing SED gets softer.

BAL and non-BAL quasars: continuum, emission, and absorption properties establish a common parent sample

“…A2: with increasing C iii] 'blueshift' (increasing bin number), the C iv blueshift also increases. Note also that the C iv EW decreases and that He iiλ1640 decreases in strength with increasing blueshift, the latter of which is in agreement with Richards et al (2011) and Baskin et al (2013Baskin et al ( , 2015. The median and median absolute deviation (MAD) of each component weight in each bin was calculated (see examples in the bottom panels of Fig.…”

“…in Baskin et al 2015, for example) but is particularly evident here. Otherwise, the two composites drawn from the lower C iv(EW) regions show no spectral differences greater than 2 per cent shortward of the C iv emission line.…”

“…shows absorber-restframe composites of the BI-defined BAL spectra in different regions of C iv-emission space. Baskin et al (2015) presented similar absorber-frame composites and found trough depth not to be correlated with He ii-EW (see their fig. 7), in agreement with what we present here in C iv-emission space.…”

“…In the linedriven wind scenario, only the spectra with the lowest He ii EW, thus fewest high-energy ionizing-photons, are able to produce the strongest disc-winds. Baskin et al (2013Baskin et al ( , 2015 have quantified the strength of the dependence of BALtrough properties as a function of the He ii-emission EW (broader and faster-moving troughs are observed when He ii is weak). The systematic behaviour of He ii-emission as a function of C iv-blueshift for non-BAL quasars has also been identified by Baskin et al (2015).…”

Spectral Evolution in High Redshift Quasars From the Final Baryon Oscillation Spectroscopic Survey Sample

“…Quasars display striking similarities such as broad emission lines and continua that follow an approximate power law relationship in the near-UV and optical wavelengths (Merloni 2016, and references therein). However, the exact properties of the continuum and emission lines vary across the population of quasars in a manner that is poorly understood (Richards et al 2011;Shen & Ho 2014;Baskin et al 2015, for instance). Our understanding of quasar diversity has improved in recent years through applications of several analysis techniques to the sample of spectroscopically observed quasars.…”