On the structure and energetics of quasar broad absorption-line outflows

Abstract

Quasar accretion-disk outflows might play an important role in galaxy evolution, but they are notoriously difficult to study due to line saturation and blending problems in the Lyα forest. We circumvent these problems by constructing median composite spectra of diverse broad absorption lines (BALs) and 'mini-BALs' in SDSS-III BOSS quasars at redshifts 2.3 ≤ z ≤ 3.5. Sorting by C iv λ1549,1551 absorption-line strength with Al iii λ1855,1863 as an additional indicator of low ionisations (LoBALs) we find the following: (1) Deeper and broader BALs are accompanied by weaker He ii λ1640 emission lines, consistent with softer ionising spectra producing more effective radiative acceleration. (2) P v λ1118,1128 absorption is present with resolved ∼1:1 depth ratios in all composites from mini-BALs to strong BALs indicating that line saturation, large total column densities log N H (cm −2 ) 22.7, and large ionisation parameters log U −0.5 are common.(3) Different observed depths in saturated lines identify inhomogeneous partial covering on spatial scales 0.006 pc, where weak/low-abundance transitions like P v form in small high-column density clumps while stronger/broader lines like C iv form in larger regions. (4) Excitedstate S iv* λ1073 and C iii* λ1176 lines in BAL outflows indicate typical densities n e 3 × 10 5 cm −3 and maximum radial distances R 23 pc from the quasars. (5) For reasonable actual distances, the median BAL outflow has minimum kinetic energy L K /L 0.005(R/1.2pc) sufficient for feedback to galaxy evolution. (6) LoBAL quasars have the largest median outflow column densities, highest velocities, and weakest He ii λ1640 emission in our study; they appear to be at one extreme in a distribution of quasar properties where softer ionising spectra drive more powerful outflows.

Radiation-pressure Waves and Multiphase Quasar Outflows

“…High average densities in our proposed outflow structure could enhance the total amount of low-ionization gas, but would also increase the strength of absorption features from the high ions. This agrees with the high degree of reddening observed for LoBAL quasars (Reichard et al 2003;Ganguly et al 2007;Dai et al 2012), and with the measurements by Allen et al (2011);Reichard et al (2003); Filiz Ak et al (2014); Hamann et al (2019), where LoBALs show absorption troughs of high-ionization species that are (at least) as large as those in HiBALs. Further potential connections between the structure of the outflows and the nature of LoBALs need to be addressed in future studies.…”

“…Interestingly, Hamann et al (2019) recently argued that LoBALs show the largest average column densities and highest velocities in the outflows. This result is consistent with our findings that larger velocities drive higher densities, which also relates to the presence of low-ionization species.…”

“…When the features are narrower, down to a few hundreds of km s −1 , but one can still 'decide' that they correspond to quasar outflows, these adopt the name of mini-BALs, and below this poorly-defined threshold the troughs are named narrow absorption lines (NALs; e.g., Hamann & Sabra 2004). This classification arises from differences in the properties measured for these absorbers but it is possible that they all trace the same type of outflows and the differences are driven by orientation, time of observation, or spatial location within the outflow (see Hamann et al 2019, and references therein). We will refer to all the absorbers in this work as troughs, and will analyze them independently of the aforementioned nomenclature.…”

“…A number of works have analyzed composite outflow spectra (e.g., Baskin et al 2013Baskin et al , 2015Hamann et al 2019) but they have mostly focused on correlations between the outflows and the emission sources. We present here a complementary approach; our analysis focuses on correlations and dependences between physical parameters describing the absorption troughs, such as their position in velocity space, which we assume to trace the velocity of the outflow, their width, and the degree of detachment between troughs and sources, together with the dependences on quasar redshift and brightness.…”

Stratified disc wind models for the AGN broad-line region: ultraviolet, optical, and X-ray properties

“…Another improvement could be to include dust opacity in the flow, so that dusty wind models for the BLR can be tested. Although we have explored the sensitivity to some model parameters, it would also be interesting to investigate possible scalings with mass, Eddington fraction or SED shape, in the light of the results of Richards et al (2011) andHamann et al (2019a). However, the use of a kinematic, prescribed model makes it hard to investigate dynamic effects -for example, changes in wind strength due to an increased Eddington fraction or a softer SED shape.…”