The physics and physical properties of quasar outflows

Abstract

We describe two studies designed to characterize the total column densities, kinetic energies, and acceleration physics of broad absorption line (BAL) outflows in quasars. The first study uses new Chandra X-ray and ground-based rest-frame UV observations of 7 quasars with mini-BALs at extreme high speeds, in the range 0.1c to 0.2c, to test the idea that strong radiative shielding (and therefore strong X-ray absorption) is needed to moderate the mini-BAL ionizations and facilitate their acceleration to extreme speeds. We find that the X-ray absorption is weak or absent, with generally N H < few × 10 22 cm −2 , and that radiative shielding is not important. We argue that the mini-BAL ionizations are controlled, instead, by high gas densities of order n H ∼ 4 × 10 8 cm −3 in small outflow substructures. If we conservatively assume that the total column density in the mini-BAL gas is N H < 10 22 cm −2 , covering >15% of the UV continuum source along our lines of sight (based on measured line depths), then the radial thickness of these outflows is only ∆R < 3 × 10 13 cm and their transverse size is >8 × 10 15 cm. Thus the outflow regions have the shape of very thin "pancakes" viewed face-on, or they occupy larger volumes like a spray of dense cloudlets with a very small volume filling factor. We speculate that this situation (with ineffective shielding and small dense outflow substructures) applies to most quasar outflows, including BALs. Our second study focuses from BALs of low-abundance ions, mainly PV 1118,1128 Å, whose significant strengths imply large column densities, N H > 10 22 cm −2 , that can further challenge models of the outflow acceleration. In spite of the difficulties of finding this line in the Lyα forest, a search through the SDSS DR9 quasar catalog reveals >50 BAL sources at redshifts z > 2.3 with strong PV BALs, which we are now using to characterize the general properties of high-column outflows.