Radiation pressure confinement - IV. Application to broad absorption
  line outflows
  A fraction of quasars present broad absorption lines, produced by outflowing
gas with typical velocities of 3000 - 10,000 km/s. If the outflowing gas fills
a significant fraction of the volume where it resides, then it will be highly
ionized by the quasar due to its low density, and will not produce the observed
UV absorption. The suggestion that the outflow is shielded from the ionizing
radiation was excluded by recent observations. The remaining solution is a
dense outflow with a filling factor $f<10^{-3}$. What produces such a small
$f$? Here we point out that radiation pressure confinement (RPC) inevitably
leads to gas compression and the formation of dense thin gas sheets/filaments,
with a large gradient in density and ionization along the line of sight. The
total column of ionized dustless gas is a few times $10^{22}$ cm$^{-2}$,
consistent with the observed X-ray absorption and detectable P V absorption.
The predicted maximal columns of various ions show a small dependence on the
system parameters, and can be used to test the validity of RPC as a solution
for the overionization problem. The ionization structure of the outflow implies
that if the outflow is radiatively driven, then broad absorption line quasars
should have $L/L_{\rm Edd} \gtrsim 0.1$.
