Physical Conditions in the Low-Ionization Component of Starburst
  Outflows: The Shape of Near-Ultraviolet and Optical Absorption-Line Troughs
  in Keck Spectra of ULIRGs
  We analyze the physical conditions in the low-ionization component of
starburst outflows (in contrast to the high-ionization wind fluid observed in
X-rays), based on new Keck/LRIS spectroscopy of partially resolved absorption
troughs in near-ultraviolet and optical spectra of Ultraluminous Infrared
Galaxies. The large velocity width and blueshift present in seven, atomic
transitions indicate a macroscopic velocity gradient within the outflowing gas.
The \mgII 2796, 2803 (and \feII 2587, 2600) doublet lines in these data
constrains the gas kinematics better than the heavily blended \nad 5892, 98
doublet. The identical shape of the \mgII 2796 absorption troughs to that of
the normally weaker transition at 2803\AA requires both transitions be
optically thick at all outflow velocities. The fraction of the galactic
continuum covered by the outflow at each velocity therefore dictates the shape
of these absorption troughs. We suggest that the velocity offset of the deepest
part of the troughs, where the covering factor of low-ionization gas is near
unity, reflects the speed of a shell of swept-up, interstellar gas at the time
of blowout. In a spherical outflow, we show that the fragments of this shell
expand slowly relative to the geometrical dilution; and the covering fraction
of low-ionization gas decreases with increasing radius. Our measurement of a
covering factor that decreases with increasing velocity can therefore be
interpreted as evidence that the low-ionization outflow is accelerating. We
also present measurements of C_f(v) in 4 species, place an upper limit of 3000
cm3 on the density of the outflowing gas, and discuss lower limits on the mass
outflow rate.
