The Mass and Momentum Outflow Rates of Photoionized Galactic Outflows
  Galactic outflows are believed to play an important role in regulating star
formation in galaxies, but estimates of the outflowing mass and momentum have
historically been based on uncertain assumptions. Here, we measure the mass,
momentum, and energy outflow rates of seven nearby star-forming galaxies using
ultraviolet absorption lines and observationally motivated estimates for the
density, metallicity, and radius of the outflow. Low-mass galaxies generate
outflows faster than their escape velocities with mass outflow rates up to
twenty times larger than their star formation rates. These outflows from
low-mass galaxies also have momenta larger than provided from supernovae alone,
indicating that multiple momentum sources drive these outflows. Only 1-20\% of
the supernovae energy is converted into kinetic energy, and this fraction
decreases with increasing stellar mass such that low-mass galaxies drive more
efficient outflows. We find scaling relations between the outflows and the
stellar mass of their host galaxies (M$_\ast$) at the 2-3$\sigma$ significance
level. The mass-loading factor, or the mass outflow rate divided by the star
formation rate, scales as M$_\ast^{-0.4}$ and with the circular velocity as
v$_\mathrm{circ}^{-1.6}$. The scaling of the mass-loading factor is similar to
recent simulations, but the observations are a factor of five smaller, possibly
indicating that there is a substantial amount of unprobed gas in a different
ionization phase. The outflow momenta are consistent with a model where star
formation drives the outflow while gravity counteracts this acceleration.
