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 per cent 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-*) 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-*(-0.4) and with the circular velocity as v(circ)(-1.6). The scaling of the mass-loading factor is similar to recent simulations, but the observations are a factor of 5 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.