Scaling Relations Between Warm Galactic Outflows and Their Host Galaxies

Abstract: We report on a sample of 51 nearby, star-forming galaxies observed with the Cosmic Origin Spectrograph on the Hubble Space Telescope. We calculate Si II kinematics and densities arising from warm gas entrained in galactic outflows. We use multi-wavelength ancillary data to estimate stellar masses (M$\ast$), star-formation rates (SFR), and morphologies. We derive significant correlations between outflow velocity and SFR$^{\sim 0.1}$, M$\ast^{\sim 0.1}$ and v$\text{circ}^{\sim 1/2}$. Some mergers drive outflows faster than these relations prescribe, launching the outflow faster than the escape velocity. Calculations of the mass outflow rate reveal strong scaling with SFR$^{\sim 1/2}$ and M$\ast^{\sim 1/2}$. Additionally, mass-loading efficiency factors (mass outflow rate divided by SFR) scale approximately as M$\ast^{-1/2}$. Both the outflow velocity and mass-loading scaling suggest that these outflows are powered by supernovae, with only 0.7% of the total supernovae energy converted into the kinetic energy of the warm outflow. Galaxies lose some gas if log(M$\ast$/M$_\odot$) < $9.5$, while more massive galaxies retain all of their gas, unless they undergo a merger. This threshold for gas loss can explain the observed shape of the mass-metallicity relation.