The Ionization and Dynamics of the Makani Galactic Wind

Abstract

The Makani galaxy hosts the poster child of a galactic wind on scales of the circumgalactic medium. It consists of a two-episode wind in which the slow, outer wind originated 400 Myr ago (Episode I; R _I = 20 − 50 kpc) and the fast, inner wind is 7 Myr old (Episode II; R _II = 0 − 20 kpc). While this wind contains ionized, neutral, and molecular gas, the physical state and mass of the most extended phase—the warm, ionized gas—are unknown. Here we present Keck optical spectra of the Makani outflow. These allow us to detect hydrogen lines out to r = 30–40 kpc and thus constrain the mass, momentum, and energy in the wind. Many collisionally excited lines are detected throughout the wind, and their line ratios are consistent with 200–400 km s ^−1 shocks that power the ionized gas, with v _shock = σ _wind . Combining shock models, density-sensitive line ratios, and mass and velocity measurements, we estimate that the ionized mass and outflow rate in the Episode II wind could be as high as those of the molecular gas: ${M}_{\mathrm{II}}^{{\rm{H}}{\rm\small{II}}}\sim {M}_{\mathrm{II}}^{{{\rm{H}}}_{2}}=(1-2)\times {10}^{9}\,{M}_{\odot }$ and ${dM}/{{dt}}_{\mathrm{II}}^{{\rm{H}}{\rm\small{II}}}\sim {dM}/{{dt}}_{\mathrm{II}}^{{{\rm{H}}}_{2}}=170-250\,{M}_{\odot }$ yr ^−1 . The outer wind has slowed, so that ${dM}/{{dt}}_{{\rm{I}}}^{{\rm{H}}{\rm\small{II}}}\sim 10\,{M}_{\odot }$ yr ^−1 , but it contains more ionized gas, ${M}_{{\rm{I}}}^{{\rm{H}}{\rm\small{II}}}=5\times {10}^{9}$ M _⊙ . The momentum and energy in the recent Episode II wind imply a momentum-driven flow ( p “boost” ∼7) driven by the hot ejecta and radiation pressure from the Eddington-limited, compact starburst. Much of the energy and momentum in the older Episode I wind may reside in a hotter phase, or lie further into the circumgalactic medium.

Keywords

• Galactic winds
• Stellar feedback
• Starburst galaxies
• Circumgalactic medium
• Shocks