The Mass Inflow and Outflow Rates of the Milky Way

Abstract

We present new calculations of the mass inflow and outflow rates around the Milky Way (MW), derived from a catalog of ultraviolet metal-line high velocity clouds (HVCs). These calculations are conducted by transforming the HVC velocities into the Galactic Standard of Rest (GSR) reference frame, identifying inflowing (v GSR < 0 km s −1 ) and outflowing (v GSR > 0 km s −1 ) populations, and using observational constraints on the distance, metallicity, dust content, covering fractions, and total silicon column density of each population. After removing HVCs associated with the Magellanic Stream and the Fermi Bubbles, we find inflow and outflow rates in cool (T ∼ 10 4 K) ionized gas of dM in /dt (0.53± 0.23) (d/12 kpc)(Z/0.2Z ) −1 M yr −1 and dM out /dt (0.16 ± 0.07) (d/12 kpc)(Z/0.5Z ) −1 M yr −1 . The apparent excess of inflowing over outflowing gas suggests that the MW is currently in an inflowdominated phase, but the presence of substantial mass flux in both directions supports a Galactic fountain model, in which gas is constantly recycled between the disk and the halo. We also find that the metal flux in both directions (in and out) is indistinguishable. By comparing the outflow rate to the Galactic star formation rate, we present the first estimate of the mass loading factor (η HVC ) of the disk-wide MW wind, finding η HVC (0.10±0.06) (d/12 kpc)(Z/0.5Z ) −1 . Including the contributions from low-and intermediate-velocity clouds and from hot gas would increase these inflow and outflow estimates.

Cool outflows in galaxies and their implications

“…Overall, most of the neutral and molecular gas entrained in the outflow will not escape the Milky Way. Instead, this material will participate in a large-scale galactic fountain (Bregman 1980;Armillotta et al 2019), feeding the Milky Way CGM (Bland-Hawthorn and Gerhard 2016;Hodges-Kluck et al 2016b;Zheng et al 2019;Werk et al 2019;Bish et al 2019;Fox et al 2019), before falling back onto the outer disk of the Galaxy, and providing new fuel for star formation. This may be a common occurrence in quenched galaxies, but the very subtle signatures of the cool, warm, hot, and relativistic fluids in this tenuous outflow will generally be challenging to detect in external galaxies.…”

The Impact of Cosmic Rays on Thermal Instability in the Circumgalactic Medium

“…The galactic fountain, through which gas is expelled from and accreted onto the galactic disk, is an integral component of galaxy evolution. The mass accretion of cold gas in the Milky Way is measured to be somewhere between 0.0002 and 0.006 M e kpc −2 yr −1 (Fox et al 2019;Werk et al 2019).…”

Large Metallicity Variations in the Galactic Interstellar Medium

“…The rate of gas accretion on the Galaxy disk currently measured (0.1-1.4 M /yr, [26,27]) is a factor of ∼ 100 larger than the minimum required to sustain the formation and survival of chemical inhomogeneities, as we estimate in the Methods. Therefore not only could pockets of low-metallicity/pristine gas exist, but they could in fact be very common.…”