The contribution of halo red giant mass loss to the high-velocity gas falling onto the Milky Way disk

Abstract

The origin of gas falling from the halo toward the disk of the Milky Way is still largely unclear. Here the amount of gas shed by the (older) halo red giants is estimated. The distribution of red giants (RGs) in the halo is not known but that of a subset of stars in the post RG phase, the sdB stars of the horizontal-branch (HB), is. Using the mid-plane density and $z$-distribution of sdB stars, the ratio of sdB stars to all HB stars, and the RG mass loss, the infall due to total mass lost by all halo RG stars at $z>1$ kpc is calculated. For the extended halo component $\dot{M}_{\rm halo RGs} \simeq 1.4 \cdot 10^{-5}$ \msun kpc$^{-2}$ yr$^{-1}$ while the thick disk component RGs contribute $\dot{M}_{\rm thick disk RGs} \simeq 5.4 \cdot 10^{-5}$ \msun kpc$^{-2}$ yr$^{-1}$, each with an uncertainty of a factor 4. The total rate of infall due to RG mass-loss is $\dot{M}_{{\rm RGs at\}z>1 {\rm kpc}} \simeq 7 \cdot 10^{-5}$ \msun kpc$^{-2}$ yr$^{-1}$, a sizeable fraction of the equally uncertain observed rate of infall of material. Since most of the RG stars in the extended halo are old, their mass loss is predominantly metal-poor, while that of the disk RGs is more metal-rich. The galactic fountain flow provides additional metal-rich infall and small galaxies being accreted contribute to the infall of gas as well.