On the Cooling Flow Problem in the Gaseous Halo of the Milky Way
  Theoretical and observational arguments suggest that there is a large amount
of hot ($\sim 10^6$ K), diffuse gas residing in the Milky Way's halo, while its
total mass and spatial distribution are still unclear. In this work, we present
a general model for the gas density distribution in the Galactic halo, and
investigate the gas evolution under radiative cooling with a series of 2D
hydrodynamic simulations. We find that the mass inflow rate in the developed
cooling flow increases with gas metallicity and the total gas mass in the halo.
For a fixed halo gas mass, the spatial gas distribution affects the onset time
of the cooling catastrophe, which starts earlier when the gas distribution is
more centrally-peaked, but does not substantially affect the final mass inflow
rate. The gravity from the Galactic bulge and disk affects gas properties in
inner regions, but has little effect on the final inflow rate either. We
confirm our results by investigating cooling flows in several density models
adopted from the literature, including the Navarro-Frenk-White (NFW) model, the
cored-NFW model, the Maller & Bullock model, and the $\beta$ model. Typical
mass inflow rates in our simulations range from $\sim 5 M_{\odot}$ yr$^{-1}$ to
$\sim 60 M_{\odot}$ yr$^{-1}$, and are much higher than the observed star
formation rate in our Galaxy, suggesting that stellar and active galactic
nucleus feedback processes may play important roles in the evolution of the
Milky Way (MW) and MW-type galaxies.
