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We assess the fraction of the Milky Way's circumgalactic medium (CGM) eluding detection due to its velocity being similar to gas in the disk. This is achieved using synthetic observations of the CGM in a simulated MW-mass galaxy that shows similar CGM kinematics to the MW and external L$\sim$L$_*$ galaxies. As viewed by a mock observer at a location similar to the Sun, only 50$\%$ (by mass) of the gas moves at high velocity ($|v_{\rm LSR}|\geq$100 km s$^{-1}$ or $|v_{\rm DEV}|\geq$50 km s$^{-1}$) in the simulated CGM and would be observable. The low velocity gas is thermodynamically similar to the high velocity gas, indicating the 50$\%$ observable fraction is applicable to spectral lines from the radio to the ultraviolet. We apply the observable mass fraction (50$\%$) to current estimates of the MW's CGM, and find a corrected total mass of 2.8$\times$10$^{8} M_{\odot}$ for gas below 10$^6$K within $\sim15$ kpc (this excludes the Magellanic System). This is less than the total mass of the CGM extending out to $\sim$150 kpc in other L$\sim$L$_*$ galaxies. However, we find similar OVI column densities when the discrepancy in path length between the MW and external galaxies is considered. The coherent spatial and kinematic distribution of low velocity gas in the simulated CGM suggests that current HI observations of the MW's CGM may miss large low velocity HI complexes. In addition, current mass estimates of the MW's CGM based on high-velocity line observations with distance constraints may miss a non-negligible fraction of gas in the outer halo which can be obscured if it moves at a velocity similar to the gas in the lower halo., Comment: 11 pages, 9 figures, 3 tables, accepted for publication in ApJ

Galactic halo gas traces inflowing star formation fuel and feedback from a galaxy's disk and is therefore crucial to our understanding of galaxy evolution. In this review, we summarize the multi-wavelength observational properties and origin models of Galactic and low redshift spiral galaxy halo gas. Galactic halos contain multiphase gas flows that are dominated in mass by the ionized component and extend to large radii. The densest, coldest halo gas observed in neutral hydrogen (HI) is generally closest to the disk (< 20 kpc), and absorption line results indicate warm and warm-hot diffuse halo gas is present throughout a galaxy's halo. The hot halo gas detected is not a significant fraction of a galaxy's baryons. The disk-halo interface is where the multiphase flows are integrated into the star forming disk, and there is evidence for both feedback and fueling at this interface from the temperature and kinematic gradient of the gas and HI structures. The origin and fate of halo gas is considered in the context of cosmological and idealized local simulations. Accretion along cosmic filaments occurs in both a hot (> 10^5.5 K) and cold mode in simulations, with the compressed material close to the disk the coldest and densest, in agreement with observations. There is evidence in halo gas observations for radiative and mechanical feedback mechanisms, including escaping photons from the disk, supernova-driven winds, and a galactic fountain. Satellite accretion also leaves behind abundant halo gas. This satellite gas interacts with the existing halo medium, and much of this gas will become part of the diffuse halo before it can reach the disk. The accretion rate from cold and warm halo gas is generally below a galaxy disk's star formation rate, but gas at the disk-halo interface and stellar feedback may be important additional fuel sources., Comment: 50 pages, 9 figures (1 in 3D, view with a current version of Adobe), to appear in ARA&A, 50, 491

We examine galaxy formation in a cosmological AMR simulation, which includes two high resolution boxes, one centered on a 3 \times 10^14 M\odot cluster, and one centered on a void. We examine the evolution of 611 massive (M\ast > 10^10M\odot) galaxies. We find that the fraction of the final stellar mass which is accreted from other galaxies is between 15 and 40% and increases with stellar mass. The accreted fraction does not depend strongly on environment at a given stellar mass, but the galaxies in groups and cluster environments are older and underwent mergers earlier than galaxies in lower density environments. On average, the accreted stars are ~2.5 Gyrs older, and ~0.15 dex more metal poor than the stars formed in-situ. Accreted stellar material typically lies on the outskirts of galaxies; the average half-light radius of the accreted stars is 2.6 times larger than that of the in-situ stars. This leads to radial gradients in age and metallicity for massive galaxies, in qualitative agreement with observations. Massive galaxies grow by mergers at a rate of approximately 2.6% per Gyr. These mergers have a median (mass-weighted) mass ratio less than 0.26 \pm 0.21, with an absolute lower limit of 0.20, for galaxies with M\ast ~ 10^12 M\odot. This suggests that major mergers do not dominate in the accretion history of massive galaxies. All of these results agree qualitatively with results from SPH simulations by Oser et al. (2010, 2012)., Comment: 18 pages, 12 figures, submitted to MNRAS