As for many other aspects of galaxy evolution, the Milky Way and its gaseous environment represent an excellent laboratory to study the details of gas-accretion processes of L⋆ galaxies in the local Universe. Although it remains a challenging task to reconstruct the 3D distribution and galactocentric kinematics of the Milky Way's circumgalactic medium from an internal vantage point in the rotating disk, the combination of multi-wavelength observations of the gas in all its phases, the measurement and modeling of the stellar composition of the Milky Way disk and its star-formation rate and history, the numerical modeling of the hydrodynamic processes that shape the properties of the Galaxy's CGM, and the deep observations of the Milky Way and its satellite galaxies in a cosmological context together provide a particularly rich database that cannot be achieved for any other galaxy in the Universe.
The above discussed observations and simulations imply that the combination of gas infall, outflows, and mergers generates a multi-phase gaseous halo that is characterized by a highly complex spatial distribution of gas structures of different age and origin. The cycle of processes that is believed to lead to the continuous feeding of the Milky Way disk with fresh gas to supplement subsequent star-formation therein can be summarized as follows.
Cold and warm gas from the intergalactic medium and from satellite galaxies enters the Milky Way halo at its virial radius and is then processed by the ambient hot coronal gas. Fragments of the originally infalling gas may reach the disk in the form of warm or cold gas streams, while the remaining gas fraction is being incorporated into the hot Galactic Corona. In this way, the Corona is continuously fed with fresh gas from outside, while it is further stirred up and heated by gas outflowing from the star-forming disk and (eventually) from the Galactic center region (i.e., it is influenced by feedback processes). From the hot Corona, warm ionized and/or warm neutral gas patches may condense out through cooling processes, and these structures will sink down to the disk through the disk-halo interface, further contributing to the overall accretion rate of the Milky Way disk.
The accretion of cold and warm gas from the Magellanic Stream is a direct result of the interaction between the Milky Way and its population of satellite galaxies (here: the Magellanic Clouds). The MS adds more than 1 billion solar masses of gaseous material to the Milky Way halo, material that either directly or indirectly feeds the MW disk to supplement star formation therein. Thus, there is sufficient cold and warm gaseous material present in the outer halo to maintain the Milky Way's star-formation rate in the far future at its current level, although it remains unclear, how much of the material from the MS will finally end up in the disk (and at what time scale). The amount of gas that is currently being accreted through the disk-halo interface, and that will determine the star-formation rate in the near future, remains uncertain, however.
Our understanding of the gas-accretion processes in the Milky Way is far from being complete. On the observational side, additional constraints on distances and 3D velocities of the HVCs, the role of low-velocity halo gas, and the mass and spatial extent of the hot coronal gas based on multi-wavelength observations are highly desired on the long way towards a complete census of the Milky Way's circumgalactic gas. On the theoretical side, more advanced hydrodynamical simulations of the Milky Way's gaseous halo, that include all relevant physical processes in a realistic cosmological environment, will be of great importance to study the dynamics of gas flows around the Galaxy. Finally, a systematic comparison between gas-accretion processes in the Milky Way and similar processes in other low- and high-redshift galaxies in the general context of galaxy evolution (with particular focus on environmental issues, feedback effects, and other important aspects) will provide crucial information that will help to better understand the cycling of gas on galactic and super-galactic scales. Many of these aspects will also be discussed in the following chapters.
Acknowledgements The author would like to thank Andy Fox, Matt Haffner, Fabian Heitsch, Jürgen Kerp, & Sebastián Nuza for providing helpful comments and supplementary material for the figures.