Title: The Cycle of Gaseous Baryons Between the Disk and Halo
Description: The gaseous halo surrounding a galaxy disk is often referred to as the circum-galactic medium (CGM). The boundary of the CGM is loosely defined as the virial radius of the galaxy. Recent observations and simulations have shown that the CGM is massive, multiphase, clumpy, and metal-enriched. The CGM plays an important role in galaxy formation and evolution – it serves as a massive baryonic reservoir, from which the disk accretes gas fuel to sustain the star-formation activities, and to which the disk deposits feedback material. This dissertation focuses on the gas distribution in the CGM of the Milky Way (MW) and the baryon cycle between the CGM and disk of our neighbor – the Triangulum Galaxy (M33). Observations of the MW’s CGM are unavoidably contaminated by foreground gas since we reside in the MW’s disk. Conventionally, a velocity cut at |VLSR|~100 km/s is used as a proxy for distance, with low-velocity (|VLSR|<100 km/s) gas being more nearby to the Galactic disk than high-velocity gas.

Using both a MW-mass simulation and all-sky QSO observations, I show that the low-velocity gas in the MW’s CGM is as massive as their high-velocity counterpart, and that the MW most likely hosts a massive CGM reservoir as those L~L* galaxies at z~0.2. I further study how baryons are cycled between the disk and CGM by observing gas accretion in M33. Using HST/COS to observe seven UV-bright stars in M33’s disk, I find that there is a layer of metal-enriched inflow moving towards M33 at a rate of dM/dt=2.9 Msun/yr. The gas inflow may be related to galactic fountain process or debris falling back down due to the potential past M31-M33 interaction. This work is among the first to unambiguously reveal the existence of a disk-wide, ionized galactic inflow beyond the Milky Way. In addition, with the same set of HST/COS sightlines, I make a serendipitous discovery of an ionized very-high-velocity cloud towards M33.