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Abstract The circumgalactic medium (CGM) is a reservoir of metals and star-forming fuel. Most baryons in the Universe are in the CGM or the intergalactic medium (IGM). The baryon cycle—how mass and metals reach the CGM from the inner regions of the galaxy and how gas from the CGM replenishes star-forming activity in the inner regions—is an essential question in galaxy evolution. In this paper, we study the flow of mass and metals in a stacked sample of 2770 isolated halos from the IllustrisTNG100 cosmological hydrodynamic simulation. The mean gas flow as a function of radius and angle is similar across a large galactic mass range when accounting for different feedback modes. Although both star formation and black holes cause powerful outflows, the flows from star formation are more angularly restricted. Black hole feedback dominates mass flow throughout the halo, while star formation feedback mainly affects the inner region. When scaling by virial radius (R_v), large dynamical changes occur at 0.2R_vfor most halos, suggesting a characteristic size for the inner galaxy. Despite kinetic-mode feedback from black holes being the primary quenching mechanism in IllustrisTNG, a small population of high-mass kinetic-mode disks are able to form stars. 

The circumgalactic medium (CGM) is a known reservoir of metals and star forming fuel. Most baryons in the universe are in the CGM or ICM. The baryon cycle- how metals reach the CGM from the inner regions of the galaxy and how gas from the CGM replenishes star forming activity in the inner regions- is an essential question in galaxy evolution. We seek to illuminate these processes by analyzing 2770 isolated halos in the IllustrisTNG simulation. This sample is divided into different classes of galaxy based star forming and AGN feedback, and morphology. By stacking halos of similar mass and history, we can identify correlations between galaxy history and the properties and dynamics of the surrounding gas. 

The structure and properties of the circumgalactic medium (CGM) between R ~30–100 kpc around nearby, massive spiral galaxies remain largely unknown. One hypothesis is that large quantities of gas are held in rotationally-supported disks of neutral hydrogen (HI) that extend out to ~100 kpc. While observations of individual galaxies have detected HI out to distances of 80 kpc, a larger sample is necessary to determine the frequency and characteristics of extended HI disks. Using the Green Bank Telescope (GBT) we conducted a comprehensive survey mapping HI along the major and minor axes of 20 mass-selected galaxies to distances of 100 kpc and a limiting column density of 2 x 1018 cm-2. We have determined the total extended HI mass and its distribution within each galaxy by fitting our data to HI distribution models. We have found rotationally-supported disks in ~50% of the sample that extend to distances between 40 and 100 kpc. 

The circumgalactic medium (CGM) is a critical repository for metals around galaxies, and serves as the meeting ground for galactic outflows and infalling material from the intergalactic medium. We study the net flow of metals through the CGM in the IllustrisTNG galaxy formation simulations, with a particular focus on the geometry and radial distribution of metal flows. Special care is taken to account for “fuzz” particles, which are often missed by traditional methods of selecting CGM particles in the Illustris simulations, but dominate the net outflow of gas around massive galaxies near the virial radius. 

The M81 galaxy group is surrounded by an HI debris field scattered by the tidal interactions of its galaxies, a situation that has obvious similarities to the Magellanic stream and illuminates the formation of in-situ stars in stellar halos during galaxy collisions. Using observations of stars across the M81 group from the Subaru Hyper Suprime-Cam, and observations of the neutral HI from the Very Large Array, we find that within this HI debris the density of young stars broadly correlates with the density of gas, as expected given the Schmidt-Kennicutt star formation law and the results of previous work. Yet, there are regions that have systematically different behaviors in distributions of stars and gas. We focus on two stretches of HI coming off NGC 3077: the Southern tidal bridge (between M81 and NGC 3077) and the Northern tidal bridge (from NGC 3077 in the direction of M82). The Southern bridge has a narrow strip of young stars down its center, and the Northern bridge is mostly devoid of stars. While the driver(s) for this kind of behavior remain uncertain, our analysis of star formation in galaxy group-scale mergers from the TNG50 hydrodynamical galaxy simulations shows that the differences between projected line-of-sight distances of the gas may be an important consideration. 

IllustrisTNG is a widely used suite of hydrodynamical simulations, but we found that most users are likely missing up to 5% of the gas mass within the virial radius of halos generated by TNG. An FoF (“friend of friends”) algorithm determines what gas in the simulation is assigned to each halo. Approximately one third of gas cells are not in any halo: this gas is called “fuzz”. However, at certain densities, temperatures, and halo-centric distances this decision becomes (perhaps necessarily) arbitrary for individual gas cells. We use a method of loading gas data that avoids this issue, and instead loads all gas in a given volume of the simulation. Preliminary findings suggest the FoF algorithm functions as a permeable, stochastic density threshold which preferentially misses low density gas. At the same time, the fuzz does not match gas of similar densities in the same region. In this poster, we compare fuzz gas and halo gas at similar radii and densities to explore the implications of this omission.