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How Do Galaxies Get Their Gas?


The growth of galactic disks requires a continuous gas supply, but direct observations of gas accretion onto galaxies remain rare. We analyze the kinematics of the low-ionization-state gas in the circumgalactic medium (CGM) and study how the gas feeds the galactic disks. We have observed quasars behind z=0.2, star-forming galaxies, and the quasar sightlines intersect the CGM within 100 kpc of the foreground galaxies. We find that the Doppler shift of the circumgalactic gas absorption shares the same sign as the quasar side of the galactic disk, but the Doppler shifts are smaller than disk rotation predicts. Our results suggest that the low-ionization-state gas in the inner CGM corotates with the galactic disks, and centrifugal forces partially support the low-ionization-state gas. However, the circumgalactic absorption spans a broader velocity range than a thin rotating disk can explain, and we model the velocity range using a simple inflow model with gas spiraling inwards near the disk plane. We also use new, high-resolution imaging to reveal the wrapping direction of galaxy spiral arms, and we combine it with the galaxy rotation curve to deduce the orientation of the galactic disk in 3D-space. Using the 3D disk orientation as an additional parameter for the inflow model, we constrain the radial inflow speed or rule out a velocity component from radial inflow in the quasar sightline. We find inflow speeds comparable to that of the cold inflowing gas we have identified around galaxies in the EAGLE cosmological simulations, where most of the EAGLE galaxies have mass inflow rates high enough to sustain the star-forming activities of the galaxies. We also find cold rotating disks with inflowing gas in EAGLE, and these disks extend to radii comparable to the impact parameters of our quasar sightlines around galaxies. Hence, using both observations and simulations, we conclude that the inner CGM has significant angular momentum, and our analysis with EAGLE further supports our inflow interpretation for the circumgalactic kinematic observations.

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