Kinematics of the Circumgalactic Medium of a z = 0.77 Galaxy from Mg ii Tomography

Galaxy evolution is thought to be driven in large part by the flow of gas between galaxies and the circumgalactic medium (CGM), a halo of metal-enriched gas extending out to ≳100 kpc from each galaxy. Studying the spatial structure of the CGM holds promise for understanding these gas flow mechanisms; however, the common method of using background quasar sight lines provides minimal spatial information. Recent works have shown the utility of extended background sources such as giant gravitationally lensed arcs. Using background lensed arcs from the CSWA 38 lens system, we continuously probed, at a resolution element of about 15 kpc², the spatial and kinematic distribution of Mg ii absorption in a star-forming galaxy at z = 0.77 (stellar mass ≈10^9.7 M _⊙, star formation rate ≈10 M _⊙ yr⁻¹) at impact parameters D ≃ 5–30 kpc. Our results present an anisotropic, optically thick medium whose absorption strength decreases with increasing impact parameter, in agreement with the statistics toward quasars and other gravitational arcs. Furthermore, we find generally low line-of-sight velocities in comparison to the relatively high velocity dispersion in the Mg ii gas (with typical σ ≈ 50 km s⁻¹). While the galaxy itself exhibits a clear outflow (with Mg ii velocities up to ∼500 km s⁻¹) in the down-the-barrel spectrum, the outflow component is subdominant and only weakly detected at larger impact parameters probed by the background arcs. Our results provide evidence of mainly dispersion-supported, metal-enriched gas recycling through the CGM.