Galaxies probing galaxies at high resolution: co-rotating gas associated with a milky way analog at z = 0.4

We present results on gas flows in the halo of a Milky-Way-like galaxy at z = 0.413 based on high-resolution spectroscopy of a background galaxy. This is the first study of circumgalactic gas at high spectral resolution toward an extended background source (i.e., a galaxy rather than a quasar). Using long-slit spectroscopy of the foreground galaxy, we observe spatially extended Hα emission with a circular rotation velocity $v_{circ}\approx 270$ km s⁻¹. Using echelle spectroscopy of the background galaxy, we detect $Mgii$ and $Feii$ absorption lines at an impact parameter $\rho =27$ kpc that are blueshifted from systemic in the sense of the foreground galaxy's rotation. The strongest absorber (${EW}_{2796}=0.90$ Å) has an estimated column density ($N_H⩾{10}^{19}$ cm⁻²) and line-of-sight velocity dispersion ($\sigma =17$ km s⁻¹) that are consistent with the observed properties of extended $Hi$ disks in the local universe. Our analysis of the rotation curve also suggests that this $r\approx 30kpc$ gaseous disk is warped with respect to the stellar disk. In addition, we detect two weak $Mgii$ absorbers in the halo with small velocity dispersions ($\sigma <10$ km s⁻¹). While the exact geometry is unclear, one component is consistent with an extraplanar gas cloud near the disk–halo interface that is co-rotating with the disk, and the other is consistent with a tidal feature similar to the Magellanic Stream. We can place lower limits on the cloud sizes ($l>0.4$ kpc) for these absorbers given the extended nature of the background source. We discuss the implications of these results for models of the geometry and kinematics of gas in the circumgalactic medium.