Galaxies Probing Galaxies at High Resolution: Co-Rotating Gas Associated with a Milky Way Analog at z=0.4

TL;DR

This study uses high-resolution spectroscopy of a background galaxy to analyze the circumgalactic gas of a Milky Way-like galaxy at z=0.413, revealing co-rotating gas, warped disks, and halo structures, providing new insights into galaxy halo gas dynamics.

Contribution

First high-resolution spectroscopic analysis of circumgalactic gas towards an extended background galaxy, revealing detailed gas kinematics and structures at z=0.4.

Findings

Detection of co-rotating Mg II and Fe II absorption at 27 kpc impact parameter.

Evidence of a warped gaseous disk aligned with the galaxy rotation.

Identification of halo gas clouds with small velocity dispersions and potential tidal features.

Abstract

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 towards an extended background source (i.e., a galaxy rather than a quasar). Using longslit spectroscopy of the foreground galaxy, we observe spatially extended H alpha emission with circular rotation velocity v=270 km/s. Using echelle spectroscopy of the background galaxy, we detect Mg II and Fe II absorption lines at 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 A has an estimated column density (N_H>10^19 cm-2) and line-of-sight velocity dispersion (sigma=17 km/s) that are consistent with the observed properties of extended H I disks in the local universe. Our analysis of the rotation curve also suggests that this r=30 kpc gaseous disk is warped with respect to the stellar disk. In addition, we detect two weak Mg II absorbers in the halo with small velocity dispersions (sigma<10 km/s). 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.