Kinematic Analysis of an Ultrastrong Mg II Absorber at z ≈ 1.13 Linked to Circumgalactic Gas Structures

We present a spectroscopic and imaging analysis of the z _gal ≈ 1.1334 ultrastrong Mg ii absorption system identified in the Very Large Telescope/UVES spectrum of a background quasar located at ρ ≈ 18 kpc from a star-forming galaxy. Low-ionization metal lines like Mg i, Fe ii, and Ca ii are also detected for this absorber. The H I lines are outside of the wavelength coverage. The Mg ii has a rest-frame equivalent width of W_r (2796) = 3.185 ± 0.032 Å, with the absorption spread across Δv ≈ 460 km s⁻¹ in several components. A component-by-component ionization modeling shows several of these components having solar and higher metallicities. The models also predict a total h i column density of log [ N (H I) / cm⁻²] ≈ 22.5, consistent with ultrastrong Mg ii absorbers being sub–damped Lyα and damped Lyα systems. The absorber is well within the virial radius of the nearest galaxy, which has a stellar mass M _* = 4.7 × 10¹⁰ M _⊙, and a star formation rate of ≈8.3 M _⊙ yr⁻¹. The absorption is along the projected major axis of the galaxy with a velocity spread that is wider than the galaxy’s disk rotation. From the kinematic analysis of the absorber and the galaxy, the origin of the absorption can be attributed to a combination of circumgalactic gas structures, some corotating with the disk and the rest moving at line-of-sight velocities outside of the disk rotation.