Directly constraining the spatial coherence of the z ∼ 1 circumgalactic medium

One of the biggest puzzles regarding the circumgalactic medium (CGM) is the structure of its cool (T ∼ 104 K) gas phase. While the kinematics of quasar absorption systems suggests the CGM is composed of a population of different clouds, constraining their extent and spatial distribution has proven challenging, both from theoretical and observational points of view. In this work, we study the spatial structure of the z ∼ 1 CGM with unprecedented detail via resolved spectroscopy of giant gravitational arcs. We put together a sample of Mg IIλλ2796, 2803 detections obtained with VLT/MUSE in 91 spatially independent and contiguous sight lines toward 3 arcs, each probing an isolated star-forming galaxy believed to be detected in absorption. We constrain the coherence scale of this gas (Clength) - which represents the spatial scale over which the Mg II equivalent width (EW) remains constant - by comparing EW variations measured across all sight lines with empirical models. We find 1.4 < Clength/kpc < 7.8 (95% confidence). This measurement, of unprecedented accuracy, represents the scale over which the cool gas tends to cluster in separate structures. We argue that, if Clength is a universal property of the CGM, it needs to be reproduced by current and future theoretical models in order for us to understand the exact role of this medium in galaxy evolution....