Investigating the circumgalactic medium through Mg  <scp>ii</scp> absorption coincidence

ABSTRACT We present a statistical measurement of the transverse coherence of Mg ii $\lambda \lambda 2796,2803$ absorption using a large sample of 9204 absorber-centric quasar sightline pairs from the Sloan Digital Sky Survey. We quantify the probability that an Mg ii absorber detected along one sightline is also present along a nearby sightline, and measure how this coincidence probability varies with projected separation from $\sim$50 kpc to $\sim$1 Mpc. The resulting coincidence curve exhibits a clear two-regime structure: the coincidence probability rises steeply to $\sim$5–8 per cent at separations below $\sim$100 kpc, but declines rapidly beyond this scale and settles into a low plateau of $\sim$1–2 per cent out to $\sim$1 Mpc. A simple geometrical single-halo model reproduces the enhanced probability at $\lesssim$100 kpc, while the large-scale plateau is well explained by the expected contribution from galaxy clustering, confirmed using both photometric galaxy counts and the two-point correlation function. A complementary stacking analysis reveals a significant excess in Mg ii equivalent width in paired sightlines lacking individual detections, implying a coherence scale of $\sim$100–200 kpc for the cool, metal-enriched CGM. Together, these results identify the transition from a halo-dominated coherence regime at small separations to a clustering-dominated regime at large scales, bridging the gap between small-scale lensing constraints and megaparsec-scale absorber clustering studies.