Evidence for Azimuthally Anisotropic MgII Absorption around DESI Luminous Red Galaxies

We use DESI DR1 luminous red galaxies (LRGs) and background quasar spectra to measure the mean MgII equivalent-width field around massive quiescent galaxies as a function of projected radius and azimuth relative to the projected major axis. Our forced-measurement approach assigns a MgII doublet-window EW to every LRG-quasar pair, including spectra without individually detected absorbers, and subtracts a redshift-matched random-control signal measured from the same normalized quasar spectra. The all-angle profile declines smoothly over $r_\text{p}=0.01$-$1.0$ proper Mpc and shows stronger inner absorption at $0.4<z_{\rm LRG}<0.75$ than at $0.75<z_{\rm LRG}<1.1$. Superposed on this radial and redshift dependence, sightlines near the LRG major axis show enhanced absorption relative to the minor axis at $r_\text{p}\simeq30$-$80$ kpc. For the full $0.4<z_{\rm LRG}<1.1$ sample, the integrated fiducial major-minus-minor difference over $r_\text{p}=0.03$-$0.077$ Mpc is $0.184\pm0.075$ Angstrom, with a position-angle randomization probability $p=0.013$. The redshift split shows no significant evolution in the anisotropy amplitude, even though the inner EW profile itself evolves. This DESI DR1 measurement provides evidence for a localized major-axis enhancement of MgII-bearing cool/warm gas around LRGs, showing that azimuthal CGM structure is measurable in massive quiescent halos as well as in star-forming systems.