Project AMIGA: The Inner Circumgalactic Medium of Andromeda from Thick Disk to Halo*

Abstract The inner circumgalactic medium (CGM) of galaxies, where disk and halo processes intersect, remains poorly characterized despite its critical role in regulating galaxy evolution. We present results from Project AMIGA Insider, mapping Andromeda’s (M31) inner CGM within 0.25 R vir (∼75 kpc) using 11 QSO sightlines, bringing our total sample to 54 sightlines from the disk to 2 R vir . We detect a clear transition between M31’s thick disk and CGM at R ≲ 30 kpc, where low/intermediate ions show thick-disk corotating components with higher column densities than the CGM ones, while high ions exhibit similar column densities in both the CGM and thick disk. Beyond this region, all ion column densities decrease with impact parameter, with steeper gradients for low ions than high ions. The inner CGM ( R ≲ 100 kpc) shows more complex gas phases and multicomponent absorption compared to the predominantly single-component outer CGM. We find no significant azimuthal dependence for any observed ions, suggesting M31’s CGM is shaped by radial processes (e.g., cooling flows, precipitation) rather than disk-aligned outflows. We estimate the total metal mass in M31’s cool (Si ii , Si iii , Si iv ) CGM within R vir to be (1.9 ± 0. 3 stat ± 0. 7 sys ) × 10 7 M ⊙ , leading to a cool gas mass of ≈ 6 × 1 0 9 ( Z / 0.3 Z ⊙ ) − 1 M ⊙ . The warmer O vi gas may contain at least 10 times more metal and gas mass. Compared to the COS-Halos L *galaxies, M31’s cool CGM shows lower Si column densities at R ≲ 0.4 R 200 and overall lower cool CGM masses, likely reflecting differences in galaxy mass and environment.