Project AMIGA: The Circumgalactic Medium of Andromeda Based on observations made with the NASA/ESA Hubble Space Telescope, obtained from the data archive at the Space Telescope Science Institute. STScI is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS 5-26555

Project AMIGA (Absorption Maps In the Gas of Andromeda) is a survey of the circumgalactic medium (CGM) of Andromeda (M31, 300 kpc) along 43 QSO sightlines at impact parameters 25 ≤ R ≤ 569 kpc (25 at R ). We use ultraviolet absorption measurements of Si ii, Si iii, Si iv, C ii, and C iv from the Hubble Space Telescope/Cosmic Origins Spectrograph and O vi from the Far Ultraviolet Spectroscopic Explorer to provide an unparalleled look at how the physical conditions and metals are distributed in the CGM of M31. We find that Si iii and O vi have a covering factor near unity for R 1.2 and 1.9 , respectively, demonstrating that M31 has a very extended ∼104-105.5 K ionized CGM. The metal and baryon masses of the 104-105.5 K CGM gas within are 108 and 4 × 1010 (Z/0.3 Z )−1 M , respectively. There is not much azimuthal variation in the column densities or kinematics, but there is with R. The CGM gas at R 0.5 is more dynamic and has more complicated, multiphase structures than at larger radii, perhaps a result of more direct impact of galactic feedback in the inner regions of the CGM. Several absorbers are projected spatially and kinematically close to M31 dwarf satellites, but we show that those are unlikely to give rise to the observed absorption. Cosmological zoom simulations of ∼L* galaxies have O vi extending well beyond as observed for M31 but do not reproduce well the radial column density profiles of the lower ions. However, some similar trends are also observed, such as the lower ions showing a larger dispersion in column density and stronger dependence on R than higher ions. Based on our findings, it is likely that the Milky Way has a ∼104-105.5 K CGM as extended as for M31 and their CGM (especially the warm-hot gas probed by O vi) are overlapping.