Project AMIGA: Extent and Distribution of the Circumgalactic Medium of Andromeda

UV absorption-line studies of single sight lines through an ensemble of galaxy halos have shown that the circumgalactic medium (CGM) plays a major role in galaxy evolution. However, these observations fail to capture the radial-azimuthal dependence of the CGM properties, which has significant diagnostic power, especially in the context of recent zoom-cosmological simulations. Here we present the first scientific results stemming from Project AMIGA, a large UV Hubble Space Telescope program designed to determine how baryons and metals are distributed in the CGM of the Andromeda (M31) galaxy. With 44 QSOs piercing the CGM from 25 to 527 kpc and 25 of them from 25 kpc to about the virial radius (300 kpc) of M31, this is the largest survey of a single galaxy beyond the Milky Way. We find high covering factors of metal ions (e.g., CII, SiII, SiIII, SiIV, CIV) that vary with the projected distance depending on their ionization stages. We show different ions trace different components of the CGM and have remarkable and different trends as a function of the projected distance from M31. We finally discuss the extent and mass of the CGM of M31 and how they compare with surveys of higher redshift galaxies using single QSOs.