Project AMIGA: The Circumgalactic Medium of Andromeda
  Project AMIGA (Absorption Maps In the Gas of Andromeda) is a large
ultraviolet Hubble Space Telescope program, which has assembled a sample of 43
QSOs that pierce the circumgalactic medium (CGM) of Andromeda (M31) from R=25
to 569 kpc (25 of them probing gas from 25 kpc to about the virial radius-Rvir
= 300 kpc-of M31). Our large sample provides an unparalleled look at the
physical conditions and distribution of metals in the CGM of a single galaxy
using ions that probe a wide range of gas phases (Si II, Si III, Si IV, C II, C
IV, and O VI, the latter being from the Far Ultraviolet Spectroscopic
Explorer). We find that Si III and O VI have near unity covering factor
maintained all the way out to 1.2Rvir and 1.9Rvir, respectively. We show that
Si III is the dominant ion over Si II and Si IV at any R. While we do not find
that the properties of the CGM of M31 depend strongly on the azimuth, we show
that they change remarkably around 0.3-0.5Rvir, conveying that the inner
regions of the CGM of M31 are more dynamic and have more complicated
multi-phase gas-structures than at R>0.5Rvir. We estimate the metal mass of the
CGM within Rvir as probed by Si II, Si III, and Si IV is 2x10^7 Msun and by O
VI is >8x10^7 Msun, while the baryon mass of the 10^4-10^5.5 K gas is ~4x10^10
(Z/0.3 Zsun)^(-1) Msun within Rvir. We show that different zoom-in cosmological
simulations of L* galaxies better reproduce the column density profile of O VI
with R than Si III or the other studied ions. We find that observations of the
M31 CGM and zoom-in simulations of L* galaxies have both lower ions showing
higher column density dispersion and dependence on R than higher ions,
indicating that the higher ionization structures are larger and/or more broadly
distributed.
