We demonstrate the presence of an extended and massive circumgalactic medium ( CGM ) around Messier 31 using archival HST COS ultraviolet spectroscopy of 18 QSOs projected within two virial radii of M31 ( R _ { vir } = 300 kpc ) .
We detect absorption from Si III at -300 \mathrel { \hbox { \hbox to 0.0 pt { \hbox { \lower 4.0 pt \hbox { $ \sim$ } } } \hbox { $ < $ } } % } v _ { LSR } \mathrel { \hbox { \hbox to 0.0 pt { \hbox { \lower 4.0 pt \hbox { $ \sim$ } } } % \hbox { $ < $ } } } -150 { km s } ^ { -1 } toward all 3 sightlines at R \mathrel { \hbox { \hbox to 0.0 pt { \hbox { \lower 4.0 pt \hbox { $ \sim$ } } } \hbox { $ < $ } } } 0. % 2 R _ { vir } , 3 of 4 sightlines at 0.8 \mathrel { \hbox { \hbox to 0.0 pt { \hbox { \lower 4.0 pt \hbox { $ \sim$ } } } \hbox { $ < $ } } } % R / R _ { vir } \mathrel { \hbox { \hbox to 0.0 pt { \hbox { \lower 4.0 pt \hbox { $ \sim$ } } } % \hbox { $ < $ } } } 1.1 , and possibly 1 of 11 at 1.1 < R / R _ { vir } \mathrel { \hbox { \hbox to 0.0 pt { \hbox { \lower 4.0 pt \hbox { $ \sim$ } % } } \hbox { $ < $ } } } 1.8 .
We present several arguments that the gas at these velocities observed in these directions originates from the CGM of M31 rather than the Local Group or Milky Way CGM or Magellanic Stream .
We show that the dwarf galaxies located in the CGM of M31 have very similar velocities over similar projected distances from M31 .
We find a non-trivial relationship only at these velocities between the column densities ( N ) of all the ions and R , whereby N decreases with increasing R .
Singly ionized species are only detected in the inner CGM of M31 at < 0.2 R _ { vir } .
At R < 0.8 R _ { vir } , the covering fraction is close to unity for Si III and C IV ( f _ { c } \sim 60 \% – 97 \% at the 90 % confidence level ) , but drops to f _ { c } \mathrel { \hbox { \hbox to 0.0 pt { \hbox { \lower 4.0 pt \hbox { $ \sim$ } } } \hbox { % $ < $ } } } 10 – 20 \% at R \mathrel { \hbox { \hbox to 0.0 pt { \hbox { \lower 4.0 pt \hbox { $ \sim$ } } } \hbox { $ > $ } } } R _ % { vir } .
We show that the M31 CGM gas is bound , multiphase , predominantly ionized ( i.e. , H II \gg H I ) , and becomes more highly ionized gas at larger R .
We estimate using Si II , Si III , and Si IV a CGM metal mass of at least 2 \times 10 ^ { 6 } M _ { \sun } and gas mass of \mathrel { \hbox { \hbox to 0.0 pt { \hbox { \lower 4.0 pt \hbox { $ \sim$ } } } \hbox { $ > $ } } } 3 % \times 10 ^ { 9 } ( Z _ { \sun } / Z ) M _ { \sun } within 0.2 R _ { vir } , and possibly a factor \sim 10 larger within R _ { vir } , implying substantial metal and gas masses in the CGM of M31 .
Compared with galaxies in the COS-Halos survey , the CGM of M31 appears to be quite typical for a L ^ { * } galaxy .