Tracing the Metal-Poor M31 Stellar Halo with Blue Horizontal Branch
  Stars
  We have analyzed new HST/ACS and HST/WFC3 imaging in F475W and F814W of two
previously-unobserved fields along the M31 minor axis to confirm our previous
constraints on the shape of M31's inner stellar halo. Both of these new
datasets reach a depth of at least F814W$<$27 and clearly detect the blue
horizontal branch (BHB) of the field as a distinct feature of the
color-magnitude diagram. We measure the density of BHB stars and the ratio of
BHB to red giant branch stars in each field using identical techniques to our
previous work. We find excellent agreement with our previous measurement of a
power-law for the 2-D projected surface density with an index of
2.6$^{+0.3}_{-0.2}$ outside of 3 kpc, which flattens to $\alpha <$1.2 inside of
3 kpc. Our findings confirm our previous suggestion that the field BHB stars in
M31 are part of the halo population. However, the total halo profile is now
known to differ from this BHB profile, which suggests that we have isolated the
metal-poor component. This component appears to have an unbroken power-law
profile from 3-150 kpc but accounts for only about half of the total halo
stellar mass. Discrepancies between the BHB density profile and other
measurements of the inner halo are therefore likely due to the different
profile of the metal-rich halo component, which is not only steeper than the
profile of the met al-poor component, but also has a larger core radius. These
profile differences also help to explain the large ratio of BHB/RGB stars in
our observations.
