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 metal-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 .