We attempt to constrain the shape of M31 ’ s inner stellar halo by tracing the surface density of blue horizontal branch ( BHB ) stars at galactocentric distances ranging from 2 kpc to 35 kpc .
Our measurements make use of resolved stellar photometry from a section of the Panchromatic Hubble Andromeda Treasury ( PHAT ) survey , supplemented by several archival Hubble Space Telescope observations .
We find that the ratio of BHB to red giant stars is relatively constant outside of 10 kpc , suggesting that the BHB is as reliable a tracer of the halo population as the red giant branch .
In the inner halo , we do not expect BHB stars to be produced by the high metallicity bulge and disk , making BHB stars a good candidate to be a reliable tracer of the stellar halo to much smaller galactocentric distances .
If we assume a power-law profile r ^ { - \alpha } for the 2-D projected surface density BHB distribution , we obtain a high-quality fit with a 2-D power-law index of \alpha = 2.6 ^ { +0.3 } _ { -0.2 } outside of 3 kpc , which flattens to \alpha < 1.2 inside of 3 kpc .
This slope is consistent with previous measurements but is anchored to a radial baseline that extends much farther inward .
Finally , assuming azimuthal symmetry and a constant mass-to-light ratio , the best-fitting profile yields a total halo stellar mass of 2.1 ^ { +1.7 } _ { -0.4 } \times 10 ^ { 9 } M _ { \odot } .
These properties are comparable with both simulations of stellar halo formation formed by satellite disruption alone , and with simulations that include some in situ formation of halo stars .