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We attempt to constrain the shape of M31's inner stellar halo by tracing thesurface density of blue horizontal branch (BHB) stars at galactocentricdistances ranging from 2 kpc to 35 kpc. Our measurements make use of resolvedstellar photometry from a section of the Panchromatic Hubble Andromeda Treasury(PHAT) survey, supplemented by several archival Hubble Space Telescopeobservations. We find that the ratio of BHB to red giant stars is relativelyconstant outside of 10 kpc, suggesting that the BHB is as reliable a tracer ofthe halo population as the red giant branch. In the inner halo, we do notexpect BHB stars to be produced by the high metallicity bulge and disk, makingBHB stars a good candidate to be a reliable tracer of the stellar halo to muchsmaller galactocentric distances. If we assume a power-law profile r^(-\alpha)for the 2-D projected surface density BHB distribution, we obtain ahigh-quality fit with a 2-D power-law index of \alpha=2.6^{+0.3}_{-0.2} outsideof 3 kpc, which flattens to \alpha<1.2 inside of 3 kpc. This slope isconsistent with previous measurements but is anchored to a radial baseline thatextends much farther inward. Finally, assuming azimuthal symmetry and aconstant mass-to-light ratio, the best-fitting profile yields a total halostellar mass of 2.1^{+1.7}_{-0.4} x 10^9 M_sun. These properties are comparablewith both simulations of stellar halo formation formed by satellite disruptionalone, and with simulations that include some in situ formation of halo stars.

The Panchromatic Hubble Andromeda Treasury II. Tracing the Inner M31 Halo with Blue Horizontal Branch Stars