Probing the Inner Halo of M31 with Blue Horizontal Branch Stars
Abstract
We attempt to constrain the density profile 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 1 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, we can constrain the exponent of the power law to a precision of 10% outside of 3 kpc. Inside of 3 kpc, we find that the profile flattens significantly. Finally, assuming azimuthal symmetry and a constant mass-to-light ratio, we calculate a total halo stellar mass. We find 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. Support for this work is provided by NASA through grant GO-12055 from the Space Telescope Science Institute.
- Publication:
-
American Astronomical Society Meeting Abstracts #221
- Pub Date:
- January 2013
- Bibcode:
- 2013AAS...22112206W