The Panchromatic Hubble Andromeda Treasury. Ii. Tracing the Inner M31 Halo With Blue Horizontal Branch Stars

Abstract

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 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 −α for the two-dimensional (2D) projected surface density BHB distribution, we obtain a high-quality fit with a 2D power-law index of α = 2.6 +0.3 −0.2 outside of 3 kpc, which flattens to α < 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 × 10 9 M . These properties are comparable with both simulations of stellar halo formation by satellite disruption alone and simulations that include some in situ formation of halo stars.

Constraints on the Formation of M31’s Stellar Halo from the SPLASH Survey

“…This is true regardless of whether tidal debris features are included in the profile, although the inclusion of tidal debris features does affect the normalization and index of the power-law fit. A similar surface brightness profile was found using the PAndAS star count data [7], and an extension of a power-law profile into the inner regions of M31 (to 3 kpc in projected distance from M31's center) was observed in blue horizontal branch stars with PHAT data [28,29].…”

PHAT. XIX. The Ancient Star Formation History of the M31 Disk

“…One well-known difference between these model sets is that the BaSTI set has an extended HB and the other sets do not. There is very little evidence for an extended HB in the PHAT photometry outside of the bulge and minor-axis fields (Rosenfield et al 2012;Williams et al 2012), and the feature is so weak compared to the red clump and RGB that our results depend very little on it.…”

The Panchromatic Hubble Andromeda Treasury. X. Ultraviolet to Infrared Photometry of 117 Million Equidistant Stars

“…Other features appear that are associated with the red clump, such as the extension to red colors caused by differential extinction, the half-magnitude extension to fainter magnitudes caused by the secondary red clump (Girardi 1999), and a more subtle extension to bluer colors and fainter F814W magnitudes caused by horizontal branch stars (Williams et al 2012). In addition, there is a clear concentration of stars at the early-AGB about 1.5 mag above the red clump.…”

“…These include improved constraints on post-AGB and AGB-manqué evolutionary phases (Rosenfield et al 2012), tracing the stellar mass distribution of the inner M31 halo (Williams et al 2012), new techniques for measuring robust ages and masses of star clusters (Beerman et al 2012) and for measuring the initial mass function from resolved stellar photometry (Weisz et al 2013), a major increase in the number of cataloged star clusters in M31 (Johnson et al 2012;Fouesneau et al 2014), evidence for a metallicity ceiling for Carbon stars (Boyer et al 2013), and additional complexity in the structural components of M31 (Dorman et al 2013). These results were based on our first generation of photometry, where measurements were made for each camera separately, and then combined at the catalog level.…”