Isolating Red Giant Stars in M31's Elusive Outer Spheroid

Deep UBRI images of a 15′ × 15′ field in the outer spheroid of the galaxy M31 obtained using the Kitt Peak National Observatory's 4 m telescope and I-band images obtained with the Keck 10 m telescope and Low Resolution Imaging Spectrograph are used to isolate a sample of candidate red giant branch stars located at a projected radial distance of 19 kpc along the minor axis. These stars are distinguished from the more numerous distant field galaxies on the basis of broadband U - B, B - R, and R - I colors and image morphology; we isolate objects whose colors are consistent with the long, but relatively narrow, locus occupied by red giants in UBRI color space (as observed in Galactic globular cluster giants and predicted by models spanning a wide range of metallicities and ages), and those whose angular sizes are consistent with the stellar point-spread function, FWHM = 0.″6–0.″9 (Keck) and FWHM = 0.″9–1.″5 (Kitt Peak). We carry out the same analysis of data on a comparison field with a Galactic latitude similar to the M31 halo field. The color-magnitude diagram of objects in the comparison field is well described by a superposition of foreground Galactic dwarf stars (in keeping with a standard empirical model of the Galaxy) against a backdrop of contaminating faint blue field galaxies (I ≳ 21, B - I ∼ 1–2.5), while the M31 halo field contains a clear excess of faint red objects (I ∼ 20–23, B - I ∼ 2–3.5) in addition to these two components. The location of this population of faint red objects in the color-magnitude diagram is as would be expected for red giant stars at the distance of M31. The surface density of RGB candidates in the R = 19 kpc M31 halo field is consistent with the findings of two recent Hubble Space Telescope (HST) studies. The data indicate that M31's halo is much denser and/or larger than that of the Galaxy: (ρ/ρ)(Λ/1.5)-ν ∼ 10 , where Λ is the ratio of the radial scale lengths of M31 and the Galaxy and ν = -3.8 is the assumed power-law index of the density profile; in fact, M31's profile may be steeper than this (ν < -3.8). The color and slope of the red giant branch in M31's outer halo are suggestive of a relatively metal-rich population, [Fe/H] ≳ -1, in agreement with the HST measurements.