David B. Reitzel — Page 2

arXiv2000

Keck Studies of M31's Stellar Halo

We present Keck 10-meter/LRIS spectra of candidate red giants in the halo of M31, located at a projected radius of R=19kpc on the minor axis. These spectroscopic targets have been selected using a combination of UBRI-based and morphological screening to eliminate background galaxies. Radial velocity measurements are used to separate M31 halo giants from foreground Milky Way dwarf stars, M31 disk stars, and residual background galaxies. The metallicity of each M31 halo giant is measured using standard photometric and spectroscopic techniques, the latter based on the strength of the CaII triplet. The various [Fe/H] estimates are in rough agreement with one another. The data reveal a large spread (>2dex) in [Fe/H] in M31's halo; there is no strong radial [Fe/H] gradient. LRIS and HIRES spectra are also presented for red giants in five dwarf spheroidal satellites of M31: AndI, AndIII, AndV, AndVI, and AndVII. There appears to be a significant metallicity spread in AndVI and possibly in AndI. The new radial velocity data on these outer dwarfs are used to constrain the total mass of M31: the best estimate is under 10^(12)Msun, somewhat less than the best estimate for the Milky Way.

astro-ph↗

arXiv1998

Isolating Red Giant Stars in M31's Elusive Outer Spheroid

Deep UBRI images of a 15'x15' field in the M31 galaxy's outer spheroid obtained with the KPNO 4-m telescope, and Keck 10-m telescope+LRIS I-band images, are used to isolate candidate red giant branch stars located at R=19 kpc (projected) along the minor axis. These stars are distinguished from the more numerous distant field galaxies by isolating objects whose colors are consistent with the long but narrow locus occupied by RGB stars in UBRI color space and whose angular sizes are consistent with the stellar PSF (FWHM~0.6"-1.5"). We carry out the same analysis of data on a comparison field with a similar Galactic latitude to the M31 halo field. The color-magnitude diagram of comparison field objects is well described by a superposition of foreground Galactic dwarf stars against a backdrop of faint blue field galaxies, 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 CMD location of this population of faint red objects is as would be expected for RGB 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 HST studies. The data indicate that M31's stellar halo is much denser/larger than that of the Galaxy: (ρ_M31/ρ_MW) (Λ/1.5)^(-ν) ~ 10, where Λis the ratio of M31's and the Galaxy's radial scale lengths and ν=-3.8 is the assumed power law index of the density profile. The color and slope of the RGB in M31's outer halo are suggestive of a relatively metal-rich population, [Fe/H] >~ -1, in agreement with the HST measurements.

astro-ph↗