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29 March 2024
 
  » arxiv » astro-ph/0605171

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The Outer Halo of M31: A New Method for Isolating Red Giant Stars and a Measurement of the Brightness Profile and Metallicity Distribution
Karoline M. Gilbert ; Puragra Guhathakurta ; Jasonjot S. Kalirai ; R. Michael Rich ; Steven R. Majewski ; James C. Ostheimer ; David B. Reitzel ; A. Javier Cenarro ; Michael C. Cooper ; Carynn Luine ; Richard J. Patterson ;
Date 8 May 2006
AbstractWe present a method for isolating a clean sample of red giant branch stars in the outer regions of the Andromeda spiral galaxy (M31) from an ongoing spectroscopic survey using the DEIMOS instrument on the Keck~II 10-m telescope. The survey aims to study the kinematics, global structure, substructure, and metallicity of M31’s halo. Although most of our spectroscopic targets were photometrically screened to reject foreground Milky Way dwarf star contaminants, the latter class of objects still constitutes a substantial fraction of the observed spectra in the sparse outer halo. Our likelihood-based method for isolating M31 red giants uses multiple criteria: (1) radial velocity, (2) intermediate-width band photometry through the DDO51 filter centered on the surface-gravity sensitive MgH/Mg b absorption features, (3) strength of the Na I 8190 Angstrom absorption line doublet, (4) location within an (I, V-I) color-magnitude diagram, and (5) comparison of photometric versus spectroscopic metallicity estimates. Training sets consisting of 462 definite M31 red giants and 169 Galactic dwarf stars are used to derive empirical probability distribution functions for each diagnostic. These functions are used to calculate the likelihood that a given star is a red giant branch star in M31 versus a Milky Way dwarf star. The ability to identify individual stars as M31 red giants gives us an unprecedented level of sensitivity in studying the properties of the galaxy’s outer halo. We present the surface brightness profile and metallicity distribution of the clean sample of outer halo M31 red giants.
Source arXiv, astro-ph/0605171
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