Separating M33 Red Giants From Foreground Milky Way Dwarfs

Details Separating M33 Red Giants From Foreground Milky Way Dwarfs Separating M33 Red Giants From Foreground Milky Way Dwarfs

Jun 2006

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2006aas...208.1414t&link_type=abstract

American Astronomical Society Meeting 208, #14.14; Bulletin of the American Astronomical Society, Vol. 38, p.95

Astronomy and Astrophysics

Astronomy

1

Scientific paper

Our goal is to reliably separate foreground Milky Way dwarfs from M33 red giants via photometry to study M33’s stellar populations (disk/halo). This cannot be done using radial velocities alone due to the large overlap between M33’s stars and Milky Way foreground stars. Our technique is based on the gravity-sensitivity of the MgI triplet and MgH absorption band at λ≈5100Å in F-K stars (Clark & McClure 1979). The strengths of these lines are quantified photometrically by the difference in magnitude between the V-band, which measures the stellar continuum, and the narrow-band DDO51 filter (λcentral=5100Å width=150Å). A (V-I, 51-V) color-color plot essentially maps onto the theoretical temperature-gravity plane, allowing for clean separation between dwarfs and giants of the same temperature for stars with 1≤(V-I)≤2. We have photometry for four fields, two each on the minor and major axes, obtained with Kitt Peak National Observatory’s 4-meter telescope and MOSAIC camera. Exposures of 45, 50, and 230 minutes in V, I, and DDO51 yield typical errors of 0.05, 0.07, and 0.05mag, respectively, for stars with I=22.5mag, which is 1.5mag fainter than the tip of M33’s red giant branch. Dwarfs and giants are separated in (51-V) by 0.2mag, giving a 3σ separation for the faintest stars in our follow-up spectroscopic survey. Using a distance of 794kpc (McConnachie et al. 2004) and a disk scale length of 1.4kpc (Ferguson et al. 2006), our observations extend to 15 scale lengths on the minor axis and 8.7 on the major axis for a disk component. For example, outside 0.67°, we find 1300 candidate M33 stars that are probably halo/thick disk stars. Our data will also be used to select targets to spectroscopically study M33’s disk/halo using DEIMOS on the Keck II telescope.We gratefully acknowledge financial support from grants NSF AST-0307863 and STScI/NASA GO-9837 to Tammy Smecker-Hane.

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