Slicing the monoceros overdensity with suprime-cam

Abstract

We derive distance, density, and metallicity distribution of the stellar Monoceros Overdensity (MO) in the outer Milky Way, based on deep imaging with the Subaru Telescope. We applied color-magnitude diagram fitting techniques in three stripes at galactic longitudes, l 130°, 150°, 170°, and galactic latitudes, +15° ≤ b ≤ +25°. The MO appears as a wall of stars at a heliocentric distance of 10.1 ± 0.5kpc across the observed longitude range with no distance change. The MO stars are more metal-rich ([Fe/H] -1.0) than the nearby stars at the same latitude. These data are used to test three different models for the origin of the MO: a perturbed disk model, which predicts a significant drop in density adjacent to the MO that is not seen; a basic flared disk model, which can give a good match to the density profile but the MO metallicity implies the disk is too metal-rich to source the MO stars; and a tidal stream model, which, from the literature, brackets the distances and densities we derive for the MO, suggesting that a model can be found that would fully fit the MO data. Further data and modeling will be required to confirm or rule out the MO feature as a stream or as a flaring of the disk