Slicing The Monoceros Overdensity with Suprime-Cam

TL;DR

This study characterizes the Monoceros Overdensity using deep Subaru imaging, analyzing its distance, density, and metallicity, and compares models of its origin including disc perturbation, flaring, and tidal stream scenarios.

Contribution

It provides detailed measurements of the MO's properties and evaluates multiple models, offering new insights into its possible origins.

Findings

MO is at ~10.1 kpc distance with no change across longitudes

MO stars are more metal-rich ([Fe/H] ~ -1.0) than nearby stars

Tidal stream models can fit the observed data

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 CMD fitting techniques in three stripes at galactic longitudes: l=130 deg, 150 deg, 170 deg; and galactic latitudes: +15 < b [deg] < +25 . The MO appears as a wall of stars at a heliocentric distance of ~ 10.1\pm0.5 kpc 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 disc model, which predicts a significant drop in density adjacent to the MO that is not seen; a basic flared disc model, which can give a good match to the density profile but the MO metallicity implies the disc is too metal rich to source the MO stars; and a tidal stream model, which bracket 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 disc.