A Megacam Survey of Outer Halo Satellites. IV. Two Foreground Populations Possibly Associated with the Monoceros Substructure in the Direction of NGC 2419 and Koposov 2

The origin of the Galactic halo stellar structure known as the Monoceros Ring is still under debate. In this work, we study this halo substructure using deep Canada–France–Hawaii Telescope wide-field photometry obtained for the globular clusters NGC 2419 and Koposov 2, where the presence of Monoceros becomes significant because of their coincident projected position. Using Sloan Digital Sky Survey photometry and spectroscopy in the area surrounding these globulars and beyond, where the same Monoceros population is detected, we conclude that a second feature, which is not likely to be associated with Milky Way disk stars along the line of sight, is present as a foreground population. Our analysis suggests that the Monoceros Ring might be composed of an old stellar population of age Gyr and a new component ~4 Gyr younger at the same heliocentric distance. Alternatively, this detection might be associated with a second wrap of Monoceros in that direction of the sky and might also indicate a metallicity spread in the ring. The detection of such a low-density feature in other sections of this halo substructure will shed light on its nature.