On rings and streams in the galactic anti-center

Abstract

We confirm that there are at least three separate low-latitude overdensities of blue F turnoff stars near the Milky Way anti-center: the Monoceros Ring, the Anti-Center Stream (ACS), and the Eastern Banded Structure (EBS). There might also be a small number of normal thick disk stars at the same location. The ACS is a tilted component that extends to higher Galactic latitude at lower Galactic longitude, 10kpc from the Sun toward the anti-center. It has a sharp cutoff on the high-latitude side. Distance, velocity, and proper motion measurements are consistent with previous orbit fits. The mean metallicity is [Fe/H] =-0.96 ± 0.03, which is lower than the thick disk and Monoceros Ring. The Monoceros Ring is a higher density substructure that is present at 15° < b < 22° at all longitudes probed in this survey. The structure likely continues toward lower latitudes. The distances are consistent with a constant distance from the Galactic center of 17.6kpc. The mean line-of-sight velocity of the structure is consistent with a thick disk rotation. However, the velocity dispersion of these stars is 15kms-1 and the metallicity is [Fe/H] =-0.80 ± 0.01. Both of these quantities are lower than the canonical thick disk. We suggest that this ring structure is likely different from the thick disk, though its association with the disk cannot be definitively ruled out. The EBS is detected primarily photometrically, near (l, b) = (225°, 30°), at a distance of 10.9kpc from the Sun. © 2012. The American Astronomical Society. All rights reserved.