A Disk Origin for the Monoceros Ring and A13 Stellar Overdensities

Understand

The Monoceros Ring (also known as the Galactic Anticenter Stellar Structure) and A13 are stellar overdensities at estimated heliocentric distances of $d \sim 11$ kpc and 15 kpc observed at low Galactic latitudes towards the anticenter of our Galaxy.

• While these overdensities were initially thought to be remnants of a tidally-disrupted satellite galaxy, an alternate scenario is that they are composed of stars from the Milky Way (MW) disk kicked out to their current location due to interactions between a satellite galaxy and the disk.
• To test this scenario, we study the stellar populations of the Monoceros Ring and A13 by measuring the number of RR Lyrae and M giant stars associated with these overdensities.
• We obtain low-resolution spectroscopy for RR Lyrae stars in the two structures and measure radial velocities to compare with previously measured velocities for M giant stars in the regions of the Monoceros Ring and A13, to assess the fraction of RR Lyrae to M giant stars ($f_{RR:MG}$) in A13 and Mon/GASS.