The Virgo Overdensity Explained

Abstract

Abstract We suggest that the Virgo Overdensity of stars in the stellar halo is the result of a radial dwarf galaxy merger that we call the Virgo Radial Merger. Because the dwarf galaxy passed very near to the Galactic center, the debris has a large range of energies but nearly zero L z angular momentum. The debris appears to extend from 5 to 50 kpc from the Sun in the Virgo region. We connect different moving groups of this merger debris to the Perpendicular and Parallel Streams (the Virgo Stellar Stream is associated with either or both of these streams), the Hercules-Aquila Cloud, and possibly the Eridanus-Phoenix Overdensity. This radial merger can explain the majority of the observed moving groups of RR Lyrae and blue horizontal branch stars that have previously been identified in Virgo. This merger also produces debris in the solar neighborhood similar to that identified as the Gaia–Enceladus merger or Gaia sausage. Orbits are provided for components of the Virgo Radial Merger progenitor and debris that appears to be related to the Cocytos Stream, which was also recovered in the Virgo region.