Update on the Nature of Virgo Overdensity

TL;DR

This study uses SDSS DR8 data to map the Virgo Overdensity, concluding it spans at least 2000 square degrees and is best explained by a minor merger event rather than halo tilt, providing new insights into its nature.

Contribution

It provides the first extensive mapping of the Virgo Overdensity using SDSS DR8 and rules out halo tilt as its cause, supporting a minor merger origin.

Findings

VOD extends over at least 2000 deg^2

Halo tilt cannot explain VOD symmetry

VOD likely results from a minor merger event

Abstract

We use the Eighth Data Release of Sloan Digital Sky Survey (SDSS DR8) catalog with its additional sky coverage of the southern Galactic hemisphere, to measure the extent and study the nature of the Virgo Overdensity (VOD; Juric et al. 2008). The data show that the VOD extends over no less than 2000 deg^2, with its true extent likely closer to 3000 deg^2. We test whether the VOD can be attributed to a tilt in the stellar halo ellipsoid with respect to the plane of the Galactic disk and find that the observed symmetry of the north-south Galactic hemisphere star counts excludes this possibility. We argue that the Virgo Overdensity, in spite of its wide area and cloud-like appearance, is still best explained by a minor merger. Its appearance and position is qualitatively similar to a near perigalacticon merger event and, assuming that the VOD and the Virgo Stellar Stream share the same progenitor, consistent with the VSS orbit determined by Casetti-Dinescu et al. (2009).