* Astronomy

Title: Update on the Nature of Virgo Overdensity
Authors: Ana Bonaca, Mario Juric, Zeljko Ivezic, Dmitry Bizyaev, Howard Brewington, Elena Malanushenko, Viktor Malanushenko, Daniel Oravetz, Kaike Pan, Alaina Shelden, Audrey Simmons, Stephanie Snedden

We use the Eighth Data Release of Sloan Digital Sky Survey (SDSS DR8) catalogue 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 deg², with its true extent likely closer to 3000 deg². 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).

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