A Kinematically-Distinct RR-Lyrae Overdensity in the Inner Regions of the Milky Way

TL;DR

This study identifies a kinematically distinct RR Lyrae overdensity in the inner Milky Way, characterized by unique metallicity and motion properties, possibly linked to omega Centauri's progenitor, but requires further modeling for confirmation.

Contribution

The paper reports the discovery and kinematic analysis of a new RR Lyrae overdensity in the inner Galaxy, suggesting a potential association with omega Centauri's progenitor.

Findings

Overdensity extends at least 20 degrees in longitude

Distinct metallicity distribution with a peak at -1.3

Proper motions indicate net vertical motion away from the plane

Abstract

We combine the Siding Spring Survey of RR Lyrae stars with the Southern Proper Motion Catalog 4, in order to detect and kinematically characterize overdensities in the inner halo of the Milky Way. We identify one such overdensity above the Galactic plane, in quadrant 4 of the Galaxy. The overdensity extends at least 20 degrees in longitude, has an average heliocentric distance of 8 kpc with a depth of 4 kpc, and is confined within 4 kpc of the Galactic plane. Its metallicity distribution is distinct from that of the field population having a peak at -1.3 and a pronounced tail to -2.0. Proper motions indicate a net vertical motion away from the plane, and a low orbital angular momentum. Qualitatively, these orbit properties suggest a possible association with omega Centauri's parent satellite. However, comparison to a specific omega Cen N-body disruption model does not give a good match with observations. Line-of-sight velocities, and more extensive N-body modelling will help clarify the nature of this overdensity.