Common origin for Hercules-Aquila and Virgo Clouds in Gaia DR2

TL;DR

This study uses RR Lyrae stars' orbital data from Gaia DR2 to show that the Hercules-Aquila Cloud and Virgo Over-density are likely remnants of a single ancient merger event, sharing similar orbital characteristics.

Contribution

It provides evidence that HAC and VOD are parts of the same accretion event, supporting the 'sausage' galaxy merger hypothesis.

Findings

HAC and VOD stars have highly eccentric orbits.

Both structures share similar regions in integrals of motion space.

HAC and VOD are likely from a single ancient merger event.

Abstract

We use a sample of ~350 RR Lyrae stars with radial velocities and Gaia DR2 proper motions to study orbital properties of the Hercules-Aquila Cloud (HAC) and Virgo Over-density (VOD). We demonstrate that both structures are dominated by stars on highly eccentric orbits, with peri-centres around ~1 kpc and apo-centres between 15 and 25 kpc from the Galactic centre. Given that the stars in the HAC and the VOD occupy very similar regions in the space spanned by integrals of motion, we conclude that these diffuse debris clouds are part of the same accretion event. More precisely, these inner halo sub-structures likely represent two complementary not-fully-mixed portions of an ancient massive merger, also known as the "sausage" event.