Spectroscopic Follow-Up of the Hercules Aquila Cloud

TL;DR

This study provides the first large spectroscopic velocity sample of the Hercules-Aquila Cloud, revealing a double-peaked velocity distribution indicative of an old accretion event, and suggests HAC is part of a larger debris structure.

Contribution

It offers the first extensive spectroscopic velocity data for HAC and compares observations with simulations to infer its formation history.

Findings

Double-peaked radial velocity distribution in HAC

HAC likely results from an old, well-mixed accretion event

HAC may be part of a larger debris structure in the Galaxy

Abstract

We designed a follow-up program to find the spectroscopic properties of the Hercules-Aquila Cloud (HAC) and test scenarios for its formation. We measured the radial velocities (RVs) of 45 RR Lyrae in the southern portion of the HAC using the facilities at the MDM observatory, producing the first large sample of velocities in the HAC. We found a double-peaked distribution in RVs, skewed slightly to negative velocities. We compared both the morphology of HAC projected onto the plane of the sky and the distribution of velocities in this structure outlined by RR Lyrae and other tracer populations at different distances to N-body simulations. We found that the behaviour is characteristic of an old, well-mixed accretion event with small apo-galactic radius. We cannot yet rule out other formation mechanisms for the HAC. However, if our interpretation is correct, HAC represents just a small portion of a much larger debris structure spread throughout the inner Galaxy whose distinct kinematic structure should be apparent in RV studies along many lines of sight.