Spectroscopic Follow-Up of the Hercules Aquila Cloud

Iulia T. Simion, Vasily Belokurov, Sergey E. Koposov, Allyson Sheffield, Kathryn V. Johnston

Research output: Contribution to journalArticlepeer-review


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.
Original languageEnglish
JournalMonthly Notices of the Royal Astronomical Society
Early online date26 Feb 2018
Publication statusPublished - 1 May 2018


  • astro-ph.GA


Dive into the research topics of 'Spectroscopic Follow-Up of the Hercules Aquila Cloud'. Together they form a unique fingerprint.

Cite this