Discovery of new retrograde substructures: the shards of ω Centauri?

• Published in: Monthly notices of the Royal Astronomical Society Vol. 478; no. 4; pp. 5449 - 5459
• Main Authors: Myeong, G C, Evans, N W, Belokurov, V, Sanders, J L, Koposov, S E
• Format: Journal Article
• Language: English
• Published: Oxford University Press 21.08.2018
• Subjects: galaxies: structure; galaxies: kinematics and dynamics
• ISSN: 0035-8711; 1365-2966
• DOI: 10.1093/mnras/sty1403

ABSTRACT We use the Sloan Digital Sky Survey (SDSS)–Gaia catalogue to search for substructure in the stellar halo. The sample comprises 62 133 halo stars with full phase space coordinates and extends out to heliocentric distances of ∼10 kpc. As actions are conserved under slow changes of the potential, they permit identification of groups of stars with a common accretion history. We devise a method to identify halo substructures based on their clustering in action space, using metallicity as a secondary check. This is validated against smooth models and numerical constructed stellar haloes from the Aquarius simulations. We identify 21 substructures in the SDSS–Gaia catalogue, including seven high-significance, high-energy and retrograde ones. We investigate whether the retrograde substructures may be material stripped off the atypical globular cluster ω Centauri. Using a simple model of the accretion of the progenitor of the ω Centauri, we tentatively argue for the possible association of up to five of our new substructures (labelled Rg1, Rg3, Rg4, Rg6 and Rg7) with this event. This sets a minimum mass of 5 × 108 M⊙ for the progenitor, so as to bring ω Centauri to its current location in action–energy space. Our proposal can be tested by high-resolution spectroscopy of the candidates to look for the unusual abundance patterns possessed by ω Centauri stars.