The Large-scale Structure of the Halo of the Andromeda Galaxy. II. Hierarchical Structure in the Pan-Andromeda Archaeological Survey

Authors

Alan W. McConnachie, National Research Council Canada
Rodrigo Ibata, Université de Strasbourg
Nicolas Martin, Université de Strasbourg
Annette M.N. Ferguson, University of Edinburgh, Institute for Astronomy
Michelle Collins, University of Surrey
Stephen Gwyn, National Research Council Canada
Mike Irwin, Institute of Astronomy
Geraint F. Lewis, The University of Sydney
A. Dougal Mackey, The Australian National University
Tim Davidge, National Research Council Canada
Veronica Arias, The University of Sydney
Anthony Conn, The University of Sydney
Patrick Côté, National Research Council Canada
Denija Crnojevic, Texas Tech University
Avon Huxor, University of Bristol
Jorge Penarrubia, University of Edinburgh, Institute for Astronomy
Chelsea Spengler, University of Victoria
Nial Tanvir, University of Leicester
David Valls-Gabaud, L'Observatoire de Paris
Arif Babul, University of Victoria
Pauline Barmby, The University of Western OntarioFollow
Nicholas F. Bate, Institute of Astronomy
Edouard Bernard, Observatoire de la Côte d'Azur
Scott Chapman, Dalhousie University
Aaron Dotter, Harvard-Smithsonian Center for Astrophysics
William Harris, McMaster University
Brendan McMonigal, The University of Sydney
Julio Navarro, University of Victoria
Thomas H. Puzia, Pontificia Universidad Católica de Chile

Document Type

Article

Publication Date

11-20-2018

Journal

Astrophysical Journal

Volume

868

Issue

1

URL with Digital Object Identifier

10.3847/1538-4357/aae8e7

Abstract

The Pan-Andromeda Archaeological Survey is a survey of >400 square degrees centered on the Andromeda (M31) and Triangulum (M33) galaxies that has provided the most extensive panorama of an L ∗ galaxy group to large projected galactocentric radii. Here, we collate and summarize the current status of our knowledge of the substructures in the stellar halo of M31, and discuss connections between these features. We estimate that the 13 most distinctive substructures were produced by at least 5 different accretion events, all in the last 3 or 4 Gyr. We suggest that a few of the substructures farthest from M31 may be shells from a single accretion event. We calculate the luminosities of some prominent substructures for which previous estimates were not available, and we estimate the stellar mass budget of the outer halo of M31. We revisit the problem of quantifying the properties of a highly structured data set; specifically, we use the OPTICS clustering algorithm to quantify the hierarchical structure of M31's stellar halo and identify three new faint structures. M31's halo, in projection, appears to be dominated by two "mega-structures," which can be considered as the two most significant branches of a merger tree produced by breaking M31's stellar halo into increasingly smaller structures based on the stellar spatial clustering. We conclude that OPTICS is a powerful algorithm that could be used in any astronomical application involving the hierarchical clustering of points. The publication of this article coincides with the public release of all PAndAS data products.