The large-scale structure of the halo of the Andromeda galaxy II. Hierarchical structure in the Pan-Andromeda Archaeological Survey

Alan W. McConnachie, Rodrigo Ibata, Nicolas Martin, Annette M. N. Ferguson, Michelle Collins, Stephen Gwyn, Mike Irwin, Geraint F. Lewis, A. Dougal Mackey, Tim Davidge, Veronica Arias, Anthony Conn, Patrick Cote, Denija Crnojevic, Avon Huxor, Jorge Penarrubia, Chelsea Spengler, Nial Tanvir, David Valls-Gabaud, Arif BabulPauline Barmby, Nicholas F. Bate, Edouard Bernard, Scott Chapman, Aaron Dotter, William Harris, Brendan McMonigal, Julio Navarro, Thomas H. Puzia, R. Michael Rich, Guillaume Thomas

Research output: Contribution to journalArticlepeer-review


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 a $L_\star$ galaxy group to large projected galactocentric radii. Here, we collate and summarise 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 Gyrs. We suggest that a few of the substructures furthest 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 dataset; 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', that can be considered as the two most significant branches of a merger tree produced by breaking M31's stellar halo into smaller and 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.
Original languageEnglish
JournalAstrophysical Journal
Issue number1
Publication statusPublished - 20 Nov 2018


  • astro-ph.GA


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