The dwarf spheroidal galaxy, Andromeda XII

(smoothed map of likely AndXII stars from CFHT-MegaCam image, with stars confirmed as members using Keck shown)

Strangers in the night: Discovery of a dwarf spheroidal galaxy on its first Local Group infall
AstroPh article
(Astrophysical Journal Letters, in press)

Scott C. Chapman (Cambridge, IoA)
Jorge Peņarrubia (U. Victoria)
Rodrigo Ibata (Obs. de Strasbourg)
Alan McConnachie (U. Victoria)
Nicolas Martin (MPIA Heidelberg)
Mike Irwin (Cambridge, IoA)
Andrew Blain (Caltech)
Bruno Letarte (Caltech)
Geraint Lewis (U. Sydney)
Fred Lo (NRAO)
Aaron Ludlow (U. Victoria)
Karen O'Neil (NRAO)
part of the
Andromeda Kinematics
nd Metallicity Experiment (M31)

As of April 2007:
-16 nights on Keck-II/DEIMOS
-13876 stars in the outer regions of M31 and its satellites with radial velocities and Calcium II Triplet [Fe/H] measurements.
-6 new dwarf satellites (AndIX,XI,XII,XIII,XV,XVI)

-Press on discoveries of:
AndXII Extended Disk Metal-Poor Halo
Simulution of late accreting satellites (coloured circles) into an environment similar to our Local Group, from Ludlow et al. (2007).

The dwarf spheroidal galaxy AndXII: First evidence for late infall.

Press Release Sky and Telescope article Science News article Space.Com article

Further theoretical context for AndXII from Sales, Navarro, Abadi, Steinmetz (astro-ph/0704.1773)
"High-velocity satellites approaching M31 in projection are unlikely to be escapers, but rather satellites on their first approach. This interpretation is probably the most appropriate for And XII [unique for the satellites of M31]. As discussed by Chapman et al. (2007), And XII is almost certainly farther than M31 but is approaching us at much higher speed (281 km/s faster) than M31. This implies that And XII is actually getting closer in projection to M31 making the interpretation of this satellite as an escaping system rather unlikely."
The figure below (from our paper ``Strangers in the night'') highlights AndXII as the one known M31 satellite which is at the limit of escaping from M31's gravitational grasp, for typical mass models of M31.

Further theoretical context, movies and simulations from Ludlow et al. (2007)
From the simulations of Ludlow et al. (2007), who have looked explicitly at high velocity structures falling into halos of similar total mass to the Local Group at late times. The distribution of dark matter particles at z=0 within 2 Mpc/h of the galaxy (black points -- 200^3 particle simulation) contrasted with red points marking the location of gravitationally bound substructures. It is clear that the filaments contain most of the matter and subhalos in the nearby volume, and they intersect at the location of the galaxy. The cosmology is flat LambdaCDM: Omega_M = 0.25, Omega_Lambda = 0.75, H_0 = 0.73. The simulation shows M_{200} = 48.4 (10^10 M_sun/h) R_{200} = 127.6 (kpc/h) C_{200} = 10.6 V_{max} = 148.2 (km/s). The simulation was undertaken with Gadget2 (Springel V., et al., 2005, MRNAS, 364, 1105).


Scott Chapman of Cambridge University
Institute of Astronomy, Madingley Road, Cambridge, CB3 0HA, UK     tel (250) 885-4735

Jorge Peņarrubia of the University of Victoria

Rodrigo Ibata of the Observatoire de Strasbourg
11, rue de l'Universit\'e, F-67000, Strasbourg, France,   tel +33 (0)3 90 24 23 91

Alan McConnachie of the University of Victoria

Nicolas Martin of the MPiA, Heidelberg, Germany

Geraint Lewis of the University of Sydney
Institute of Astronomy, School of Physics, A29, University of Sydney, NSW 2006, Australia
Tel : +61 2 9351 5184 (7726)   Mbl: 0424 254 551

Mike Irwin of Cambridge University
Institute of Astronomy, Madingley Road, Cambridge, CB3 0HA, UK  Tel : +44 (0)1223 337524

Andrew Blain of Caltech
Last revised: 25th of April, 2007