The Stellar Halo of Andromeda (M31)

Scott C. Chapman (Caltech)
Rodrigo Ibata (Obs. de Strasbourg)
Geraint Lewis (U. Sydney)
Annette Ferguson (U. Edinburgh)
Mike Irwin (Cambridge, IoA)
Alan McConnachie (U. Victoria)
Nial Tanvir (U. Hertfordshire)
Andromeda Kinematics and
Metallicity Experiment (M31)

As of Oct.2005:
-14 nights on Keck-II/DEIMOS
-9776 stars in M31 with radial velocities and Calcium II Triplet [Fe/H] measurements.
-7 publications
-Press: Extended Disk Metal-Poor Halo

Andromeda's Stellar Halo Shows Galaxy's Origin
to Be Similar to That of Milky Way

Press Release

Details on the discovery of Andromeda's metal-poor halo

Astrophysical Journal: A kinematically selected, metal-poor spheroid in the outskirts of M31

For the last decade, astronomers have thought that the Andromeda galaxy, our nearest galactic neighbor, was rather different from the Milky Way. However, our group's observations with the Keck telescope in Hawaii have allowed detailed studies of the motions and metals of nearly 10,000 stars in Andromeda, showing that the galaxy's stellar halo is "metal-poor." This means that the stars lying in the outer bounds of the galaxy are pretty much lacking in elements heavier than hydrogen, and suggests a primeval origin to the halo stars. As this situation is mimicked by our own Milky Way, this suggests the two galaxies are probably quite similar in the way they evolved, at least over their first several billion years.
The study could lead to new insights on the nature of dark matter. This is the first time we've been able to obtain a panoramic view of the motions of stars in the halo of a galaxy. These stars allow us to weigh the dark matter, and determine how it decreases with distance. While no one yet knows what dark matter is made of, its existence is well established because of the mass that must exist in galaxies for their stars to orbit the galactic centers the way they do. Current theories of galactic evolution, in fact, assume that dark-matter wells acted as a sort of "seed" for today's galaxies, with the dark matter pulling in smaller groups of stars as they passed nearby.
If the favored hierarchical cosmological model, LCDM, is correct, then the Milky Way, Andromeda, and other similarly sized galaxies with dark matter halo masses of ~10^12 solar masses should have accreted and subsequently tidally destroyed ~200 low-mass galaxies in the past ~12 Gyr (as described by Bullock and Johnston 2005).
The majority of the stellar mass associated with the accreted halo of stars is built up quite early, within the first 3-4 billion years. This halo grows initially by the accretion of metal poor proto-galactic fragments (very little galaxies which have not had time to build up appreciable elements heavier than hydrogen), and a metal-poor stellar halo is expected as the 'primeval galaxy'.
This primeval halo has been seen in the Milky Way, but until now has not been found in Andromeda which shows a very metal-rich structure out to large radii (~100kpc). This has resulted in a 10 year mystery about how Andromeda could have such a different structure and origin to the Milky Way, confounding theoretical understanding of how big spiral galaxies form.

Halo 1 Click for larger image
Halo 2 Click for larger image
Halo 3 Click for larger image
Halo 5 Click for larger image Halo 6 Click for larger image

External Surface Brightness Images from the models of Bullock & Johnston (2005)
300 kpc x 300 kpc boxes, showing five different realizations of a stellar halo in a Milky Way or M31 type galaxy. The ancient metal-poor stellar spheroid is seen in all cases as the "ball" of stars dominant within the central 100kpc of the images. This structure forms within the first 3-5billions years of all simulations, and underlies our understanding of how big spiral galaxies originate. The 'tidal' structures visible at larger radius represent the relatively late accretions of dwarf galaxies (within the last 5-10 billion years).
Color bar scales from 40 Mag/arcsec^2 (black) to 24 Mag/arcsec^2 (white)
[The eye can just pick out dark blue at 30 Mag/arcsec^2]
Click on thumbnail for larger image.

Movie of the accretion buildup of the stellar halo over the first 5 billion years of formation. Proto-galactic fragments fall into the 10^12 solar mass Dark Matter halo in random orbits, yielding a spheroidal distribution of stars which is pressure supported.

click here for movie (Credit, J. Bullock and K. Johnston

Movie of the hierarchical formation of a spiral galaxy (like M31 or the Milky Way) starting at redshift z=50 (a few 100,000 years after the big bang). Face-on (left) and edge-on (right) projections are shown. The ancient spherical halo of stars is obvious at late times as a red ball encircling the disk and bulge of the galaxy. Late accretions of dwarf galaxies are still on-going at present.

click here for movie (Credit, M. Steinmetz)



Scott Chapman of the California Institute of Technology
ms105-24, 1200 E. California Blvd., Pasadena, CA 91125, U.S.A,   tel (626) 319 7003

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

Annette Ferguson of the University of Edinburgh
Institute for Astronomy, Blackford Hill, Edinburgh EH9 3HJ, UK

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

Nial Tanvir of the University of Hertfordshire
Physical Sciences, Univ. of Hertfordshire, College Lane, Hatfield, AL10 9AB, UK
Last revised: 25th of Feb., 2006