The magnificent Andromeda Galaxy!

Top Row: Andromeda (M31): the old (left) and the new (middle and right)!
The 10m Keck telescopes (bottom left) have transformed our understanding of M31's mechanics. 
(Above left) The classical view of M31 with the Palomar telescope's photographic plates.
(Above middle) A modern wide-field CCD false-colour image of the outermost regions of M31 from the INT2.5m telescope (Ibata et al. 2001, Ferguson et al. 2002) reveals unprecedented substructures surrounding the classical view of the disk (innermost red region). These images can be used to analyze the amount of metals processed in the stars (above right, colour-coded with red as more metal poor).
(Below right) Keck spectroscopy (with masks shown as colored rectangles on the outer regions of M31) has shown that all the messy fragments in this image (except the giant stellar stream to the southeast) participate in the same rotation as the innermost disk.

Giant Disk in Andromeda's Outskirts Found with Keck!!!

Want Details?

These results are the subject of 7 scientific papers papers currently available:
The Giant Disk
The Halo
A diffuse companion to M31 ... And-IX
The outskirts of M31 & M33
The Giant Stellar Stream to the South East
The NGC205 Stellar Stream
The minor axis profile of M31

Caltech Press Release

Other Press Articles

Andromeda (or M31) keeps providing clues to how galaxies form and evolve in the Universe

M31 appears to have a stellar disk more than 3 times bigger in diameter than previously thought!
Scott Chapman from the California Institute of Technology and Rodrigo Ibata from the Observatoire de Strasbourg in France have led a team of astronomers in a project to map out the detailed motions of stars in the outskirts of the Andromeda galaxy. Their recent observations with the Keck telescopes show that the tenuous sprinkle of stars that extend outward from the galaxy are actually part of the main disk itself. This means that the spiral disk of Andromeda is three times larger in diameter than previously estimated.

Velocities of 10,000 stars in outer regions of Andromeda were measured with Keck/DEIMOS. More than half of these stars some distance from the disk which were once thought to be merely the "halo" of stars in the region and not part of the disk itself were used to provide evidence that there is a vast extended stellar disk that extends to greater than 70kpc in radius. These stars are clearly moving at velocities that demonstrate they are in orbit around the center of the galaxy.

An INT2.5m telescope (located in the Canary Islands) colour-coded map of M31 is shown below, highlighting the irregular "halo" of stars (yellow) which loops and bubbles around the 'classical' disk of M31 (red). The Keck velocity measurements have helped to determined  the nature of the inhomogenous halo (the fact that the outer fringes of the disk are clumpy and blobby). The outer regions of Andromeda are clearly dominated by a huge rotating disk, but at the same time must be the result of satellite galaxies long ago slamming together, giving rise to this inhomogeneous "messy" morphology. This giant disk discovery will be very hard to reconcile with computer simulations of forming galaxies - giant rotating disks do not arise from the accretion of small galaxy fragments.

The extended stellar disk has gone undetected in the past because the stars that appear in this halo-like outer region could not be known to be a part of the disk until their motions were calculated. In addition, the inhomogenous "fuzz" that makes up the extended disk does not look like a disk - it appears to be a fragmented, messy halo built up from many previous galaxies crashing into Andromeda, and it was assumed that stars in this region would be going every which way. Finding all these stars in an orderly rotation was the last explanation anyone would think of.

Further work will be needed to determine whether the extended disk is merely a quirk of the Andromeda galaxy, or is perhaps typical of other galaxies.


Finding that the bulk of the complex structure in Andromeda's outer region is rotating with the disk is a blessing for studying the true underlying stellar halo of the galaxy.  All the inhomogeneous fuzz seen in the image above can be lifted off and removed from consideration, yielding a sample of about 1000 stars which appear as a metal-poor ([Fe/H] = -1.4) 'pure' stellar halo sample. Using this new information, careful measurements of the random motions of the stars in the stellar halo have been made, probing it's mass and the form of the elusive dark matter which surrounds it.

Our new measurements of the stellar velocities in M31's halo show that M31's halo is remarkably similar to our own Milky Way!

The results
are made possible by technological advances in astrophysics. In this case, the Keck/DEIMOS multi-object spectrograph affixed to the Keck-II telescope possesses  the mirror size and light-gathering capacity to image stars that are very faint, and the  spectrographic sensitivity to obtain highly accurate radial velocities.
Special thanks to the Keck staff, and especially Greg Wirth who has greatly facilitated these complicated observations!


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 nov, 2006