The magnificent Andromeda Galaxy!

The division of Andromeda's outer sprinkling of stars into giant rotating disk and and pressure supported halo components:

TOP: The velocities of 10,000 stars used for studying the giant disk are shown in the frame of the rotating inner disk of M31. The dominant population of stars lag the rotation of gas as traced in the radio by the 21cm HI fine structure line, very similar to that found for stars in the inner disk. The remainder of stars form the stellar halo.

(Right panel) Radial  velocity as  a function  of major  axis  distance (from
Ibata et al. 2005). The upper  panel shows the Heliocentric radial velocity of the stars, with the dashed line marking the systemic velocity of M31. The bottom panel shows the same data corrected for the expected disk rotation given by our model of M31's rotation. Now the dashed line marks the location of stars on circular orbits. In most of these fields the dominant population of stars is seen with velocities lagging the circular velocity by velocities between 50 km/s to ~100 km/s.

(Left panel) The stars which remain in the sample after windowing out all the disk stars we denote the halo component (from
Chapman et al. 2005). These stars can be analyzed field by field ... each is consistent (best fit maximum likelihood) with a velocity dispersion of a central velocity dispersion of 150km/s, and a decreasing velocity gradient of about 1km/s per kpc. The windowed sum of 827 of these stars are shown in the panel with a best fit Gaussian function overlaid in red: a convincing detection of the random motions in the outer stellar halo of M31!

One mystery about the halo (
Irwin et al. 2005) concerns the apparent similarity in metallicity between the giant rotating disk and the flatter outer profile identified as the stellar halo. One would expect the halo component to have lower metallicity if it formed from stars in the early Universe. We can check the metallicity of our halo component underlying the disk using the Calcium Triplet equivalent width (the strength of the Calcium absorption lines). By averaging the spectra in each of our 54 halo fields observed with Keck/DEIMOS, we find a metal-poor [Fe/H] = -1.4, significantly lower than found in the disk (Fe/H] = -1.0 Ibata et al. 2005). This suggests that the global colours of all M31 stars through these outer region wash out the metallicity difference in the weak halo component which underlies the stronger disk. The spectral metallicity is illustrated in the figure below for several of our sample fields, 10-30 stars averaged.

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

Understanding it all:

The History of the Universe and Merger Scenarios for the Giant Disk

history of the Universe

(Top) From the Big Bang at left, the hierarchical growth of structures in the Universe are illustrated from the photonic imprint in the Cosmic Microwave Background (CMB) which reveals the seeds of dark matter halos that will suck in the galaxy fragments and gas. These overdensities will build the galaxies like M31 and the Milky Way. Their sites in the web of structure to the right are represented by the medium sized bright loci. The largest bright concentrations will turn into even larger galaxy associations than the M31/MW local group by the present day.
(Bottom) An illustration of how disk galaxies are thought to form from smooth accretion at high redshifts (z>5), merge in a gas rich collision scrambing the stars motions into an spheroidal shape, and then smoothly accreting more gas to reform a disk again.

Three ways that the giant rotating disk of Andromeda might form (outlined from the giant disk paper).
(Left)  Formation induced by a single large merger.
The stellar population of the accreted galaxy would end up populating the bulge or other spheroidal component of M31. The interaction itself could have triggered star-formation on a massive scale in the pre-existing gaseous disk of M31 (maybe like the gas-rich submm galaxies), with gas accreted from the satellite contributing to the enrichment. This possibility could explain the disk-like structure and kinematics, and has the testable prediction that its constituent stars should be no older than the interaction itself.
(Middle) Formation from stars accreted in a single large merger.
This scenario would be similar to the previous one. Here we envisage the assimilation of a whole galaxy in a
single merging event. The ``major merjer'' would have globally reshaped M31. Thin disks are easily heated up by mergers so this scenario makes a testable prediction that stars in the inner disk of M31 that formed before the merger should have a much larger scale-height than the younger thin disk.
(Right) Formation by the accretion of many small sub-galactic structures.  
This scenario would explain naturally the lumpy sub-structure seen at the edge of the extended disk, as well as the differences in the velocity dispersion between fields. However, the chemical homogeneity of the material argues against this possibility, since the different accreted galaxies would likely have a range of different metallicities, depending on their mass and star-formation history. A further problem with this scenario is the disk-like kinematics: a substantial mass is required for dynamical friction to affect the orbit of a galaxy satellite (>10^8 solar masses) which has to become circularised before being disrupted in order to give rise to a final structure with disk-like kinematics. Another problem is the disk-like distribution of debris. While this may be the most obvious way to get the observed messy morphology of M31's outer regions, forming the extended disk observed in M31 in this way will certainly be a challenge for modellers.

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Last revised: 25th of May, 2005