The division of Andromeda's outer sprinkling of
stars into giant rotating disk and and pressure supported halo
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
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
(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
(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
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
(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.
Last revised: 25th of May, 2005