Andromeda's Stellar Halo Shows Galaxy's Origin
to Be Similar to That of Milky Way
Details on the
discovery of Andromeda's metal-poor halo
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.
Surface Brightness Images from the models of Bullock & Johnston
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
[The eye can just pick out dark blue at 30 Mag/arcsec^2]
Click on thumbnail for larger image.
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.
here for movie (Credit, J. Bullock and K. Johnston
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.
here for movie (Credit, M. Steinmetz)