|
|  |
Galaxies are the basic structural units of the Universe and are
composed of a minority component of normal matter, i.e. stars and gas,
and a majority component of cold `dark matter -- the presence of which
is inferred only from its gravitational effects. Over the past
decades, astronomers have strived to construct a model of galaxy
formation which can simultaneously account for large scale
distribution of galaxies in the Universe, as well as the detailed
properties of individual galaxies. Supercomputer simulations have
enabled theorists to follow the evolution of matter in the Universe
from a short time after the Big Bang until the present epoch (see
 The largest N-body simulation of the
universe). These simulations indicate that galaxy formation in
the presence of cold dark matter occurs from the bottom up; that is,
the first galaxies to form are small dwarf systems and these
subsequently merge together to form progressively larger systems. In
this scenario, galaxy formation can be viewed as a continual process.
Large galaxies, like our own Milky Way, are predicted to have
cannibalized roughly a hundred small dwarf galaxies which have fallen
within their gravitational influence (a process which is expected to
continue to the present day). During destruction, stars from the
satellite galaxy are pulled out into long tidal streams which can
remain coherent for several billion years and leave a long-lasting
observable signature of the mass accretion (see Figure 1).
This picture of galaxy formation makes a number of well-defined
predictions which can be tested against observations. There is a
particular need to test the model on small 'galactic-sized' scales
where complicated astrophysics, such as star formation and feedback,
play an important role alongside gravity. The most detailed
information we can gain about how a galaxy has formed and evolved
comes from resolving it into individual stars and studying the
distributions of these stars in space, as well as in age, heavy metal
content and velocity. Old and intermediate-age stars are especially
useful since they probe conditions more than 5 billion years ago when
they were born and when galaxies were in an early stage of evolution.
Studies of this `fossil record' have traditionally focused on our own
Milky Way, where the quantity and quality of available observations is
unsurpassed. With new telescopes and instruments, it is becoming
possible to extend this type of work to other nearby galaxies. This
is of great importance in order to understand both how representative
our Milky Way is and how the nature of the assembly process varies
with galaxy mass, morphological type and local environment.
Scientists at the Max-Planck-Institute for Astrophysics are
participating in an international collaboration to explore the fossil
record of galaxy formation and evolution in our two nearest giant
neighbours, M31 and M33. Located at roughly 800 kiloparsecs (2.5
million lightyears) from us, these galaxies are close enough to
resolve into individual stars with ground-based telescopes such as the
Isaac Newton Telescope on La Palma. The researchers are conducting
wide-field imaging surveys of red giant branch stars in the faint
outer regions of these galaxies and finding some surprising results.
Figure 2 shows that the stellar distribution in the far outer regions
of M31 is distinctly inhomogeneous. There is a large stream of stars
extending to the south-east; distance estimates indicate this stream
begins 100 kpc behind M31. There is also a loop of stars which appears
to emanate from the dwarf satellite system NGC205, as well as a
variety of other stellar overdensities. The Advanced Camera for
Surveys on board the Hubble Space Telescope has been used to obtain
detailed follow-up observations of many of these features and reveals
distinct variations in the age and metallicity mix of the constituent
stars. Taken together, these results indicate that the outer regions
of M31 contain tidal debris from at least one, and possibly more,
cannibalized dwarf galaxies. On the other hand, an identical study of
the lower mass galaxy M33 reveals a very smooth stellar distribution
at large radius and no obvious signs of any substructure (see Figure 3). Contrary to
simple expectations, M33 appears to be a system which may not have
accreted any significant (luminous) mass for much of its lifetime.
Understanding the origin of the differences between M31 and M33, as well as which
behaviour is more typical, will require both additional observations and
improved theoretical predictions. The MPA researchers have already been
awarded observing time with ESO's VLT and Subaru's SuprimeCam to
conduct similar observations for a larger sample of nearby galaxies.
Annette Ferguson
Further Information:
 Isaac Newton Group of Telescopes
The Hubble Space Telescope
The Subaru Telescope
A. M. N. Ferguson, M. J. Irwin, R.A. Ibata, G. F. Lewis & N. R. Tanvir:
Evidence for Stellar Substructure in the Halo and Outer Disk of M31, 2002, AJ, 124, 1452
A. M. N. Ferguson, R. A. Johnson, D. C. Faria, M. J. Irwin, R. A. Ibata, K. V. Johnston,
G. F. Lewis & N. R. Tanvir:
The Stellar Populations of M31 Halo Substructure, 2005, ApJL, submitted
A. M. N. Ferguson, M. J. Irwin, R.A. Ibata, G. F. Lewis, A. McConnachie & N. R. Tanvir:
A Global Map of the Stellar Populations In and Around M33, 2005, MNRAS, submitted
|
