Structure of the Galaxy
Modelling the Galaxy
James BinneyMost of the light in the Universe comes from stars located in galaxies like that in which we live. In this sense our own Galaxy is typical of the galaxies that dominate the universe, and it would be a large step forward in our understanding of the Universe as a whole if we had a complete knowledge how the Galaxy is structured, and how it came to be the way it is. Dynamical models of the Galaxy are the key to gaining this knowledge.
It is helpful to imagine that the Galaxy is made up of components. The
dominant component at the location of the Sun is the stellar disk, of
which the Sun is a tiny part. Further in, the Galaxy is dominated by
the bulge/bar, while further out the dominant component is believed to
be the dark halo, about which we know very little. The dark halo is
especially problematic for a Galaxy modeller because it has so far
only been detected through its gravitational field. It is thought to
be made up of a kind of elementary particle that has never been
detected on Earth. The properties of these particles can only be
speculated about, so it is hard to design an experiment to detect
them. Perhaps for this reason, efforts to detect the particles that
make up the Galaxy's dark halo as they stream through the Earth have
so far been unsuccessful - but they could bear fruit any day!
A major goal of a Galaxy modeller is to map the gravitational
field of
the dark halo, from which we can infer the dark halo's mass
distribution. This has to be done
by first mapping the gravitational field of the whole Galaxy, and then
estimating the gravitational fields of the visible components,
principally the bulge/bar and the disk. The difference between the
gravitational field of the whole Galaxy and the combined fields of the
visible components yields the field of the dark halo.
The
gravitational field has to be measured by modelling the dynamics of
objects that move in the field. This is most easily done in the case
of gas clouds, which may be assumed to move on closed orbits. The
simplest closed orbits are circles in an axisymmetric potential, and
our knowledge of the Galactic potential has mostly been obtained from
observations of gas clouds on the assumption that they move on
circular orbits in the Galactic plane. Notable features of the data
for the Galaxy's interstellar gas are incompatible with the hypothesis
that clouds move on circular orbits. These features are neatly
explained if the Galaxy interior to about 3 kpc is dominated by a
rotating bar (Binney
et al 1991), and over the last ten years a
sustained effort has been made to interpret observations of
interstellar gas on the assumption that clouds move on closed orbits
in the rotating non-axisymmetric field of the bar.
Microlensing surveys provide another important line of evidence that inside the solar circle the Galaxy's gravity is dominated by stars rather than dark matter, contrary to the predictions of Cold Dark Matter cosmology. Some years ago we argued that the current estimates of the microlensing optical depth tau (the observational signature of the amount of stellar mass) were incompatible with other measures of the gravitational field (Bissantz et al 1997; Binney et al. 2000). Subsequently estimates of tau have come down to values similar to those predicted by models in where there is negligible dark matter in the central few kpc of the Galaxy. At the present time we lack a convincing upper limit on the amount of dark matter that can be tolerated in the inner galaxy.
It's tremendously important to probe the gravitational
potential away from the plane of the disk. This is hard to do because
away from the plane gas can't settle to closed orbits. One possibility
is to exploit tidal streams - ribbons of material pulled from orbiting
bodies such as globular clusters - which reveal a section of an orbit
through the halo. Unfortunately, very few narrow tidal streams are
known.
There is a huge and every-growing body of data on the
kinematics of individual stars - we are collaborators in the ongoing
RAVE survey of radial velocities. To make effective use of these data
to constrain the Galactic potential, we need a dynamical model of the
Galaxy in which each component has a well-defined distribution
function. In September 2005 we hosted a workshop
at which astronomers from around Europe concerted plans to build such a
model in readiness for the completion of the current radial-velocity
surveys. A decade ago we develped a unique approach to galaxy modelling
- the torus programme - and we plan to play a leading role in using
this technique to interpret upcoming galaxy surveys and eventually
ESA's cornerstone mission Gaia.
We played a significant role in the scientific exploitation of
ESA's last astrometric mission Hipparcos (Dehnen
& Binney 1998; Dehnen
1998), and we showed that a significant fraction of
solar-neighbourhood stars are concentrated into streams. It has since
been shown that these streams contain stars born at different
times, so they are not dissolving star clusters. A likely possibility
is that they reflect the shuffling of stars at the corotation
resonances of transient spiral structure that we predicted a few years
ago (Sellwood
& Binney 2002). More work needs to be done to see whether this
conjecture is correct, and the discover whether an analogous phenomenon
radially mixes the interstellar medium. If so, a major revision of our
understanding of the chemical evolution of galactic disks is required.
The Galaxy's interstellar medium poses two long-standing puzzles: (i) why does it form a warped rather than a flat layer outside the solar radius, and (ii) why are clouds seen at unexpected velocities along many lines of sight. We are working on both these problems in the wider extragalactic context.