Max Planck Institute for Astronomy - Annual Report 2005

72
III.3 The Galaxy-Dark Matter Connection
Halo occupation statistics, which describe the statistical
link between galaxies and their dark matter
haloes, play a crucial role in modern astrophysics.
They provide insights into the complicated physical
processes associated with galaxy formation, and they
allow us to use the observed distribution of galaxies
to put tight constraints on cosmological parameters. A
new Nachwuchsgruppe at the MPIA is involved in developing
new methods to constrain these occupation
statistics, and to use them to constrain both cosmology
and galaxy formation.
Understanding the origin and evolution of galaxies is
one of the most fascinating unsolved problems in astrophysics.
It involves physical processes from the scale of the
Universe itself (i.e., cosmology) down to the micro-physics
that describe the formation of individual stars. Currently
popular cosmologies consider a Universe that consists of
baryonic matter, cold dark matter and some form of vacuum
energy. Shortly after the Big-Bang, quantum processes
are thought to have created small perturbations in the matter
distribution (both the dark and baryonic matter), which
act as the seeds for structure formation. Since an overdense
region exerts a larger than average gravitational force on
its surroundings, there will be a net infall of material into
the overdense region. This causes the overdense region to
become more overdense, and thus to grow in amplitude.
At some point, the overdensity reaches a critical level at
which the perturbation starts to collapse. The associated
dark matter experiences what is called violent relaxation,
which results in the formation of a virialized dark matter
halo. At the same time, the baryonic material is shock
heated to high temperature while it settles in hydrostatic
equilibrium in the potential well of the dark matter halo.
Subsequently, atomic processes cause the gas to cool, thereby
radiating away its kinetic energy. As a result, the gas
falls to the center of the dark matter halo and collapses into
a dense condensation of gas, which ultimately experiences
star formation, and forms a galaxy. This picture, however,
is not complete, as dark matter haloes don't evolve as isolated
systems. Instead, their formation is hierarchical, in
that small haloes continuously merge together to build ever
larger haloes. The galaxies in these haloes either merge
together into a larger galaxy, or they survive as satellite
galaxies, orbiting in the background potential provided by
the dark matter.
Although this standard picture has been around for
almost three decades, we are still far from a proper understanding
of the intricate processes involved in transforming
the hot gas of the early Universe into the stars that comprise
the luminous galaxies of today. This is largely due to
the fact that many of the physical processes involved are
still poorly understood, in particular the »micro-physics«
associated with star formation and its associated feedback
on the interstellar medium.
In principle, we could learn a great deal about galaxy
formation if we could somehow determine the average relation
between halo mass and galaxy properties: e.g., how
many galaxies are there, on average, per halo, and how
does this scale with halo mass? How are galaxy properties
such as luminosity, star formation history or morphology
related to the mass of the halo in which they reside? These
halo occupation statistics describe the galaxy-dark matter
connection, which reflect the direct imprint of the various
physical processes associated with galaxy formation.
Constraining the statistical link between galaxies and dark
matter haloes therefore constrains the physics of galaxy
formation.
The halo occupation statistics are also of crucial importance
if we ever want to use the distribution of galaxies in
order to constrain cosmological parameters. An important
goal of modern day cosmology is to determine the distribution
of matter in the universe, which is strongly cosmology
dependent. Unfortunately, the majority of the matter is dark
matter, which is invisible. However, according to the galaxy
formation paradigm described above, galaxies reside
inside dark matter haloes, and we can thus use the light of
galaxies as a tracer of the dark matter mass distribution.
Unfortunately, galaxies are not a fair, unbiased tracer
of the mass distribution; if a certain region contains twice
as much light (from galaxies) than another region of the
same volume, this does not necessarily mean that it also
contains twice as much mass. This non-linear relation
between light and mass is called »galaxy bias«. It arises in
part from the fact that dark matter haloes themselves are
a biased tracer of the dark matter mass distribution. This
is easy to understand from the fact that dark matter haloes
form out of overdensities in the initial matter field. Haloes
therefore only trace overdense regions, not the underdense
ones. Fortunately, this halo bias is well understood, and for
a given cosmology we know fairly accurately how haloes
of a given mass are biased. The only missing ingredient
for specifying galaxy bias is therefore the link between
galaxies and dark matter haloes, which brings us back to
the halo occupation statistics.
In summary, a detailed description of the galaxy-dark
matter connection, in terms of the halo occupation statistics,
can provide useful constraints on both galaxy formation
and cosmology. In the following we describe some
highlights of our work that aims at establishing a self-consistent,
coherent picture of the statistical link between
galaxies and their dark matter haloes.