Galaxy Formation and Evolution

We take a multi-faceted approach to study galaxy populations in the
local universe, using the completed Two Degree Field Galaxy
Redshift Survey (2dFGRS), the ``Millennium Run'' LCDM N-body
simulation, and a semi-analytic model of galaxy formation. Our
investigation covers both small and large scale aspects of the
galaxy distribution. This work can be broken into three sections,
outlined below. Using the 2dFGRS we explore the higher-order
clustering properties of local galaxies to quantify both (i) the
linear and non-linear bias of the distribution relative to the
underlying matter field, and (ii) the nature of hierarchical
scaling in the clustering moments of the galaxy distribution. This
last point is the expected signature of an initially Gaussian
distribution of matter density fluctuations that evolved under the
action of gravitational instability. We show in Chapters 2, 3, and
4 that the 2dFGRS higher-order clustering moments are indeed
hierarchical, which we measure up to sixth order for galaxies
brighter than M_bJ-5log10 h=-17 and which sample the survey volume
out to z~0.3. The moments are found to be well described by the
negative binomial probability distribution function, and we rule
out, at high significance, other models of galaxy clustering, such
as the lognormal distribution. This result holds in redshift space
on all scales where we obtain a good statistical signal, typically
0.5< R (h^-1 Mpc) < 30 (i.e. from strongly non-linear to
quasi-linear regimes). Interestingly, we find that the moments on
larger scales can be significantly altered by two massive
superclusters present in the 2dFGRS. The skewness of the galaxy
distribution is found to have a weak dependence on galaxy
luminosity. We show that a simple linear biasing model provides an
inadequate description of the higher order results, suggesting that
non-linear biasing is present in the clustering moments of the
2dFGRS. The large-scale distribution of structure within the 2dFGRS
allows us to study the properties of the galaxy population as a
function of local environment. In Chapter 5 we measure the
luminosity function of early and late-types galaxies in survey
regions ranging from sparse voids to dense clusters to reveal the
dominant population in each. Fitting each luminosity function with
a Schechter function allows us to quantify how the bright and faint
populations transform with changing density contrast. We find that
(i) the population in voids is dominated by late types, with a
noticeable deficit of intermediate and bright galaxies relative to
the mean, and (ii) cluster regions have an excess of very bright
early-type galaxies relative to the mean. When directly comparing
faint early and late type galaxies in void and cluster regions, the
cluster population shows comparable abundances of both types,
whereas in voids the late types dominate by almost an order of
magnitude. Of interest to many galaxy formation models is our
measurement that reveals that the faint-end slope of the overall
luminosity function depends at most weakly on density environment.
Finally, in Chapter 6, we develop a self-consistent model of galaxy
formation and couple this to the Millennium Run LCDM N-body
simulation. This simulation represents a significant step forward
in both size and resolution, allowing us to follow the the complete
evolutionary histories of approximately 20 million galaxies down to
luminosities as faint as the Small Magellanic Cloud in a volume
comparable to that sampled by the 2dFGRS. In our galaxy formation
model we supplement previous treatments of the growth and activity
of central black holes with a new model for `radio' feedback from
those active galactic nuclei that lie at the centre of a
quasistatic X-ray emitting atmosphere in a galaxy group or cluster.
With this we can simultaneously explain (i) the low observed mass
drop-out rate in cooling flows, (ii) the exponential cut-off at the
bright end of the galaxy luminosity function, and (iii) the fact
that the most massive galaxies tend to be bulge-dominated systems
in clusters and contain systematically older stars than lower mass
galaxies. This success occurs because static hot atmospheres form
only in the most massive structures, and radio feedback (in
contrast, for example, to supernova or starburst feedback) can
suppress further cooling and thus star formation without itself
requiring star formation. Matching galaxy formation models with
such observations has previously proved quite challenging.