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