Galaxy Evolution

Some Recent Results:

We study how galaxies evolve using varied and complimentary methods: Detailed studies of local galaxies allow the reconstruction of their formation history, whilst observing distant galaxies provides a direct measure of how galaxy populations evolve with time.

The Local Universe

The formation of hot stellar systems (bulges and elliptical galaxies) is the centrepoint of much of our work, forming a link to the dynamical studies described here. In particular, we find quite different properties for 'classical' and 'pseudo'-bulges , consistent with, respectively, hierarchical, merger-driven and secular, internal-torque driven origins. We find that the light profiles of elliptical galaxies are consistent with a merger history in which black holes and AGN-feedback have had dramatic impact on the observed galaxy properties.

We also study nearby disks: Detailed observation of our neighbouring galaxy Andromeda (M31) have provided tight constraints on the dust distribution and stellar populations in the disk . Meanwhile, the outer profiles of disks fall into different categories, putting constraints on the dynamical and star formation processes which apply at low density.

The Higher Redshift Universe

In the distant Universe our work focuses on large samples of galaxies from deep surveys, and more complete samples of individual clusters and groups. Together with local samples these provide an overview of how galaxy populations populate their dark matter haloes, and have evolved both structurally and in their stellar populations across most of cosmic time.

The most evolved galaxies are found in the cores of rich, local clusters. We examine how these populations have evolved over more than 50 % of cosmic time, utilising photometric and spectroscopic data from the EDisCS project (ESO Distant Cluster Survey). We examine the evolution of mass to light ratio for early-type cluster galaxies, utilizing the fundamental plane evolution and environmental dependence and, importantly, accounting for size evolution. We have also examined how the morphological composition of the galaxy population trace their underlying haloes, utilizing a well-studied sample of spectroscopically-selected groups at z~0.4. Whilst bright elliptical galaxies tend to live in halo cores but are otherwise ubiquitous, lenticular (S0) galaxies exist with similar fractions in groups and clusters, but are relatively absent at lower density. This provides hard constraints for studies of morphological evolution. We are following up X-ray detected groups at z~0.2-0.9 with highly complete spectroscopy to understand how a bright IGM relates to group and galaxy properties. High quality photometric redshifts have allowed us to probe the galaxy population to unprecidented depth up to z~1 within the COSMOS survey. The resultant stellar mass function requires a 'bimodal' bright plus faint fit of both passive and star forming populations, mass dependence of galaxy formation efficiency. Finally, we have developed a multiscale density framework to describe galaxy environments in detail, but without the assumptions which go into the construction of group catalogues.