GAMA: towards a physical understanding of galaxy formation

Large international surveys of the low-redshift universe, such as the 2-degree Field Galaxy Redshift Survey (2dFGRS: Colless 2001, 2003) and the Sloan Digital Sky Survey (SDSS: York 2000; SDSSDR6: Adelman-McCarthy 2008) have transformed our quantitative understanding of galaxies, galaxy populations and large-scale structure. This in turn has contributed directly to the emergence of a concordance cosmological model (e.g. Spergel 2003, 2007; Cole 2005): a flat, dark-energy dominated collisionless cold dark matter model (Lambda-CDM) which provides both the bedrock and blueprint for our galaxy formation models. Known as hierarchical-CDM, this structure-formation model provides a physically motivated, fully numerical and verifiable description of the observable universe on Mpc (and greater) scales. However, on the scales of clusters, groups and galaxies, the picture is less clear. There are significant inconsistencies between the basic mechanism (the hierarchical merging of dark matter haloes) and the empirical evidence (in particular the complex yet fragile sub-structure of individual galaxies); exactly how a merger-based process can give rise to such fine sub-structure lies at the heart of the debate. It is on these scales (between 1 kpc and 1 Mpc) that the poorly understood interplay between the dark and baryonic matter becomes crucial as the dark matter haloes virialize and merge, and the baryons decouple (from the dark matter) and crystallize into observable galaxies. The complexity of the physics and the range of length, time and mass scales involved (from atomic to cosmological) prevents full numerical modelling at this time. As a consequence, this regime can now be investigated only by phenomenological modelling which, by definition, is guided and informed by empirical datasets (e.g. Baugh 2006).