The star formation histories, masses, and morphologies of galaxies are well known to be strongly dependent on their environment, with massive, passively evolving spheroids dominating cluster cores, while in field regions, galaxies are typically low-mass, star-forming, and disk-dominated. These correlations between galaxy properties and their local environment have been quantified through the classic morphology-density and SF-density relations. However, despite much effort, it still remains unclear whether these environmental trends are: (i) the direct result of the initial conditions in which the galaxy forms, whereby massive galaxies preferentially form earlier in the highest overdensities in the primordial density field; or (ii) produced later by the direct interaction of the galaxy with one or more aspects of the environment. Several physical mechanisms have been proposed that could cause the transformation of galaxies such as ram-pressure stripping (where gas in the galaxy is stripped by the "wind" produced by the galaxy's passage through the intra-cluster medium), mergers, galaxy harassment (repeated close, high-velocity encounters with other cluster galaxies), and suffocation (where just the diffuse gas in the outer galaxy halo is stripped, preventing further accretion of gas onto the galaxy). Much attention has also been focussed on galaxy-scale feedback processes, for example quenching of star-formation by extreme feedback from quasar winds which may expel all the cold gas from the galaxy, or via less-extreme "radio feedback" from low-luminosity AGN, which may prevent further cooling of gas.

The relative importance of these processes on galaxies may also depend crucially on the mass of the galaxy in question, as AGN feedback should become increasingly effective with galaxy mass as a result of the tight correlation between black hole and bulge masses, while low-mass galaxies with their shallow potential wells should be more susceptible to disruption by environmental processes. To this end we have been looking at how star-formation within galaxies depends on both their local environment and mass, in particular attempting to distinguish between galaxy-scale and external quenching processes, using the spectroscopic data from the SDSS, firstly for galaxies in the vicinity of the A2199 supercluster region, and later for the entire SDSS DR4 dataset.

• Galaxy mean stellar ages as a function of spatial position in the A2199 supercluster environment, for dwarf (left plot) and giant galaxies (right). In the above plots we have estimated the mean stellar age of each galaxy in the A2199 supercluster based on its spectral indices, and these are shown by symbols of varying colours from blue for galaxies dominated by young stars (age < 3Gyr) to red for galaxies dominated by evolved stellar populations (age > 8 Gyr). The black contours indicate the local luminosity-weighted surface density of supercluster galaxies. We see that in the groups and clusters, both dwarf and giant galaxies have predominately old stellar populations. In the field however, the stellar ages of dwarf and giant galaxies are quite different. Whereas giant galaxies show a complete interspersed mixture of old and young populations in the field, dwarf galaxies in these environments are exclusively dominated by young stars.

• The fraction of galaxies which are currently passively-evolving (EW[Halpha]<2A) as a function of both luminosity and local environment. In high-density regions corresponding to cluster and group environments, galaxies are predominately passively-evolving independent of luminosity. However in the rarefied field, the fraction of passively-evolving galaxies is a strong function of luminosity, dropping from ~50% for M* galaxies to zero by Mr=-18.

The dominant processes that quench star-formation in galaxies depend crucially on their masses. For massive galaxies, the gradual SF- and morphology-density relations which extend well outside the cluster virial radii, and the finding of many passive ellipticals in the isolated field, indicate the star-formation histories and morphologies of massive galaxies are largely defined by their build-up through mergers and the probable consequent feedback from AGN. In contrast the star-formation histories of dwarf galaxies appear completely defined by their local environment, as passive dEs are ONLY found as satellites to massive halos, whether that be a cluster, group or massive galaxy. This implies that internal processes (e.g. merging, AGN/SN feedback, gas exhaustion through star-formation) are unable to shut off star-formation in dwarf galaxies. Instead dwarf galaxies arrive in dense environments still gas-rich and are transformed in-situ by processes such as ram-pressure stripping and harassment, and indeed observationally most star-forming dwarf galaxies in local clusters are currently being transformed. The ultimate consequences of these processes are to transform infalling gas-rich low-mass spirals into passively-evolving dEs, but their immediate effects may well be to induce starbursts. We have found observational evidence for infalling galaxies undergoing starbursts when they encounter the cluster environment for the first time, and are now focussing on understanding what processes may be responsible for triggering these starbursts, and quantifying the importance of triggered starbursts for galaxy evolution within clusters.

Researchers: Chris Haines, Somak Raychaudhury, Smriti Mahajan