Starbursts in clusters

A variety of different physical processes have been proposed to explain the strong correlations seen in the local Universe between the properties of galaxies and their surrounding environment. In particular, the strong concentration in high-density environments, such as cores of clusters of galaxies, of passive, metal-rich spheroidal galaxies (ellipticals and lenticulars, or S0s). This association of these pressure-supported stellar systems with high-density regions today suggests that dynamical interactions may in part responsible be responsible for these trends, but whether these are the sole explanation and exactly when these processes act are still unknown.

Efforts to infer the star-formation histories of local elliptical galaxies from analysis of the spectra of their stars suggests that they formed most of their stars 8-11 billion years ago, at redshifts above z ~ 1. Moreover, the fact that these galaxies are metal-rich also suggests that this activity may well have been obscured by dust. These analyses also indicate that more massive ellipticals having older stellar populations. Thus the available evidence suggests that young spheroidal galaxies in regions which collapse to become clusters, must have formed their stars faster (and so at a higher star-formation rates) than those in less-dense regions. Thus, while clusters today are less active than the surrounding field, their progenitor structures at earlier times are actually expected to be more more active than surrounding lower-density regions.

These suggestions are supported by the discovery in observations in mid- or far-infrared, or submillimetre wavebands of galaxy clusters at z ~ 0.5 - 2, of a population ofdusty strongly star-forming galaxies. These very actively star-forming systems are almost completely absent from such high-density environments in the local Universe, but their number densities exhibit very strong redshift evolution which means that the cores of z < 1 clusters host significant numbers of luminous, but dusty, star-forming galaxies (an example of which is shown in the Figure, where the contours highlight strongly star-forming galaxies detected through their luminous submillimetre emission).

Our work focuses on bringing together dynamical and multiwavelength tools to investigate the properties of strongly star-forming galaxies in high density environments from z ~ 0.5 to z ~ 2.0 and beyond. We use a combination of dust-insensitive selection and three-dimensional imaging spectroscopy to first select samples of starbursts in these environments and to then disentangle and understand their dynamics, distribution of star formation and gas masses. By comparing our observations to predictions of numerical simulations of galaxy formation we aim to distinguish between the physical mechanisms responsible for the rapid evolution in this population.

Staff involved with this project at Durham include Richard Bower, Mark Swinbank. and Ian Smail.