Compact groups of galaxies: their nature, properties and assembly history
by
Gary Mamon(Institut d'Astrophysique de Pari)
→
Europe/Rome
Description
ABSTRACT: Compact groups of galaxies are selected to be nearby, isolated, very dense systems of 4 or more galaxies in a range of 3 magnitudes. Their high densities and low velocity dispersions make them the ideal sites for galaxy interactions and mergers, and the present-day massive galaxies may have assembled within compact groups. Yet, half the galaxies in the popular, visually selected, Hickson catalog are spiral, with no statistical signs of galaxy mergers or luminosity segregation. This has led to a debate on whether compact group samples are strongly contaminated by chance alignments of galaxies along the line of sight. I will address the nature of compact groups through the use of mock compact group catalogues extracted from the outputs of semi-analytical models of galaxy formation, themselves run on the halo merger trees of the Millennium cosmological simulations. The use of such mock compact groups allows the quantification of the fraction of compact groups that are chance projections in comparison to those that are physically dense in 3D. The mock compact groups are roughly 10 times more frequent than observed, suggesting a strong incompleteness of the Hickson catalog. I will present the properties of a new, complete, sample of compact groups, extracted from 2MASS in a similar, but automatic fashion as the Hickson catalog. I will highlight the strong differences between the 2MASS-selected compacts groups and the Hickson sample and ask whether star forming galaxies have normal specific star formation rates, are bursty, or anemic. Finally, I will study the mass assembly of compact groups, using the semi-analytical models to compare different possible formation channels.
