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The interface between galaxies and galaxy groups

Final Activity Report Summary - GALAXY GROUPS (The interface between galaxies and galaxy groups)

Small groups of galaxies represent the most common galaxy environment and contain the bulk of all matter in the nearby Universe. They are also sites of strong galaxy evolution, providing an environment where galaxies can interact with one another and with the hot, X-ray emitting gas pervading the space between the galaxies in groups. Despite these important features, the global properties of groups are relatively poorly known, and much remains to be understood regarding the co-evolution of groups and their galaxies and the processes governing it.

The main objective of this project was to improve our understanding of the formation and evolution of galaxy groups and the galaxies they contain. The project used observational data collected by ground- and space-based telescopes to investigate these issues, combined with results of numerical calculations including computer simulations of structure formation in the Universe.

Groups of galaxies can be identified on the basis of the hot X-ray gas between their galaxies, or by the requirement that a certain number of galaxies are close together on the sky or in three-dimensional space. It was long suspected that groups selected on the basis of the first criterion may not be representative of the overall group population, thus only constituting the 'tip of the iceberg'. A detailed X-ray and optical study of nine groups (to be expanded to 25) was carried out, confirming that a high fraction of optically identified groups appeared to contain only modest quantities of hot, X-ray emitting gas. This could be telling us that most groups we see today are still in the process of being formed, so their gas is still too rarefied or cold to emit X-rays.

A major study of the temperature and heavy element abundances in a sample of 15 galaxy groups was also concluded. This study provided the first demonstration that the global proportion of heavy elements, such as iron and silicon, in the hot gas in groups was lower than that in more massive galaxy clusters, probably at least in part because some of these elements were lost from groups altogether at very early stages of group formation. This had important implications for our understanding of the origin and amount of heavy elements in the nearby Universe, and for theories of structure formation on group and cluster scales.

The work also included the discovery and detailed study of the first clear example of a spiral galaxy being stripped of its interstellar gas by the hot intergalactic medium in a group, apparently because of a combination of:
1. star formation activity ejecting matter from the galaxy; and
2. the ram pressure felt by the galaxy as it moved through the hot intragroup gas.

This combination could be an important mechanism for the formation of so-called lenticular galaxies, the origin of which is still not understood. A follow-up project was investigating the role of this mechanism in a larger sample of eight groups where the galaxy configuration was particularly compact and where the galaxies had already lost large amounts of gas. The results suggested that other mechanisms, such as galaxies interacting strongly with one another under their mutual gravity, should also play a role for driving gas out of galaxies in groups. Both studies provided important new insight into the processes that dictated the evolution of galaxies in groups and clusters.

A final result to mention was the first, albeit tentative, discovery of a giant 'atmosphere' of hot gas surrounding a massive spiral galaxy. Models of galaxy formation predicted that large spiral galaxies should be immersed in halos of hot gas left over from the galaxy formation process. Hot gas was previously detected around spiral galaxies in which vigorous star formation was ejecting matter from the galaxy, but until now hot halos because of infall of intergalactic matter were not detected. Our discovery suggested that such halos do exist but are very faint.