Over the last years, evidence increasingly points to the Universe being dominated by dark matter, and only 5 % is ordinary baryonic matter. Unlike dark matter, however, baryons — such as neutrons and protons — are detectable. In addition, their transformations and associated energy release are key elements for tracing the evolution of galaxies and their stars. EU-funded scientists posited that most of the baryonic matter lies in the little-explored intergalactic medium. They initiated the 'Diffuse baryons in space' (DIFFUSE BARYONS) project to examine the properties of baryonic matter in the halos of galaxies. As a depository of all matter ejected from galaxies, halos carry within them unique information about the history of energy and metal production. Because of their low brightness, a detailed study of their physical properties was possible only with large surveys, both from the ground and space. DIFFUSE BARYONS researchers explored the absorption and emission properties of cool gas at the outskirts of galaxies. By observing pairs of quasars and galaxies, they found traces of neutral hydrogen and ionised magnesium extending out to distances of up to 100 kiloparsecs. Halos of cool gas were more extended around active galaxies. The higher the activity in the galaxy, the more extended the envelope around the galaxies seemed to be. This is a new signature of star formation activity in galaxies accompanied by outflows ending up in the extended halo and even escaping into the intergalactic medium, enriching it with metals. Taking their research a step further, DIFFUSE BARYONS looked at the effect that the high gas density close to quasars has on the dynamics of gas outflows further away in the intergalactic medium. Their calculations showed that the thermal state of gas is very different to that usually assumed at large scales. The coexistence of a wide range of ionisation levels is expected to have major implications for interpreting astronomical data. With the financial support of the European Commission, the astrophysics group at the University of Haifa, Israel, was greatly expanded during the lifetime of the DIFFUSE BARYONS project. Several graduate students, postdoctoral fellows and research assistants worked together to deepen our understanding of diffuse intergalactic gas under extreme astrophysical conditions.
Baryons, galaxies, dark matter, space, quasars, star formation