Across the Universe, galaxies are distributed along what’s called the cosmic web, a complex network of filaments made up of ordinary and dark matter. And where those filaments intersect, galaxy clusters – collections of hundreds or even thousands of galaxies bound to each other by the force of gravity – tend to form. They are the biggest and densest clusters in the Universe and are the subject of much research by astrophysicists. But precisely how filaments contribute to galactic evolution is still poorly understood.
To address this open question a team of researchers, including F. Combes and P. Salomé (LERMA, Paris Observatory) has first characterized the large scale environment of the nearest galaxy cluster, Virgo, that is the benchmark cluster in the local Universe. By using several up-to-date databases the authors built the largest catalog of ∼7 000 galaxies in and around Virgo, up to ∼ 12 virial radii in projection. In a first study, the team releases the full catalog of galaxies and their structural properties, as well as the associated environmental properties including 3D positions, distances from the filament spines, environmental densities. In a second study the authors build a mass complete subsample of 250 galaxies in the major filaments around Virgo, and follow them up in their gas content, star formation rate, morphological type and evolution stage.
Theirs is the largest study conducted to date on this topic, with a sample big enough for scientists to be able to precisely estimate their gas content – and especially the amount of cold, dense atomic hydrogen that stars are made out of. Measurements were taken using the decametric radio telescope in Nançay, France, and the IRAM-30m telescope in Pico Veleta, Spain.
A transitional environment
Filaments appear as an intermediate environment between the field and the cluster. If a cluster size is about several tens of million light years, filaments can be even ten times longer, while remaining quite thin, around a few million light years. The authors find that long filaments tend to be thin and with a low density contrast while shorter filaments show a larger scatter in their structural properties. Overall, filaments appear as a transitional environment between the field and cluster in terms of local densities, galaxy morphologies, and fraction of barred galaxies. Denser filaments have a higher fraction of early type galaxies, suggesting that the morphology-density relation is already in place in the filaments, before galaxies fall into the cluster itself.
The team finds a progression from field galaxies to filament and cluster ones for decreasing star formation activity and increasing fraction of galaxies in the quenching phase, increasing proportion of early-type galaxies, decreasing gas content, and increasing levels of gas deficiency. The authors thus demonstrate that processes that lead to star formation quenching are already at play in filaments. They depend mostly on the local galaxy density, while the distance to the filament spine is a secondary parameter. As the environmental density increases, the gas depletion time decreases, since the gas content decreases faster than the star formation rate.
This suggests that gas depletion significantly precedes star formation quenching. By comparison with comparable samples of field and cluster galaxies the authors show that early type galaxies do not only live in the dense cores of cluster, but they are present in significant fraction already in filaments, and with suppressed levels of star formation, while they are rare in isolation. These results open new challenges for theoretical models to explain the formation of the most massive galaxies in the Universe, and their co-evolution with their large scale structure environments.
References
Virgo Filaments : I- Processing of gas in cosmological filaments around Virgo cluster, Castignani, G., Combes, F., Jablonka, P., Finn, R.A., Rudnick, G., Vulcani, B., Desai, V., Zaritsky, D., Salomé, P. : 2022 A&A 657, A9
https://arxiv.org/abs/2101.04389
Virgo Filaments : II- Catalog and First Results on the Effect of Filaments on galaxy properties, Castignani, G., Vulcani, B., Finn, R. A., Combes, F. et al. : ApJ in press,
https://arxiv.org/abs/2110.13797