From X rays to far infraredgalaxy cluster ZwCL0024+1652 under the multiwavelength limelight

Laburpena

Clusters of Galaxies are the largest gravitationally bound systems known. Discovered by Charles Messier in the XVIII century, they started to be systematically studied two hundred years later, when Abell and Zwicky undertook a series of surveys to identify concentrations of galaxies in the accessible Universe. These initial studies concluded that clusters of galaxies were formed by objects with the same visual colors and used them to establish memberships. This has been since then one of the biggest issues in this field: the accurate separation of cluster population versus projected foreground or background objects. One other issue is to establish the dynamical status of both the cluster itself and the sources within. From the latter, the former can be inferred, even by crude assumptions on the typical mass of the galaxies, since the velocity dispersion of the members and the cluster radius are linked via the Virial Theorem. However, early observations from spaceborne telescopes discovered significant extended X–ray emission from the cluster cores that was soon identified as Bremsstrahlung radiation in the di use intracluster plasma. The detection of such hot gas led to the calculation of the potential well needed to keep it bound to the system and the amount of gas required. Both estimates, from optical and X–ray data disagreed by up to (and even beyond) 70% in some cases. At the same time, the characteristics of the cluster population were studied and compared to field galaxies. It was found that cluster members favoured elliptical morphologies, larger masses and red colours, versus the dominant fraction of blue mid size spirals in the field. Moreover, the fraction of blue galaxies was found to vary along the clustercentric distance and with redshift, increasing this blue fraction directly with both. It was established that clusters of galaxies harboured much more mass that that directly observable in optical wavelengths and that their members had undergone or were undergoing transformations that made their evolutionary path diverge from their counterparts in the field. To appropriately address those issues a key observable was demanded: accurate redshifts. However, that was found hard to get. On the one hand, photometric redshifts by themselves lack of the precision needed to establish whether a galaxy is within the cluster or not. On the other, spectroscopic redshifts are extremely demanding in terms of observation time and the selection of objects imply some a-priori criteria that may significantly bias the result, focusing in typical cluster members and eventually overlooking objects in the ends of the distribution function of luminosities and colors...