Galaxy groups within voids

Abstract

Context. Despite cosmic voids being vast and almost empty, galaxy aggregations do exist within them, although they are much sparser than in denser regions like walls, filaments, and clusters. The internal matter distribution within voids might have an impact on the properties and evolution of void galaxies. Aims. In this work, we aim to identify and characterise a sample of galaxy groups within voids in the local Universe (z < 0.08), taking into account the peculiarities of these vast and empty structures. Methods. The void galaxies used in this study were selected from a well-defined void galaxy sample, from which the parent sample of the Calar Alto Void Integral-field Treasury surveY (CAVITY) legacy project was drawn. To identify galaxy groups, we applied a friend-of-friend (FoF) ─like group finder algorithm to the selected sample, ensuring a certain degree of gravitational binding among group members. The same algorithm was applied to identify a control sample of groups not in clusters nor voids, referred as NCNV groups. Results. The catalogue of groups consists of 1367 physically bound groups, with a total of 3040 galaxies, plus 14 672 galaxy singlets. Most of the galaxies in voids are singlets (59%). In contrast, most of the NCNV galaxies in the control sample are in groups (60%). To consider the dynamical stage of the groups, we used the parameters harmonic radius (RH), radial velocity dispersion (σvr2), dimensionless crossing time (H0tc), and group virial mass (Mvir). We also used the total optical (r band) luminosity, Lr, to estimate the mass-to-light ratio (M/L) of the groups. We studied the relations of void properties and these parameters with the group richness. Conclusions. Galaxy groups can be found in any void in the local Universe, with no dependency of group richness on the density of voids. The densest groups in the studied sample of voids are composed of six galaxies; therefore, voids generally contain small groups, in comparison to denser structures such as filaments, walls, and galaxy clusters. Galaxy groups within voids are typically loose groups, in an early stage of their evolution.