The parallelism between galaxy clusters and early-type galaxies

Context. This is the third study of a series dedicated to the observed parallelism of properties between galaxy clusters and groups (GCGs) and early-type galaxies (ETGs). Aims. Here we investigate the physical origin of the mass–radius relation (MRR). Methods. Having collected literature data on masses and radii for objects going from globular clusters (GCs) to ETGs and GCGs, we set up the MR plane and compare the observed distribution with the MRR predicted by theoretical models for both the monolithic and hierarchical scenarios. Results. We argue that the distribution of stellar systems in the MR plane is due to complementary mechanisms: (i) on one hand, as shown in Paper II, the relation of the virial equilibrium intersects with a relation that provides the total luminosity as a function of the star formation history; (ii) on the other hand, the locus predicted for the collapse of systems should be convolved with the statistical expectation for the maximum mass of the halos at each cosmic epoch. This second aspect provides a natural boundary limit explaining both the curved distribution observed in the MR plane and the existence of a zone of avoidance. Conclusions. The distribution of stellar systems in the MR plane is the result of two combined evolutions, that of the stellar component and that of the halo component.

Taking apart the dynamical clock

Context. In globular clusters (GCs), blue straggler stars (BSS) are heavier than the average star, so dynamical friction strongly affects them. The radial distribution of BSS, normalized to a reference population, appears bimodal in a fraction of Galactic GCs, with a density peak in the core, a prominent zone of avoidance at intermediate radii, and again higher density in the outskirts. The zone of avoidance appears to be located at larger radii the more relaxed the host cluster, acting as a sort of dynamical clock. Aims. We use a new method to compute the evolution of the BSS radial distribution under dynamical friction and diffusion. Methods. We evolve our BSS in the mean cluster potential under dynamical friction plus a random fluctuating force, solving the Langevin equation with the Mannella quasi symplectic scheme. This is a new simulation method that is much faster and simpler than direct N-body codes, but retains their main feature: diffusion powered by strong, if infrequent, kicks. Results. We compute the radial distribution of initially unsegregated BSS normalized to a reference population as a function of time. We trace the evolution of its minimum, corresponding to the zone of avoidance. We compare the evolution under kicks extracted from a Gaussian distribution to that obtained using a Holtsmark distribution. The latter is a fat-tailed distribution which correctly models the effects of close gravitational encounters. We find that the zone of avoidance moves outwards over time, as expected based on observations, only when using the Holtsmark distribution. Thus, the correct representation of near encounters is crucial to reproduce the dynamics of the system. Conclusions. We confirm and extend earlier results that showed how the dynamical clock indicator depends on dynamical friction and on effective diffusion powered by dynamical encounters. We demonstrated the high sensitivity of the clock to the details of the mechanism underlying diffusion, which may explain the difficulties in reproducing the motion of the zone of avoidance across different simulation methods.

The 2MASS redshift survey galaxy group catalogue derived from a graph-theory based friends-of-friends algorithm

ABSTRACT We present the galaxy group catalogue for the recently completed 2MASS Redshift Survey (2MRS; Macri et al. 2019) which consists of 44 572 redshifts, including 1041 new measurements for galaxies mostly located within the Zone of Avoidance. The galaxy group catalogue is generated by using a novel, graph-theory based, modified version of the friends-of-friends algorithm. Several graph-theory examples are presented throughout this paper, including a new method for identifying substructures within groups. The results and graph-theory methods have been thoroughly interrogated against previous 2MRS group catalogues and a Theoretical Astrophysical Observatory (TAO) mock by making use of cutting-edge visualization techniques including immersive facilities, a digital planetarium, and virtual reality. This has resulted in a stable and robust catalogue with on-sky positions and line-of-sight distances within 0.5 and 2 Mpc, respectively, and has recovered all major groups and clusters. The final catalogue consists of 3022 groups, resulting in the most complete ‘whole-sky’ galaxy group catalogue to date. We determine the 3D positions of these groups, as well as their luminosity and comoving distances, observed and corrected number of members, richness metric, velocity dispersion, and estimates of R200 and M200. We present three additional data products, i.e. the 2MRS galaxies found in groups, a catalogue of subgroups, and a catalogue of 687 new group candidates with no counterparts in previous 2MRS-based analyses.

Fix Probabilities from LOP Geometry

A simple scheme is presented for mapping the 2D probability density for an observer's position, defined by any number of lines of position (LOPs) on the surface of the Earth, assuming that the LOPs result from uncorrelated observations that have normally distributed errors. Although the mapping can be used to determine the position fix corresponding to the LOPs (which is consistent with other methods), its intended use is computing the total probability that the observer is located within (or outside) some specified area of interest, such as a zone of avoidance around a navigational hazard. Numerical experiments with areas where the average total interior probability is known, such as the triangles and polygons formed by nearly convergent LOPs, show that the method provides correct answers. The numerical experiments also revealed that theoretical probabilities associated with commonly used error ellipses are overstated for navigational solutions based on small numbers of LOPs.

A kinematic confirmation of the hidden Vela supercluster

ABSTRACT The Universe region obscured by the Milky Way is very large and only future blind large H i redshift, and targeted peculiar surveys on the outer borders will determine how much mass is hidden there. Meanwhile, we apply for the first time two independent techniques to the galaxy peculiar velocity catalogue CosmicFlows−3 in order to explore for the kinematic signature of a specific large-scale structure hidden behind this zone: the Vela supercluster at cz ∼18 000 km s−1. Using the gravitational velocity and density contrast fields, we find excellent agreement when comparing our results to the Vela object as traced in redshift space. The article provides the first kinematic evidence of a major mass concentration (knot of the Cosmic Web) located in the direction behind Vela constellation, pin pointing that the Zone of Avoidance should be surveyed in detail in the future.