The richest clusters in the Coma and Leo superclusters: Properties and evolution

TL;DR

The paper investigates the richest clusters in the Coma and Leo superclusters (A1656, A1367, A1185) to illuminate their growth from turnaround to virialisation using PPS, a spherical-collapse framework, and multi-wavelength data from SDSS DR10 and DESI. Substructure is quantified with Gaussian mixture modeling, while cluster connectivity is assessed via Bisous filaments; galaxy content is analyzed across clusters, their regions of influence, and surrounding large-scale environments. The study finds persistent substructure and multiple filaments around all three clusters, with regions of influence at $R_{inf}\approx 4\,h^{-1}$ Mpc and density contrast at the border $\Delta\rho_{inf}\approx 50$–60, turnaround at $z_{ta}\approx 0.4$–0.5, and virialisation in about $3.3$ Gyr; supercluster regions are predicted to virialise in ~10 Gyr and are unlikely to merge. Tight scaling relations link cluster masses to masses embedded in their regions of influence and turnaround ($M_{inf} \approx 1.6 M_{cl} + 0.2$, $M_{ta} \approx 2.2 M_{cl} + 0.3$), suggesting self-similar mass distribution even as cluster properties vary widely due to different evolutionary histories. The results underscore substantial diversity in cluster assembly and environment-driven galaxy evolution, and they motivate larger, joint observational-simulation studies to map cluster growth across cosmic time.

Abstract

We study the substructure, connectivity, and galaxy content of galaxy clusters A1656 and 1367 in the Coma supercluster and of A1185 in the Leo supercluster with the aim of understanding the evolution of clusters from turnaround to virialisation. We used data from the SDSS DR10 MAIN galaxy sample and from DESI cluster catalogues. The projected phase space diagram and the distribution of mass were used to identify regions of various infall stages (early and late infall, and regions of ongoing infall, i.e. regions of influence), their characteristic radii, embedded mass, and density contrasts in order to study the evolution of clusters with the spherical collapse model. We determined the substructure of clusters using normal mixture modelling and their connectivity by counting filaments in the cluster's regions of influence, analysed galaxy content of clusters, and derived scaling relations between cluster masses. All three clusters have a substructure with two to five components and up to six filaments connected to them. The radii of regions of influence are $R_\mathrm{inf} \approx 4$ Mpc, and the density contrast at their borders is $Δρ_{inf} \approx 50 - 60$. The scaling relations between the masses of clusters have a very small scatter. The galaxy content of the clusters and of their regions of influence vary from cluster to cluster. In superclusters the percentage of quiescent galaxies is higher than in low-density regions between superclusters. The collapse of the regions of influence of clusters started at redshifts $z \approx 0.4 - 0.5$. Clusters will be virialised in $\approx 3.3$ Gyrs. Clusters in superclusters will not merge, and their present-day turnaround regions will be virialised in $\approx 10$ Gyrs. The large variety of properties of clusters suggests that they have followed different paths during evolution.

Figures (12)

• Figure 1: Star formation rate ($\log \mathrm{SFR}$) versus $D_n(4000)$ index for galaxies with $M_r = -18.30$. Horizontal lines show $D_n(4000)$ limits for galaxies with stellar populations of different ages, $1.35$, $1.75$, and $2.0$ (see text for details), and the vertical line shows the limit $\log \mathrm{SFR} = -0.5$ that separates star-forming and quenched galaxies.
• Figure 2: Mass--radius relation from the spherical collapse model. Different lines correspond to the masses for different density contrasts, $\Delta\rho$ (Eq. \ref{['eq:sph']}). Lines $M_{60}$--$M_{30}$ correspond to the density contrasts, $\Delta\rho_{inf}$, at the borders of the spheres of influence as found for clusters. Lines $M_{13.1}$, $M_{8.73}$, and $M_{5.41}$ corresponds to the turnaround, future collapse, and zero gravity density contrasts. All lines are calculated for the redshift $z = 0.03$.
• Figure 3: Upper panel: Sky distribution of groups (filled circles and crosses) and single galaxies (empty circles) in the region of the Coma and the Leo superclusters. Colours of symbols show groups in different global luminosity-density regions (orange $D8 \geq 5$, blue $5 > D8 \geq 1.5$, and grey crosses $D8 < 1.5$). To avoid strong projections, we only plot single galaxies in superclusters ($D8 \geq 5$). Filament member galaxies in groups are shown with magenta x-s and single galaxies with green x-s. Violet stars show members of long filaments with length $F_{\mathrm{len}}^{l} \geq 5$$h^{-1}$ Mpc. In groups we show the location of the brightest group galaxies only. Dark red circles: $L_{gr} \geq 15\times10^{10} h^{-2} L_{\sun}$. Member galaxies of clusters A1656, A1367, and A1185 are shown in red, green, and blue. Black circles mark the location of groups from WH24 catalogue, circle sizes are proportional to group richness. Lower panels: Number density distribution of galaxies in groups and single galaxies between clusters A1656 and A1367 (left panel), and A1367 and A1185 (right panel). Here, $d_\parallel$ denotes the coordinate on a straight line between the clusters, and red and blue lines show the linear density and linear number density distributions, respectively. The cross-section radius, where the densities were evaluated, was $8\,{\rm Mpc\,h^{-1}}$. The vertical dashed and dotted lines show influence and turnaround radii locations away from their respective end-point clusters.
• Figure 4: Upper row: Distribution of galaxies in clusters and in their environment in the plane of the sky. Left: A1656. Middle: A1367. Right: A1185. Symbols of different colours show galaxies in different components of the cluster: the redder the colour, the higher the median values of the $D_n(4000)$ index of galaxies are in a component. Black crosses show galaxies of short filaments ($F_{\mathrm{len}}^{s} \leq 5$$h^{-1}$ Mpc), and dark blue crosses show galaxies in long filaments ($F_{\mathrm{len}}^{l} > 5$$h^{-1}$ Mpc). Dark red circles show clusters from WH24m catalogue. Circle sizes are proportional to the number of galaxies in a cluster. Orange circles show the regions of influence, with $R_{inf}$. Middle row: Distribution of the clustercentric distances, $D_c$ (upper panels), and the PPS diagram (lower panels). Symbols are as in the upper panels. Vertical lines show the cluster radius, $R_\mathrm{cl}$, and the radius of the sphere of influence, $R_\mathrm{inf}$. In the lower panel, the dark blue region is the early infall region, and light blue is the late infall region. Lower row: Density contrast, $\delta \rho$, versus clustercentric distance. Vertical line shows the radius of the sphere of influence, $R_\mathrm{inf}$, and horizontal lines mark the characteristic density contrasts, as shown in the Figure. The blue area shows turnaround region, and the light blue area shows future collapse region.
• Figure 5: Star formation rate $log SFR$ (left) and $D_{n}(4000)$ index for galaxies in clusters, in the regions of influence (i) of clusters, in the high-density regions ($D85$, excluding clusters; global luminosity-density $D8 \geq 5$), and in the low-density regions ($D805$, $D8 < 5$ in the units of mean luminosity-density) of our sample and filament members (with distance from the nearest filament axis $D_{fil} <= 0.5$$h^{-1}$ Mpc) and non-member galaxies ($D_{fil} > 0.5$$h^{-1}$ Mpc). Different samples are marked in the figure. We only considered filaments with length $L_{fil} \geq 3$$h^{-1}$ Mpc as more reliable. The horizontal line in the left panel corresponds to $log SFR = -0.5$, and lines in the right panel correspond to $D_{n}(4000) = 1.55$, $1.75$, and $2.0$.