The role of supercluster filaments in shaping galaxy clusters

TL;DR

This work tests the hypothesis that galaxy cluster elongation aligns with surrounding filaments by mapping 2D filaments from optical galaxies and comparing them to X-ray-derived cluster shapes in two nearby superclusters (SSC and HRSC). Using DisPerSE to identify filaments and a probabilistic Hough Transform to measure filament inclinations, the study finds that a large fraction of clusters are connected to filaments and that cluster major axes preferentially align with connected filaments near the cluster centers, with the signal weakening beyond $1.6\,r_{200}$. X-ray ellipses quantify cluster shapes, and combined with optical filament maps, reveal an observational signature of anisotropic, filament-driven accretion shaping cluster potentials. The results support the scenario in which filaments are major channels for mass inflow at low redshift and demonstrate a practical observational proxy for the accretion direction in clusters, informing galaxy evolution and merger histories.

Abstract

In a hierarchical $Λ$CDM Universe, cosmic filaments serve as the primary channels for matter accretion into galaxy clusters, influencing the shape of their dark matter halos. We investigate whether the elongation of galaxy clusters correlates with the orientation of surrounding filaments, providing the first observational test of this relationship in large supercluster regions. We identified and characterized cosmic filaments in two dimensions within the two superclusters that are part of the low-redshift sub-survey of the Chilean Cluster Galaxy Evolution Survey (CHANCES): the Shapley supercluster and the Horologium-Reticulum supercluster. We analyzed the alignment between filament directions -- traced by galaxy distributions -- and the triaxiality of cluster gravitational potentials -- traced by X-ray emission- using publicly available optical and X-ray data. We have found that most (82%) of the X-ray clusters are associated with and interconnected by the optically detected filaments. The clusters-filaments alignment analysis shows that the elongation of most clusters is well aligned with nearby filaments, providing observational confirmation of theoretical predictions, with the alignment progressively reducing at larger cluster-centric distances ($> 1.6 r_{200}$). Overall, our results support the notion that filaments are the main source of galaxy accretion at redshift below 0.1 and additionally provide evidence that matter accretion through filaments shapes the gravitational potential of galaxy clusters. We propose this measurement as a simple observational proxy to determine the direction of accretion in clusters, which is key to understanding both galaxy evolution and the merger history of galaxy clusters.

Figures (5)

• Figure 1: Large-scale structure identification using DisPerSE in two dimensions within the SSC. Greyscale hexagons show the density map of photometric members in the supercluster. Red lines indicate optically detected filaments, while different symbols correspond to critical points (see legend). Large-scale X-ray detections in the SSC area are in magenta contours. The cyan circles indicate clusters detected in the 0.2-2.3 keV as extended X-ray sources in the Shapley area and $z$ range considered for this work Bulbul2024, while purple circles are the large-scale emission in the band 0.6-2.3 identified as galaxy clusters used in this work. Green rectangle indicates the CHANCES coverage in SSC and the area considered for the optical-X-ray comparative analysis shown in Sect. \ref{['sec:method']}.
• Figure 2: Same as Fig. \ref{['fig:Shapley']} but for HRSC. Green polygon indicates the area that CHANCES will observe.
• Figure 3: Left: Two-dimensional reconstruction of the filament network using the spectroscopic sample (green segments) and photometric members (red segments) in the SSC core. The gray map indicates the density of photometric members in the area, while blue circles are the spectral members. Orange circles indicate known X-ray clusters in the SSC Haines2018a. Right: Probability distribution of the distances between skeletons of filament networks from photometric to spectroscopic (orange curve) and spectroscopic to photometric (green curve) members within the ShaSS area. Vertical dashed orange and green lines show the median distance between skeletons obtained from the two comparisons.
• Figure 4: Schematic representation of the definition of filament segments connected to the X-ray cluster and the computation of their projected inclinations using the PHT technique. Red circles indicate the segment points $[S1,S2,…,S7]$ that are part of the detected filament network. Dashed magenta lines represent 0.8 and 1.2 times the $r_{200}$ around the X-ray peak, respectively. Dark red segment points $[S2,S3,S4,S5]$, located between the dashed curves, are used to compute the mean projected inclination of the filament segment connected to the galaxy cluster (magenta ellipse). The PHT operator is centered at the position of $S4$, with each line representing a possible local alignment angle of the segment points, based on the positions of the points within the dashed circumference. Line thickness corresponds to the probability of alignment at that specific angle, with the thickest blue line denoting the most probable inclination angle at $S4$. The same procedure is applied at $S2$, $S3$ and $S5$ to ultimately compute the mean projected alignment angle using Eq. \ref{['eq:arctan2']}, represented as the red vector $\hat{v}_{fill}$. The inclination angle of the X-ray cluster is represented as the magenta vector $\hat{v}_{x}.$
• Figure 5: Distribution of the alignment between optical filaments connected to galaxy clusters and the major axis of each cluster as inferred from its X-ray emission. Fractions and 68% confidence intervals were computed in equal-sized angular bins, following the method described in damsted2023codex. Different colors indicate different projected cluster–filament distances used to compute the projected filament inclination. The gray-dashed horizontal line represents the uniform distribution, while the black-dotted vertical line indicates an alignment threshold of less than 25 degrees.