Large-scale Filaments Research Articles

Exploring galaxy properties of eCALIFA with contrastive learning

Contrastive learning (CL) has emerged as a potent tool for building meaningful latent representations of galaxy properties across a broad spectrum of wavelengths, ranging from optical and infrared to radio frequencies. These latent representations facilitate a variety of downstream tasks, including galaxy classification, similarity searches in extensive datasets, and parameter estimation, which is why they are often referred to as foundation models for galaxies. In this study, we employ CL on the latest extended data release from the Calar Alto Legacy Integral Field Area (CALIFA) survey, which encompasses a total of 895 galaxies with enhanced spatial resolution that reaches the limits imposed by natural seeing (FWHMPSF ∼ 1.5). We demonstrate that CL can be effectively applied to Integral Field Unit (IFU) surveys, even with relatively small training sets, to construct meaningful embedding where galaxies are well separated based on their physical properties. We discover that the strongest correlations in the embedding space are observed with the equivalent width of Hα, galaxy morphology, stellar metallicity, luminosity-weighted age, stellar surface mass density, the [NII]/Hα ratio, and stellar mass, in descending order of correlation strength. Additionally, we illustrate the feasibility of unsupervised separation of galaxy populations along the star formation main sequence, successfully identifying the blue cloud and the red sequence in a two-cluster scenario, and the green valley population in a three-cluster scenario. Our findings indicate that galaxy luminosity profiles have minimal impact on the construction of the embedding space, suggesting that morphology and spectral features play a more significant role in distinguishing between galaxy populations. Moreover, we explore the use of CL for detecting variations in galaxy population distributions across different large-scale structures, including voids, clusters, and filaments and walls. Nonetheless, we acknowledge the limitations of the CL framework and our specific training set in detecting subtle differences in galaxy properties, such as the presence of an AGN or other minor scale variations that exceed the scope of primary parameters such as the stellar mass or morphology. Conclusively, we propose that CL can serve as an embedding function for the development of larger models capable of integrating data from multiple datasets, thereby advancing the construction of more comprehensive foundation models for galaxies.

Energetic Particles and High-Energy Processes in Cosmological Filaments and Their Astronomical Implications

Large-scale cosmic filaments connect galaxies, clusters, and voids. They are permeated by magnetic fields with a variety of topologies. Cosmic rays with energies up to 1020eV can be produced in astrophysical environments associated with star-formation and AGN activities. The fate of these cosmic rays in filaments, which cannot be directly observed on Earth, are rarely studied. We investigate the high-energy processes associated with energetic particles (cosmic rays) in filaments, adopting an ecological approach that includes galaxies, clusters/superclusters, and voids as key cosmological structures in the filament ecosystem. We derive the phenomenology for modelling interfaces between filaments and these structures, and investigate how the transfer and fate of energetic cosmic ray protons are affected by the magnetism of the interfaces. We consider different magnetic field configurations in filaments and assess the implications for cosmic ray confinement and survival against hadronic pion-producing and photo-pair interactions. Our analysis shows that the fate of the particles depends on the location of their origin within a filament ecosystem, and that filaments act as ‘highways’, channelling cosmic rays between galaxies, galaxy clusters, and superclusters. Filaments can also operate as cosmic ‘fly paper’, capturing cosmic ray protons with energies up to 1018eV from cosmic voids. Our analysis predicts the presence of a population of ∼1012–1016eV cosmic ray protons in filaments and voids accumulated continually over cosmic time. These protons do not suffer significant energy losses through photo-pair or pion production, nor can they be cooled efficiently. Instead, they form a cosmic ray fossil record of the power generation history of the Universe.

Observation of a Large-Scale Filament Eruption Initiated by Two Small-Scale Erupting Filaments Pushing Out from Below

Observation of a Large-Scale Filament Eruption Initiated by Two Small-Scale Erupting Filaments Pushing Out from Below

Relation between the Local Width and Linear Halo Mass Density of Cosmic Filaments

Large-scale cosmic filaments may have played an important role in shaping the properties of galaxies. Meanwhile, cosmic filaments are believed to harbor a substantial portion of the missing baryons at redshift z < 2. To inspect the role of filaments in these issues, many properties of filaments need to be examined, including their lengths, thicknesses, and density profiles. However, measuring some of these properties poses challenges. This study concentrates on estimating filament width/thickness, investigating potential correlations between the local width of filaments and the properties of dark matter halos within filaments. We find that the local width of filaments generally increases with the mass of dark matter halos embedded in filaments per unit length, roughly following a second-order polynomial, although with notable scatter. We probe and discuss means that may refine our findings. After further verification and improvements, this relation could be applied to filament samples constructed from the observed galaxy distribution, aiding in understanding the roles of cosmic filaments in galaxy evolution and uncovering the missing baryons.

Galaxy triplets alignment in large-scale filaments

ABSTRACT Leveraging the data sets of galaxy triplets and large-scale filaments obtained from the Sloan Digital Sky Survey, we scrutinize the alignment of the three sides of the triangles formed by galaxy triplets and the normal vectors of the triplet planes within observed large-scale filaments. Our statistical investigation reveals that the longest and median sides of the galaxy triplets exhibit a robust alignment with the spines of their host large-scale filaments, while the shortest sides show no or only weak alignment with the filaments. Additionally, the normal vectors of triplets tend to be perpendicular to the filaments. The alignment signal diminishes rapidly with the increasing distance from the triplet to the filament spine, and is primarily significant for triplets located within distances shorter than 0.2 Mpc$/h$, with a confidence level exceeding $20\sigma$. Moreover, in comparison to compact galaxy triplets, the alignment signal is more conspicuous among the loose triplets. This alignment analysis contributes to the formulation of a framework depicting the clustering and relaxation of galaxies within cosmological large-scale filament regimes, providing deeper insights into the intricate interactions between galaxies and their pivotal role in shaping galaxy groups.

Large-Scale, Stretchable, Self-Protective, and Multifunctional Perovskite Luminescent Filament with Ultra-High Stability.

Lead halide perovskites possess great application potential in flexible displays and wearable optoelectronics owing to their prominent optoelectronic properties. However, the intrinsic instability upon moisture, heat, and ultraviolet (UV) light irradiation hinders their development and application. In this work, an ultra-stable CsPbX3 (X = Cl, Br, I) perovskite luminescent filament (PLF) with high stretchability (≈2400%) and luminescence performance (photoluminescence quantum yield (PLQY) of 24.5%, tunable emission spectrum, and high color purity) is introduced by a facile environmental-friendly wet-spinning technology via solvent extraction. Benefiting from the in situ encapsulation of the hydrophobic thermoplastic polyurethane (TPU) and the chelation of Lewis base CO in TPU with Lewis acid Pb2+, the CsPbBr3 PLF demonstrates ultra-high photoluminescence (PL) stability when stored in ambient air and high humidity circumstance, annealed at 50 °C, and dipped in water for 30 days, illuminated under ultraviolet light for 300 min, and immersed in organic solvents and solutions with pH of 1-13 for 5 min, respectively. Impressively, it retains 80% of its initial PL after being recycled five times. Overall, the CsPbX3 PLF demonstrates promising prospects in multifunctional applications, including organic dyes and tensile strain sensing, flexible pattern displays, secondary anti-counterfeiting, and hazard warning systems.

Characterizing cool, neutral gas, and ionized metals in the outskirts of low-z galaxy clusters

ABSTRACT We present the first detection of cool, neutral gas in the outskirts of low-z galaxy clusters using a statistically significant sample of 3191 z ≈ 0.2 background quasar-foreground cluster pairs by cross-matching the Hubble Spectroscopic Legacy Archive quasar catalogue with optically- and SZ-selected cluster catalogues. The median cluster mass of our sample is ≈1014.2 $\rm {\rm M}_{\odot }$ with a median impact parameter (ρcl) of ≈5 Mpc. We detect significant Lyα, marginal C iv, but no O vi absorption in the signal-to-noise ratio weighted mean stacked spectra with rest-frame equivalent widths of 0.096 ± 0.011, 0.032 ± 0.015, and &amp;lt;0.009 Å (3σ) for our sample. The Lyα REW shows a declining trend with increasing ρcl (ρcl/R500), which is well explained by a power-law with a slope of −0.79 (−0.70). The covering fractions (CFs) measured for Lyα (21 per cent), C iv (10 per cent), and O vi (10 per cent) in cluster outskirts are significantly lower than in the circumgalatic medium (CGM). We also find that the CGM of galaxies that are closer to cluster centres or that are in massive clusters is considerably deficient in neutral gas. The low CF of the Lyα along with the non-detection of Lyα signal when the strong absorbers ($N(\rm{H\,{\small I}}) &amp;gt; 10^{13}$ cm−2) are excluded, indicate the patchy distribution of cool gas in the outskirts. We argue that the cool gas in cluster outskirts in combination arises from the circumgalactic gas stripped from cluster galaxies and to large-scale filaments feeding the clusters with cool gas.

Observational Evidence of the Merging of Filaments and Hub Formation in G083.097+03.270

We uncover a hub–filament system correlated with massive young stellar associations in G083.097+03.270. Diagnosed with simultaneous 12CO, 13CO, and C18O line observations, the region is found to host two distinct and elongated filaments having separate velocity components, interacting spatially and kinematically, that appear to have seeded the formation of a dense hub at the intersection. A large velocity spread at the hub, in addition to a clear bridging feature connecting the filaments in velocity, indicate the merging of filaments. Along the filament axis, the velocity gradient reveals a global gas motion with an increasing velocity dispersion inward to the hub signifying turbulence. Altogether, the clustering of Class I sources, a high excitation temperature, a high column density, and the presence of a massive outflow at the central hub suggest enhanced star formation. We propose that the merging of large-scale filaments and velocity gradients along filaments are the driving factors in the mass accumulation process at the hub that have sequentially led to the massive star formation. With two giant filaments merging to coincide with a hub therein with ongoing star formation, this site serves as a benchmark for the “filaments to clusters” star-forming paradigm.

Breaking of Large-Scale Filament due to Magnetic Reconnection and Consequent Partial Eruption

Following our previous work, we studied the partial eruption of a large-scale horse-shoe-like filament that had been observed in a decaying active region on the solar disk for more than 4.5 days. The filament became active after it was broken into two pieces, P1 and P2 seen in Hα, by magnetic reconnection between the magnetic field around it and that of a newly emerging active region nearby. P1 eventually erupted 13 hr after the breaking and escaped from the Sun, developing to a fast coronal mass ejection, and P2 stayed. But the mass in P1 falling down to P2 in the eruption suggests that the global magnetic fields over P1 and P2 were still connected to each other prior to the eruption. The reconnection process breaking the filament occurred outside the filament, and P1 and P2 were located almost at the same altitude, so the fashion of the filament partial eruption studied here differs from that of the “double-decker model” and that of reconnection inside the filament. Analyzing the decay indices of the background fields above P1 and P2, n 1 and n 2, showed that the altitude where n 1 exceeds the critical value of n c = 1.5 for the loss of equilibrium or the torus instability is lower than that where n 2 > n c , and that n 1 > n 2 always holds at all altitudes. Combining this fact with that the eruption occurred 13 hr after filament was broken by reconnection, we conclude that the eruption of P1 was triggered by the loss of equilibrium or the torus instability in the configuration, and magnetic reconnection breaking the filament helped weaken the confinement of the background field on P1, allowing P1 to erupt. Detailed features of the eruption and the corresponding physical scenario were also discussed.

JCMT/SCUBA-2 uncovers an excess of 850 μm counts on megaparsec scales around high-redshift quasars

We conducted a systematic survey of the environment of high-redshift quasars at submillimeter wavelengths to unveil and characterize the surrounding distribution of dusty submillimeter galaxies (SMGs). We took sensitive observations with the SCUBA-2 instrument on the James Clerk Maxwell Telescope for 3 enormous Lyman-alpha nebulae (ELANe) and 17 quasar fields in the redshift range 2 &lt; z &lt; 4.2 selected from recent Lyman alpha (Lyα) surveys. These observations uncovered 523 and 101 sources at 850 μm and 450 μm, respectively, with signal-to-noise ratios (S/N) &gt; 4 or detected in both bands at S/N &gt; 3. We ran self-consistent Monte Carlo simulations to construct 850 μm number counts and unveil an excess of sources in 75% of the targeted fields. Overall, regions around ELANe and quasars are overabundant with respect to blank fields by a factor of 3.4 ± 0.4 and 2.5 ± 0.2, respectively (weighted averages). Therefore, the excess of submillimeter sources is likely part of the megaparsec-scale environment around these systems. By combining all fields and repeating the count analysis in radial apertures, we find (at high significance, ≳5σ) a decrease in the overdensity factor from &gt; 3 within ∼2 cMpc to ∼2 in the annulus at the edge of the surveyed field (∼10 cMpc), which suggests that the physical extent of the overdensities is larger than our maps. We computed preferred directions for the overdensities of SMGs from the positions of the sources and used them to orient and create stacked maps of source densities for the quasars’ environment. This stacking unveils an elongated structure reminiscent of a large-scale filament with a scale width of ≈3 cMpc. Finally, the directions of the overdensities are roughly aligned with the major axis of the Lyα nebulae, suggesting that the latter trace, on scales of hundreds of kiloparsecs, the central regions of the projected large-scale structure described by the SMGs on megaparsec scales. Confirming member associations of the SMGs is required to further characterize their spatial and kinematic distribution around ELANe and quasars.

Large-scale velocity-coherent filaments in the SEDIGISM survey: Association with spiral arms and the fraction of dense gas

Context. Filamentary structures in the interstellar medium are closely related to star formation. It is possible that the dense gas mass fraction (DGMF) or clump formation efficiency in large-scale filaments determine whether or not they end up hosting star formation activity. Aims. We aim to automatically identify large-scale filaments, as well as characterizing them, investigating their association with Galactic structures, and studying their DGMFs. Methods. We used a modified minimum spanning tree (MST) algorithm to chain parsec-scale 13CO clumps previously extracted from the Structure, Excitation, and Dynamics of the Inner Galactic InterStellar Medium (SEDIGISM) survey. The MST connects nodes in a graph such that the sum of edge lengths is at a minimum. A modified MST also ensures the velocity coherence between nodes, so that the identified filaments are coherent in position-position-velocity (PPV) space. Results. We generated a catalog of 88 large-scale (&gt;10 pc) filaments in the inner Galactic plane (with −60° &lt; l &lt; 18° and |b| &lt; 0.5°). These SEDIGISM filaments are larger and less dense than MST filaments previously identified from the Bolocam Galactic Plane Survey (BGPS) and the APEX Telescope Large Area Survey of the Galaxy (ATLASGAL). We find that eight of the filaments run along spiral arms and can be regarded as the “bones” of the Milky Way. We also found three bones associated with the Local spur in PPV space. By compiling 168 large-scale filaments with available DGMF across the Galaxy, namely, an order of magnitude more than previously investigated, we find that DGMFs are not correlated with Galactic location. We also find that bones have higher DGMFs than other filaments.

ATOMS: ALMA Three-millimeter Observations of Massive Star-forming regions – XV. Steady accretion from global collapse to core feeding in massive hub-filament system SDC335

ABSTRACT We present ALMA Band-3/7 observations towards ‘the Heart’ of a massive hub-filament system (HFS) SDC335, to investigate its fragmentation and accretion. At a resolution of ∼0.03 pc, 3 mm continuum emission resolves two massive dense cores MM1 and MM2, with $383(^{\scriptscriptstyle +234}_{\scriptscriptstyle -120})$ M⊙ (10–24 % mass of ‘the Heart’) and $74(^{\scriptscriptstyle +47}_{\scriptscriptstyle -24})$ M⊙, respectively. With a resolution down to 0.01 pc, 0.87 mm continuum emission shows MM1 further fragments into six condensations and multi-transition lines of H2CS provide temperature estimation. The relation between separation and mass of condensations at a scale of 0.01 pc favors turbulent Jeans fragmentation where the turbulence seems to be scale-free rather than scale-dependent. We use the H13CO+ J = 1 − 0 emission line to resolve the complex gas motion inside ‘the Heart’ in position-position-velocity space. We identify four major gas streams connected to large-scale filaments, inheriting the anti-clockwise spiral pattern. Along these streams, gas feeds the central massive core MM1. Assuming an inclination angle of 45(± 15)° and a H13CO+ abundance of 5(± 3) × 10−11, the total mass infall rate is estimated to be 2.40(± 0.78) × 10−3 M⊙ yr−1, numerically consistent with the accretion rates derived from the clump-scale spherical infall model and the core-scale outflows. The consistency suggests a continuous, near steady-state, and efficient accretion from global collapse, therefore ensuring core feeding. Our comprehensive study of SDC335 showcases the detailed gas kinematics in a prototypical massive infalling clump, and calls for further systematic and statistical studies in a large sample.

Relaxed blue ellipticals: accretion-driven stellar growth is a key evolutionary channel for low mass elliptical galaxies

ABSTRACT How elliptical galaxies form is a key question in observational cosmology. While the formation of massive ellipticals is strongly linked to mergers, the low mass (M⋆/M⊙ &amp;lt; 109.5) regime remains less well explored. In particular, studying elliptical populations when they are blue, and therefore rapidly building stellar mass, offers strong constraints on their formation. Here, we study 108 blue low-mass ellipticals (which have a median stellar mass of 108.7 M⊙) at z &amp;lt; 0.3 in the COSMOS field. Visual inspection of extremely deep optical HSC images indicates that less than 3 per cent of these systems have visible tidal features, a factor of 2 less than the incidence of tidal features in a control sample of galaxies with the same distribution of stellar mass and redshift. This suggests that the star formation activity in these objects is not driven by mergers or interactions but by secular gas accretion. We combine accurate physical parameters from the COSMOS2020 catalogue, with measurements of local density and the locations of galaxies in the cosmic web, to show that our blue ellipticals reside in low-density environments, further away from nodes and large-scale filaments than other galaxies. At similar stellar masses and environments, blue ellipticals outnumber their normal (red) counterparts by a factor of 2. Thus, these systems are likely progenitors of not only normal ellipticals at similar stellar mass but, given their high star formation rates, also of ellipticals at higher stellar masses. Secular gas accretion therefore likely plays a significant (and possibly dominant) role in the stellar assembly of elliptical galaxies in the low-mass regime.

The Cassiopeia Filament: A Blown Spur of the Local Arm

We present wide-field and high-sensitivity CO(1–0) molecular line observations toward the Cassiopeia region, using the 13.7 m millimeter telescope of the Purple Mountain Observatory. The CO observations reveal a large-scale highly filamentary molecular cloud within the Galactic region of 132.°0 ≥ l ≥ 122.°0 and −1.°0 ≤ b ≤ 3.°0 and the velocity range from approximately +1 to +4 km s−1. The measured length of the large-scale filament, referred to as the Cassiopeia Filament, is ∼390 pc. The observed properties of the Cassiopeia Filament, such as length, column density, and velocity gradient, are consistent with those synthetic large-scale filaments in the inter-arm regions. Based on its observed properties and location on the Galactic plane, we suggest that the Cassiopeia Filament is a spur of the Local arm, which is formed due to the galactic shear. The western end of the Cassiopeia Filament shows a giant arc-like molecular gas shell, which extends in the velocity range from roughly −1 to +7 km s−1. Finger-like structures, with systematic velocity gradients, are detected in the shell. The CO kinematics suggest that the large shell is expanding at a velocity of ∼6.5 km s−1. Both the shell and finger-like structures outline a giant bubble with a radius of ∼16 pc, which is likely produced by the stellar wind from the progenitor star of a supernova remnant. The observed spectral line widths suggest that the whole Cassiopeia Filament was quiescent initially until its west part was blown by the stellar wind and became supersonically turbulent.

Tracking Halo Orbits and Their Mass Evolution around Large-scale Filaments

We have explored the dynamical and mass evolution of halos driven by large-scale filaments using a dark-matter-only cosmological simulation with the help of a phase-space analysis. Since a nonnegligible number of galaxies are expected to fall into the cluster environment through large-scale filaments, tracking how halos move around large-scale filaments can provide a more comprehensive view on the evolution of cluster galaxies. Halos exhibit orbital motions around filaments, which emerge as specific trajectories in a phase space composed of halos’ perpendicular distance and velocity component with respect to filaments. These phase-space trajectories can be represented by three cases according to their current states. We parameterize the trajectories with halos’ initial position and velocity, maximum velocity, formation time, and time since first crossing, which are found to be correlated with each other. These correlations are explained well in the context of the large-scale structure formation. The mass evolution and dynamical properties of halos seem to be affected by the density of filaments, which can be shown from the fact that halos around denser filaments are more likely to lose their mass and be bound within large-scale filaments. Finally, we reproduce the mass segregation trend around filaments found in observations. The mass segregation has been developed because halos that formed earlier approached filaments earlier, grew efficiently, and ended up being more massive. We also found that dynamical friction helps to retain this segregation trend.

New insights into the Triangulum Australis supercluster of galaxies

The Triangulum Australis cluster is one of about a dozen nearby massive cluster systems which contribute to the gravitational pull behind the so-called Great Attractor that is dominated by the nearby Shapley Supercluster mass, which conforms the galaxy velocity flows observed in that general direction. Here, we study the structure and dynamical mass of the Triangulum Australis cluster together with the neighbouring cluster AS0794. We present a set of 131 velocities collected in the regions of the two clusters with the 2.5 m Du Pont telescope at Las Campanas Observatory (Chile). For the Triangulum Australis cluster we find a dynamical mass of about Mv = 4.2 (±1.3) × 1015 M⊙ and for AS0794 a value of about Mv = 1.7 (±1.3) × 1013 M⊙. These values are consistent with the observed X-ray luminosities of these clusters. Combined with velocities already known we reanalyse the structure and dynamics of this general region, finding that both clusters, together with at least eight other ones, form a large supercluster, centered on TriAus (which dominates in terms of mass). We find that this supercluster is part of a large-scale structure filament linked to the Shapley supercluster (SSC). Uncertainties remain on the richness and detailed structure of this filament and the TriAus supercluster because parts of it remain hidden behind the Galaxy disk.

Suzaku Observations of the Cluster Outskirts and Intercluster Filament in the Triple Merger Cluster A98

We present Suzaku observations of the Abell 98 (A98) triple galaxy cluster system and the purported intercluster filament. The three subclusters are expected to lie along a large-scale cosmic filament. With partial azimuthal coverage of the northernmost cluster, we find that the inferred entropy profile of this relatively low-mass cluster (kT ≈ 2.8 keV) adheres to expectations from models of self-similar pure gravitational collapse in the region of the virial radius. There is evidence of extended structure beyond r 200 to the north of the northernmost cluster, along the merger axis, with properties consistent that are expected for the warm–hot intergalactic medium ( keV and n e = 7.6 × 10−5 ± 3.6 × 10−5 cm−3). No such emission is detected at the same radius in regions away from the merger axis, consistent with the expectation that the merger axis of this triple system lies along a large-scale cosmic filament. In the bridge region between A98N and A98S, there is evidence of filamentary emission at the 2.2σ level, as well as a tentative detection of cool gas (kT ∼ 1 keV). The entropy profile of this intercluster filament suggests that the A98 system is most likely aligned closer to the plane of the sky rather than along the line of sight. The structure to the north of the system, as well as in-between A98N and A98S, is indicative that the clusters are connected to a larger-scale structure spanning at least 4 Mpc.

Multiphase turbulence in galactic haloes: effect of the driving

ABSTRACT Supernova explosions, active galactic nuclei jets, galaxy–galaxy interactions, and cluster mergers can drive turbulence in the circumgalactic medium (CGM) and the intracluster medium (ICM). However, the exact nature of turbulence forced by these sources and its impact on the different statistical properties of the CGM/ICM and their global thermodynamics is still unclear. To investigate the effects of different types of forcing, we conduct high-resolution (10083 resolution elements) idealized hydrodynamic simulations with purely solenoidal (divergence-free) forcing, purely compressive (curl-free) forcing, and natural mixture forcing (equal fractions of the two components). The simulations also include radiative cooling. We study the impact of the three different forcing modes (sol, comp, and mix) on the morphology of the gas, its temperature and density distributions, sources and sinks of enstrophy, i.e. solenoidal motions, as well as the kinematics of hot (∼107 K) X-ray emitting and cold (∼104 K) H α emitting gas. We find that compressive forcing leads to stronger variations in density and temperature of the gas as compared to solenoidal forcing. The cold phase gas forms large-scale filamentary structures for compressive forcing and misty, small-scale clouds for solenoidal forcing. The cold phase gas has stronger large-scale velocities for compressive forcing. The natural mixture forcing shows kinematics and gas distributions intermediate between the two extremes, the cold-phase gas occurs as both large-scale filaments and small-scale misty clouds.

The eROSITA view of the Abell 3391/95 field: Case study from the Magneticum cosmological simulation

Context.Clusters of galaxies reside at the nodes of the cosmic web, interconnected by filamentary structures that contain tenuous diffuse gas, especially in the warm-hot phase. Galaxy clusters grow by mergers of smaller objects and gas that are mainly accreted through these large-scale filaments. For the first time, the large-scale cosmic structure and a long gas-emission filament have been captured by eROSITA on board the Spectrum-Roentgen-Gamma mission in a direct X-ray observation of the A3391/95 field.Aims.We investigate the assembly history of an A3391/95-like system of clusters and the thermo-chemical properties of the diffuse gas in it by connecting simulation predictions to the eROSITA observations with the aim to constrain the origin and nature of the gas in the pair-interconnecting bridge.Methods.We analysed the properties of a system resembling A3391/95, extracted from the (352h−1cMpc)3volume of the Magneticum Pathfinder cosmological simulations atz= 0.07. We tracked the main progenitors of the pair clusters and of surrounding groups back in time to study the assembly history of the system and its evolution.Results.Similarly to the observed A3391/95 system, the simulated cluster pair is embedded in a complex network of gas filaments, with structures aligned over more than 20 projected Mpc, and the whole region collapses towards the central overdense node. The spheres of influence (3 ×R200) of the two main clusters already overlap atz= 0.07, but their virial boundaries are still physically separated. The diffuse gas located in the interconnecting bridge closely reflects the warm-hot intergalactic medium, with a typical temperature of ~1 keV and an overdensityδ ~100 with respect to the mean baryon density of the Universe, and a lower enrichment level compared to the intra-cluster medium in clusters. We find that most of the bridge gas collapsed from directions roughly orthogonal to the intra-cluster gas accretion directions, and its origin is mostly unrelated to the two cluster progenitors. We find clear signatures in the surrounding groups of infall motion towards the pair, such as significant radial velocities and a slowdown of gas compared to dark matter. These findings further support the hypothesis that the Northern Clump (MCXC J0621.7-5242) cluster infalls along a cosmic gas filament towards Abell 3391 and might be merging with it.Conclusions.We conclude that in this configuration, the pair clusters of the A3391/95-like system are in a pre-merger phase and have not yet interacted. The diffuse gas in the interconnecting bridge is mostly warm filament gas and not tidally stripped cluster gas.

The eROSITA view of the Abell 3391/95 field: The Northern Clump

Context.Galaxy clusters grow through mergers and the accretion of substructures along large-scale filaments. Many of the missing baryons in the local Universe may reside in such filaments as the warm-hot intergalactic medium (WHIM).Aims.SRG/eROSITA performance verification observations revealed that the binary cluster Abell 3391/3395 and the Northern Clump (the MCXC J0621.7–5242 galaxy cluster) are aligning along a cosmic filament in soft X-rays, similarly to what has been seen in simulations before. We aim to understand the dynamical state of the Northern Clump as it enters the atmosphere (3 ×R200) of Abell 3391.Methods.We analyzed joint eROSITA,XMM-Newton, andChandraobservations to probe the morphological, thermal, and chemical properties of the Northern Clump from its center out to a radius of 988 kpc (R200). We utilized the ASKAP/EMU radio data, the DECam optical image, and thePlanck y-map to study the influence of the wide-angle tail (WAT) radio source on the Northern Clump’s central intracluster medium. Using eROSITA data, we also analyzed the gas properties of the Northern Filament, the region between the virial radii of the Northern Clump and the A3391 cluster. From the Magneticum simulation, we identified an analog of the A3391/95 system along with an infalling group resembling the Northern Clump.Results.The Northern Clump is a weak cool-core cluster centered on a WAT radio galaxy. The gas temperature over 0.2–0.5R500iskBT500=1.99 ± 0.04 keV. We employed the mass-temperature(M – T) scaling relation and obtained a mass estimate ofM500= (7.68 ± 0.43) × 1013M⊙andR500= (63 6 ± 12) kpc. Its X-ray atmosphere has a boxy shape and deviates from spherical symmetry. We identify a southern surface brightness edge, likely caused by subsonic motion relative to the filament gas in the southern direction. At ~R500, the southern atmosphere (infalling head) appears to be 42% hotter than its northern atmosphere. We detect a downstream tail pointing toward the north with a projected length of ~318 kpc, plausibly the result of ram pressure stripping. Through a two-temperature fit, we identify a cooler component in the Northern Filament withkBT= 0.68- 0.64+ 0.38keV andne= 1.99-1.24+0.88× 10-5cm-3, which are consistent within the expected ranges of WHIM properties. The analog group in the Magneticum simulation is experiencing changes in its gas properties and a shift between the position of the halo center and that of the bound gas, while approaching the main cluster pair.Conclusions.The Northern Clump is a dynamically active system and far from being relaxed. Its atmosphere is affected by an interaction with the WAT and by gas sloshing or its infall toward Abell 3391 along the filament, consistent with the analog group-size halo in the Magneticum simulation.