New Supercluster Discovered

The most familiar kind of structure above the scale of a galaxy is a galaxy cluster. But beyond galaxy clusters are "superclusters" and from there to filaments of the "comic web".

A new Spectrum-Roentgen-Gamma mission (SRG), extended ROentgen Survey with an Imaging Telescope Array (eROSITA) equatorial depth survey (eFEDS) has found a new "supercluster" in its initial calibration runs. It followed up on its findings with radio observations from Low Frequency Array (LOFAR) radio telescope and upgraded Giant Meterwave Radio Telescope (uGMRT) and optical results from the Hyper Suprime-Cam (HSC) confirming the supercluster and helping it to characterize the X-ray, optical and radio properties and its member clusters.

[W]e detect a previously unknown supercluster consisting of a chain of eight galaxy clusters at z=0.36. . . . We further investigate the gas in the bridge region between the cluster members for a potential WHIM  [Warm Hot Intergalactic Medium] detection. . . . We do not find significant differences in the morphological parameters and properties of the intra-cluster medium of the clusters embedded in this large-scale filament compared to eFEDS clusters. We also provide upper limits on the electron number density and mass of the warm-hot intergalactic medium as provided by the eROSITA data. These limits are consistent with previously reported values for the detections in the vicinity of clusters of galaxies. In LOFAR and uGMRT follow-up observations of the northern part of this supercluster we find two new radio relics that are the result of major merger activity in the system. These early results show the potential of eROSITA to probe large-scale structures such as superclusters and the properties of their members. Our forecasts show that we will be able to detect 450 superclusters with 3000 member clusters located in the eROSITA_DE region at the final eROSITA all-sky survey depth, enabling statistical studies of the properties of superclusters and their constituents embedded in the cosmic web.

The introduction to the paper (whose abstract above I have edited as it is rather incomprehensibly written) (most citations omitted) provides context and summarizes the history of supercluster astronomy to date:

Cosmic structures evolve hierarchically from high density peaks in the primordial density field and form galaxies, galaxy groups, and clusters of galaxies under the action of gravity. In the complex large-scale structure formation scenario, these galaxies, groups, and clusters are connected to each other via filamentary structures, called the cosmic web, and form large superclusters. Due to the large crossing times, superclusters are neither virialized nor relaxed, although individual structures located within superclusters, such as galaxy clusters, can be gravitationally bound and virialized. Superclusters contain a variety of structures with a range of masses, from massive and dense clusters of galaxies to low-density bridges, filaments, and sheets of matter, and hence are ideal laboratories in which to study the physical processes that affect the evolution of member galaxies, groups, and clusters. It has been suggested that the supercluster environment strongly influences the evolution of the properties of its constituents. 

For instance, simulations predict that the shape of clusters in superclusters is predominantly elongated, and the elongation is typically along the filament direction. Additionally, comprising a significant amount of baryons in the form of galaxies and diffuse gas, the filaments connecting clusters of galaxies have typical diameters of ∼ few Mpc, and coherence lengths of the order of ∼ 5 Mpc but can extend up to ∼ 20–25 Mpc. Detections of the baryons located in the warm-hot intergalactic medium (WHIM) locked in the filaments could reveal important clues regarding the missing baryon problem in the low-redshift Universe. 

A number of superclusters have been found in deep optical surveys of galaxies. In X-rays, the first flux-limited supercluster catalog was compiled by Chon et al. (2013). Based on the REFLEX II cluster sample, they located 164 superclusters, of which only a couple above redshift of 0.35. Although a number of multiwavelength supercluster catalogs exist in the literature, only a few superclusters and their members have been studied in depth observationally, in particular Shapley supercluster at low redshift, and two high redshift clusters. 

The Shapley supercluster, discovered by Shapley (1930), is one of the most studied superclusters of galaxies in the sky. It is a concentration of more than 20 galaxy clusters in a volume of about 10−3 Gpc3 , and about 20 deg2 in the plane of the sky at a redshift of 0.039. Ettori et al. (2000) combined ROSAT PSPC and BeppoSAX X-ray data to study in detail 3 members of the Shapley supercluster using resolved spatial and spectral analysis. They conclude that A3562, a member of the Shapley supercluster, is not relaxed and has evidence of merging activity. At high redshifts (z > 0.4) the number of known superclusters is extremely small, and even fewer have been investigated in detail. Horner & Donahue (2003) studied the supercluster MS0302 at z = 0.42 composed of 3 massive galaxy clusters. This system was discovered in an X-ray follow up observation using the Einstein X-ray observatory of optically detected cluster members. They measured the X-ray properties, temperature, and luminosity of the members of the system. Recently, Adami et al. (2018) presented the discovery of 35 superclusters found in the 50 deg2 XXL survey (Pierre et al. 2016). Of these clusters, Pompei et al. (2016) studied in detail a supercluster at z = 0.43 consisting of six galaxy clusters. They determined the temperature, luminosity, and total mass of its members, using their internally calibrated weak lensing mass – X-ray temperature scaling relation. From their morphological analysis, they find that 2 of these 6 clusters appear to be disturbed, indicating that they are likely in a merging state. Suzaku observations of A1689 and A1835 investigated correlations between the X-ray properties in cluster outskirts (R500 < R < R200) and their surrounding large scale structure, and found that the outskirts X-ray temperatures in the regions connected to the filamentary structure are higher than those connected to void regions. 

Detailed examination of the connecting bridges of the member clusters of galaxies in superclusters has been an active area of research due to its connection with the missing baryon problem. Detections of the warm-hot intergalactic medium in these regions is quite challenging with current X-ray instrumentation due to the relatively low density (< 10−4 particle cm−3 , corresponding to an over-density of 10-100 times the cosmic value1 ) and low temperature (105 − 107 K) of this gas. Recent dedicated deep X-ray observations suggest the presence of such gas in the interconnecting bridges or filaments between clusters of galaxies, e.g. A3391/95, A222/3, A2744, A1750, and A133. An alternative way to detect this low density gas is through the thermal Sunyaev Zel’dovich (tSZ) effect. Planck Collaboration et al. reported detection of such a gas in the A399-A401 cluster pair with an estimate for the gas temperature of kT = 8 × 107 K and for the electron density of ne = 3.7 × 10−4 cm−3 . It should be noted that these potential detections probe the densest and hottest ends of the WHIM, where the intracluster gas interacts with the colder primordial low-density WHIM gas. Recently, by stacking the Planck Compton y-parameter map of the tSZ signal of galaxy pairs, de Graaff et al. (2019) reported a 2.9σ detection of the WHIM gas with a gas density of ρ = 5.5 ± 2.9 ρb, where ρb is the mean matter density of the Universe, and a gas temperature of kT = (2.7 ± 1.7) ×106 K. Recently, diffuse synchrotron radio emission was detected in the region connecting the pairs A1758N/A1758S and A399/A401 using the LOw Frequency ARray (LOFAR). The origin of the radio synchrotron emission in radio bridges is not well understood, however turbulence generated by the stochastic acceleration of relativistic electrons could be an explanation for the observed diffuse radio emission in A399–A401. 

Statistical multi-wavelength studies of the properties of member structures embedded in superclusters are crucial to develop an understanding of the evolution of the large scale structure. Here we report the discovery of a new supercluster at a redshift of 0.36 in the eROSITA Final Equatorial Depth Survey (eFEDS) performed during the Performance Verification (PV) program. eFEDS is a 140 deg2 field located in an equatorial region, with R.A. from ∼127 to ∼145, and Dec. from ∼-2 to ∼5. It was observed in scanning mode by eROSITA with nominal exposure of about 2.3 ks. In this paper, we examine the X-ray and radio properties of the member clusters of galaxies in their large scale environment and the WHIM gas in the interconnecting region joining the observations from eROSITA in the X-ray, Hyper Suprime-Cam (HSC) in the optical, and LOFAR and uGMRT in the radio band. . . . 

Throughout this paper we assume a concordance ΛCDM cosmology with Ωm = 0.3, ΩΛ = 0.7, and H0 = 70 km s−1 Mpc−1.

1 The critical density of the Universe: ρc = 3H 2 (z) 8πG