Galaxy Groups Research Articles

RXCJ1111.6+4050 galaxy cluster: Observational evidence of a transitional fossil group

We present a detailed kinematical and dynamical study of the galaxy cluster RXCJ1111.6+4050 (RXCJ1111), at z = 0.0756 using 104 new spectroscopic redshifts of galaxies observed at the Telescopio Nazionale Galileo and SDSS DR16 public archive. Our analysis is performed in a multiwavelength context in order to study and compare mainly optical and X-ray properties using XMM-Newton data. We find that RXCJ1111 is a galaxy cluster showing a velocity distribution with clear deviations from Gaussianity, that we are able to explain by the presence of a substructure within the cluster. The two cluster components show velocity dispersions of 644 ± 56 km s−1 and 410 ± 123 km s−1, which yield dynamical masses of M200 = 1.9 ± 0.4 × 1014 M⊙ and 0.6 ± 0.4 × 1014 M⊙ for the main system and substructure, respectively. The 2D spatial distribution of galaxies and X-ray surface brightness of RXCJ1111 presents an elongation in the North–South direction. These observational facts, together with a gradient of 250−350 km s−1 Mpc−1 in the velocity field, following the NNE–SSE direction, suggest that the merger axis between the main system and substructure is slightly tilted with respect to the line-of-sight. The substructure is characterized by a magnitude gap Δm12 ≥ 1.8, so it fits the “fossil-like” definition of a galaxy group. From the X-ray observations, we estimate a M500, X = 1.68 ± 0.25 × 1014 M⊙, which is in good agreement with the dynamical masses when two galaxy components are considered separately. This suggests that the mass estimates obtained from X-ray and velocity dispersion are compatible even for non-relaxed clusters, at least when we are able to identify and separate galaxy clumps and derive masses by considering the virialized regions. We propose a 3D merging model and find that the fossil group is in an early phase of collision with the RXCJ1111 main cluster and placed at ∼8° ( ± 3° ) from line-of-sight. This merging model would explain the slight increase found in the TX with respect to what we would expect for relaxed clusters. Due to the presence of several brightest galaxies, after this collision, the substructure would presumably lose its fossil condition. Therefore, RXCJ1111 represents the observational evidence that the fossil stage of a system can be temporary and transitional.

A Chandra Study of the NGC 7618/UGC 12491 Major Group Merger at Apogee: Multiple Cold Fronts, Boxy Wings, Filaments, and Arc-shaped Slingshot Tails

Analyses of major group mergers are key to understanding the evolution of large-scale structure in the Universe and the microphysical properties of the hot gas in these systems. We present imaging and spectral analyses of deep Chandra observations of hot gas structures formed in the major merger of the NGC 7618 and UGC 12491 galaxy groups and compare the observed hot gas morphology, temperatures, and abundances with recent simulations. The morphology of the observed multiple cold front edges and boxy wings are consistent with those expected to be formed by Kelvin–Helmholtz instabilities and gas sloshing in inviscid gas. The arc-shaped slingshot tail morphologies seen in each galaxy suggest that the dominant galaxies are near their orbital apogee after having experienced at least one core passage at a large impact parameter.

The galaxy counterpart and environment of the dusty damped Lyman-αabsorber atz= 2.226 towards Q 1218+0832

We report on further observations of the field of the quasar Q 1218+0832. Geier et al. (2019, A&A, 625, L9) presented the discovery of the quasar resulting from a search for quasars reddened and dimmed by dust in foreground damped Lyman-αabsorbers (DLAs). The DLA is remarkable by having a very large H Icolumn density close to 1022cm−2. Its dust extinction curve shows the 2175 Å bump known from the Local Group. It also shows absorption from cold gas exemplified by C Iand CO molecules. For this paper, we present narrow-band observations of the field of Q 1218+0832 and also use an archivalHubbleSpace Telescope (HST) image to search for the galaxy counterpart of the DLA. No emission from the DLA galaxy is found in either the narrow-band imaging or in the HST image. In the HST image, we could probe down to an impact parameter of 0.3 arcsec and a 3-σdetection limit of 26.8 mag per arcsec2. In the narrow-band image, we probed down to a 0 arcsec impact parameter and detected nothing down to a 3-σdetection limit of about 3 × 10−17erg s−1cm−2. We did detect a bright Lyman-αemitter 59 arcsec south of Q 1218+0832 with a flux of 3 × 10−16erg s−1cm−2. We conclude that the DLA galaxy must be located at a very small impact parameter (< 0.3 arcsec, 2.5 kpc) or it is optically dark. Also, the DLA galaxy most likely is part of a galaxy group.

The Galaxy Number Density Profile of Halos

More precise measurements of galaxy clustering will be provided by the next generation of galaxy surveys, such as DESI, WALLABY, and the Square Kilometre Array. To utilize this information to improve our understanding of the Universe, we need to accurately model the distribution of galaxies in their host dark matter halos. In this work, we present a new galaxy number density profile of halos, which makes predictions for the positions of galaxies in the host halo, different to the widely adopted Navarro–Frenk–White (NFW) profile, since galaxies tend to be found more in the outskirts of halos (nearer the virial radius) than an NFW profile. The parameterized galaxy number density profile model of halos is fit and tested using the Dark Sage semi-analytic model of galaxy formation. We find that our galaxy number density profile model of halos can accurately reproduce the halo occupation distribution and galaxy two-point correlation function of the Dark Sage simulation. We also derive the analytic expressions for the circular velocity and gravitational potential energy for this profile model. We use the SDSS Data Release 10 galaxy group catalog to validate this galaxy number density profile model of halos. Compared to the NFW profile, we find that our model more accurately predicts the positions of galaxies in their host halo and the galaxy two-point correlation function.

Ram-pressure stripped radio tail and two ULXs in the spiral galaxy HCG 97b

ABSTRACT We report LOFAR and Very Large Array (VLA) detections of extended radio emission in the spiral galaxy HCG 97b, hosted by an X-ray bright galaxy group. The extended radio emission detected at 144 MHz, 1.4 GHz, and 4.86 GHz is elongated along the optical disc and has a tail that extends 27 kpc in projection towards the centre of the group at GHz frequencies or 60 kpc at 144 MHz. Chandra X-ray data show two off-nuclear ultra-luminous X-ray sources (ULXs), with the farther one being a plausible candidate for an accreting intermediate-mass black hole (IMBH). The asymmetry observed in both CO emission morphology and kinematics indicates that HCG 97b is undergoing ram-pressure stripping, with the leading side at the south-eastern edge of the disc. Moreover, the VLA 4.86 GHz image reveals two bright radio blobs near one ULX, aligning with the disc and tail, respectively. The spectral indices in the disc and tail are comparable and flat (α > −1), suggesting the presence of recent outflows potentially linked to ULX feedback. This hypothesis gains support from estimates showing that the bulk velocity of the relativistic electrons needed for transport from the disc to the tail is approximately ${\sim}1300\, \rm km~s^{-1}$. This velocity is much higher than those observed in ram-pressure stripped galaxies ($100{-}600\, \rm km~s^{-1}$), implying an alternative mechanism aiding the stripping process. Therefore, we conclude that HCG 97b is subject to ram pressure, with the formation of its stripped radio tail likely influenced by the putative IMBH activities.

The localization of galaxy groups in close proximity to galaxy clusters using cosmic web nodes

ABSTRACT We investigate the efficacy of using the cosmic web nodes identified by the DisPerSE topological filament finder to systematically identify galaxy groups in the infall regions around massive clusters. The large random motions and infall velocities of galaxies in the regions around clusters complicate the detection and characterisation of substructures through normal group-finding algorithms. Yet understanding the co-location of galaxies within filaments and/or groups is a key part of understanding the role of environment on galaxy evolution, particularly in light of next-generation wide-field spectroscopic surveys. Here we use simulated massive clusters from TheThreeHundred collaboration and compare the derived group catalogues, (haloes with σv > 300 h−1 km s−1) with the critical points from DisPerSE, ran on haloes with more than 100 particles. We find that in 3D, 56 per cent of DisPerSE nodes are correctly identified as groups (purity) while 68 per cent of groups are identified as nodes (completeness). The fraction of matches increases with group mass and with distance from the host cluster centre. This rises to a completeness of 100 per cent for the most massive galaxy groups (M > 1014 M⊙) in 3D, or 63 per cent when considering the projected 2D galaxy distribution. When a perfect match occurs between a cosmic web node and a galaxy group, the DisPerSE node density (δ) serves as an estimate of the group’s mass, albeit with significant scatter. We conclude that the use of a cosmic filament finder shows promise as a useful and straightforward observational tool for disentangling substructure within the infall regions of massive clusters.

MaNGA DynPop – IV. Stacked total density profile of galaxy groups and clusters from combining dynamical models of integral-field stellar kinematics and galaxy–galaxy lensing

ABSTRACT We present the measurement of total and stellar/dark matter decomposed mass density profile around a sample of galaxy groups and clusters with dynamical masses derived from integral-field stellar kinematics from the MaNGA survey in Paper I and weak lensing derived from the DECaLS imaging survey. Combining the two data sets enables accurate measurement of the radial density distribution from several kpc to Mpc scales. Intriguingly, we find that the excess surface density derived from stellar kinematics in the inner region cannot be explained by simply adding an NFW dark matter halo extrapolated from lensing measurement at a larger scale to a stellar mass component derived from the NASA-Sloan Atlas (NSA) catalogue. We find that a good fit to both data sets requires a stellar mass normalization about three times higher than that derived from the NSA catalogue, which would require an unrealistically too-heavy initial mass function for stellar mass estimation. If we keep the stellar mass normalization to that of the NSA catalogue but allow a varying inner dark matter density profile, we obtain an asymptotic slope of γgnfw = $1.82_{-0.25}^{+0.15}$ and γgnfw = $1.48_{-0.41}^{+0.20}$ for the group bin and the cluster bin, respectively, significantly steeper than the NFW case. We also compare the total mass inner density slopes with those from TNG300 and find that the values from the simulation are lower than the observation by about 2σ level.

QUIJOTE Scientific Results – XVII. Studying the anomalous microwave emission in the Andromeda Galaxy with QUIJOTE-MFI

ABSTRACT The Andromeda Galaxy (M31) is the Local Group galaxy that is most similar to the Milky Way (MW). The similarities between the two galaxies make M31 useful for studying integrated properties common to spiral galaxies. We use the data from the recent QUIJOTE-MFI Wide Survey, together with new raster observations focused on M31, to study its integrated emission. The addition of raster data improves the sensitivity of QUIJOTE-MFI maps by almost a factor 3. Our main interest is to confirm if anomalous microwave emission (AME) is present in M31, as previous studies have suggested. To do so, we built the integrated spectral energy distribution of M31 between 0.408 and 3000 GHz. We then performed a component separation analysis taking into account synchrotron, free–free, AME, and thermal dust components. AME in M31 is modelled as a log-normal distribution with maximum amplitude, AAME, equal to 1.03 ± 0.32 Jy. It peaks at ${\nu _{\rm AME}}=17.2\pm 3.2{\rm \, GHz}{}$ with a width of WAME = 0.58 ± 0.16. Both the Akaike and Bayesian information criteria find the model without AME to be less than 1 per cent as probable as the one taking AME into consideration. We find that the AME emissivity per 100 $\mu$m intensity in M31 is ${\epsilon _{\rm AME}^{\rm 28.4\, GHz}}=9.6\pm 3.1\,\mu$K MJy−1 sr, similar to that of the MW. We also provide the first upper limits for the AME polarization fraction in an extragalactic object. M31 remains the only galaxy where an AME measurement has been made of its integrated spectrum.

Galaxy And Mass Assembly: the xSAGA galaxy complement in nearby galaxy groups

ABSTRACT Groups of galaxies are the intermediate density environment in which much of the evolution of galaxies is thought to take place. In spectroscopic redshift surveys, one can identify these as close spatial-redshift associations. However, spectroscopic surveys will always be more limited in luminosity and completeness than imaging ones. Here, we combine the Galaxy And Mass Assembly (GAMA) group catalogue with the extended Satellites Around Galactic Analogues (xSAGA) catalogue of machine learning identified low-redshift satellite galaxies. We find 1825 xSAGA galaxies within the bounds of the GAMA equatorial fields (mr < 21), 1562 of which could have a counterpart in the GAMA spectroscopic catalogue (mr < 19.8). Of these, 1326 do have a GAMA counterpart with 974 below z = 0.03 (true positives) and 352 above z = 0.03 (false positives). By cross-correlating the GAMA group catalogue with the xSAGA catalogue, we can extend and characterize the satellite content of GAMA galaxy groups. We find that most groups have <5 xSAGA galaxies associated with them, but richer groups may have more. Each additional xSAGA galaxy contributes only a small fraction of the group’s total stellar mass (${\ll}10$ per cent). Selecting GAMA groups that resemble the Milky Way halo, with a few (<4) bright galaxies, we find that xSAGA can add a magnitude fainter sources to a group and that the Local Group does not stand out in the number of bright satellites. We explore the quiescent fraction of xSAGA galaxies in GAMA groups and find a good agreement with the literature.

The RESOLVE and ECO Gas in Galaxy Groups Initiative: The Group Finder and the Group H i–Halo Mass Relation

We present a four-step group-finding algorithm for the Gas in Galaxy Groups (G3) initiative, a spin-off of the z ∼ 0 REsolved Spectroscopy Of a Local VolumE (RESOLVE) and Environmental COntext (ECO) surveys. In preparation for future comparisons to intermediate redshift (e.g., the LADUMA survey), we design the group finder to adapt to incomplete, shallow, or nonuniform data. We use mock catalogs to optimize the group finder’s performance. Compared to friends-of-friends (with false-pair splitting), the G3 algorithm offers improved completeness and halo-mass recovery with minimal loss of purity. Combining it with the volume-limited H i census data for RESOLVE and ECO, we examine the H i content of galaxy groups as a function of group halo mass. Group-integrated H i mass M H I,grp rises monotonically over halo masses M halo ∼ 1011–1014.5 M ⊙, pivoting in slope at M halo ∼ 1011.4 M ⊙, the gas-richness threshold scale. We present the first measurement of the scatter in this relation, which has a median of ∼0.3 dex and is asymmetric toward lower M H I,grp. We discuss interesting tensions with theoretical predictions and prior measurements of the M H I,grp–M halo relation. In an appendix, we release RESOLVE DR4 and ECO DR3, including updates to survey redshifts, photometry, and group catalogs, as well as a major expansion of the ECO H i inventory with value-added data products.

Outskirts of Abell 1795: Probing gas clumping in the intracluster medium

Contact. The outskirts of galaxy clusters host complex interactions between the intracluster and circumcluster media. During the evolution of clusters, ram-pressure stripped gas clumps from infalling substructures break the uniformity of the gas distribution, potentially leading to observational biases at large radii. However, assessing the contribution of gas clumping poses observational challenges and requires robust X-ray measurements in the background-dominated regime of the cluster outskirts. Aims. The main objectives of this study are to isolate faint gas clumps from field sources and from the diffuse emission in the Abell 1795 galaxy cluster, then to probe their impact on the observed surface brightness and thermodynamic profiles. Methods. We performed an imaging analysis on deep Chandra ACIS-I observations of the Abell 1795 cluster outskirts, extending out to ∼1.5r200 with full azimuthal coverage. We built the 0.7 − 2.0 keV surface brightness distribution from the adaptively binned image of the diffuse emission and looked for clumps in the form of > + 2σ surface brightness outliers. Our classification of the clump candidates was based primarily on Chandra and SDSS data. Benefiting from the point source list resolved by Chandra, we extracted the thermodynamic profiles of the intracluster medium from the associated Suzaku XIS data out to r200 using multiple point source and clump candidate removal approaches. Results. We identified 24 clump candidates in the Abell 1795 field, most of which are likely to be associated with background objects, including active galactic nuclei, galaxies, and clusters or groups of galaxies, as opposed to intrinsic gas clumps. These sources had minimal impact on the surface brightness and thermodynamic profiles of the cluster emission. After correcting for clump candidates, the measured entropy profile still deviates from a pure gravitational collapse, suggesting complex physics at play in the outskirts, which may include potential electron–ion non-equilibrium and non-thermal pressure support.

Compact groups from semi-analytical models of galaxy formation – IV: Effect of group assembly on the evolution of their galaxies

ABSTRACT Using over 3000 compact groups (CGs) of galaxies extracted from mock catalogues built from semi-analytical models (SAMs) of galaxy formation, we study whether the CG assembly channel affects the z = 0 properties of galaxies and their evolution. The evolution of CG galaxy properties with time is a clear function of their stellar masses. For instance, high-stellar-mass CG galaxies have lived their last 8 Gyr with little cold gas content while maintaining their reservoir of hot gas, while low-mass CG galaxies still preserve some of their cold gas content at the present but they have completely drained their hot gas reservoir. Beyond that, we find that the evolution of CG galaxies is also a function of the assembly history of the CGs: with more extreme losses of gas content, faster mass gain rates for black holes and more marked suppression of star formation as a function of cosmic time as we go from recent to early CG assembly. Thus, CGs constitute another laboratory for galaxy assembly bias, as the later assembling groups have later star formation. Our results show that classifying CGs according to their assembly channel is a way of distinguishing different paths by which galaxies transform their properties throughout their history.

The miniJPAS survey

Context. Samples of galaxy clusters allow us to better understand the physics at play in galaxy formation and to constrain cosmological models once the mass, position (for clustering studies), and redshift are known. In this context, large optical datasets play a crucial role. Aims. We investigate the capabilities of the Javalambre-Physics of the Accelerating Universe Astrophysical Survey (J-PAS) in detecting and characterizing galaxy groups and clusters. We analysed the data of the miniJPAS survey, obtained with the JPAS-Pathfinder camera and covering 1 deg2 centred on the AEGIS field to the same depths and with the same 54 narrow-band filters plus two broader-band near-UV and near-IR filters anticipated for the full J-PAS survey. Methods. We used the Adaptive Matched Identifier of Clustered Objects (AMICO) algorithm to detect and characterize groups and clusters of galaxies down to S/N = 2.5 in the redshift range 0.05 < z < 0.8. Results. We detected 80, 30, and 11 systems with signal-to-noise ratios higher than 2.5, 3.0, and 3.5, respectively, down to ∼1013 M⊙ h−1. We derive mass-proxy scaling relations based on Chandra and XMM-Newton X-ray data for the signal amplitude returned by AMICO, the intrinsic richness, and a new proxy that incorporates the galaxies’ stellar masses. This proxy is made possible thanks to the J-PAS filters and shows a smaller scatter with respect to the richness. We fully characterize the sample and use AMICO to derive a probabilistic membership association of galaxies with the detected groups that we test against spectroscopy. We further show how the narrow-band filters of J-PAS provide a gain of up to 100% in signal-to-noise ratio in detection and an uncertainty on the redshift of clusters of only σz = 0.0037(1 + z), placing J-PAS between broad-band photometric and spectroscopic surveys. Conclusions. The performance of AMICO and J-PAS with respect to mass sensitivity, mass-proxies quality, and redshift accuracy will allow us to derive cosmological constraints not only based on cluster counts, but also based on clustering of galaxy clusters.

AGN number fraction in galaxy groups and clusters atz< 1.4 from the Subaru Hyper Suprime-Cam survey

AbstractOne of the key questions on active galactic nuclei (AGN) in galaxy clusters is how AGN could affect the formation and evolution of member galaxies and galaxy clusters in the history of the Universe. To address this issue, we investigate the dependence of AGN number fraction (fAGN) on cluster redshift (zcl) and distance from the cluster center (R$/$R200). We focus on more than 27000 galaxy groups and clusters at 0.1 < zcl < 1.4 with more than 1 million member galaxies selected from the Subaru Hyper Suprime-Cam. By combining various AGN selection methods based on infrared (IR), radio, and X-ray data, we identify 2688 AGN. We find that (i) fAGN increases with zcl and (ii) fAGN decreases with R$/$R200. The main contributors to the rapid increase of fAGN towards high-z and cluster center are IR- and radio-selected AGN, respectively. These results indicate that the emergence of the AGN population depends on the environment and redshift, and galaxy groups and clusters at high z play an important role in AGN evolution. We also find that cluster–cluster mergers may not drive AGN activity in at least the cluster center, while we have tentative evidence that cluster–cluster mergers could enhance AGN activity in the outskirts of (particularly massive) galaxy clusters.

The Ie-Re plane and the 3D-kappa space of stellar systems

Contact. This paper is the fourth in a series dedicated to the observed parallelism of properties passing from globular clusters to early-type galaxies. To a lesser extent, it also covers galaxy clusters and groups. Aims. Here, we investigate the Ie-Re plane and the 3D-kappa space defined by Bender, Burstein and Faber, as potential diagnostic tools in studies of the past evolution of these stellar systems. In the space of the parameters characterizing a stellar system such as the luminosity, L, stellar mass, Ms, half-light (mass) radius, Re, central velocity dispersion, σc, surface brightness, Ie, and so on, the Ie-Re plane is one of possible projections that was thoroughly investigated over the years with many important results. The 3D-kappa space relies on three variables that are suitable combinations of the logarithms of the above parameters. Among others, perhaps the most important result from this new space is the discovery of the fundamental plane of early type galaxies. In this paper, we intend to explore in more detail the potential capability of the joined investigation of the Ie-Re plane and 3D-kappa space. Methods. Based on the collected literature data on the mass, half-mass (light) radius, velocity dispersion, and surface brightness in different bands for the objects under investigation, we set up the Ie-Re plane and the 3D-kappa space. We then compared the observed distributions of these objects with those predicted by simple theoretical galaxy models. Results. We explored the effects of different mass-radius relationships, star formation, infall, and mass assembling histories on the diagnostic planes under examination. We also investigated variations in the 3D-kappa space as a function of the redshift. Conclusions. We show that the distribution of the stellar systems on the various diagnostic planes can cast light on the mass-radius relation and the history of star formation in stellar systems going from globular clusters to early type galaxies.

The M 101 galaxy group as a node in a nearby cosmic filament

We performed a search for faint low-surface-brightness dwarf galaxies around the major spiral galaxy M 101 and in the large rectangular area within SGL = [30–80]° and SGB = [10–37]° spanning a chain of galaxies: M 63, M 51, M 101, and NGC 6503. We based our search on data from DESI Legacy Imaging Surveys. We discovered six new probable dwarf members of the complex. We present a list of 25 prospective members of the M 101 group and estimate the total mass and the total-mass-to-K-band-luminosity ratio of the group as (1.02 ± 0.42)×1012 M⊙ and (16.0 ± 6.5) M⊙/L⊙, respectively. We note that the average dark mass-to-luminosity ratio in the groups around M 63, M 51, and M 101 is (12 ± 4) M⊙/L⊙, which is almost an order of magnitude lower than the global cosmic ratio, (102 ± 5) M⊙/L⊙.

An Initial Analysis of a Strongly Lensed QSO Candidate Identified by LAMOST

From 2011 to 2021, LAMOST has released a total of 76,167 quasar data. We try to search for gravitationally lensed QSOs by limiting coordinate differences and redshift differences of these QSOs. The name, brightness, spectrum, photometry and other information of each QSO will be visually checked carefully. Special attention should be paid to check whether there are groups of galaxies, gravitationally lensed arcs, Einstein crosses, or Einstein rings near the QSOs. Through careful selection, we select LAMOST J160603.01+290050.8 (A) and LAMOST J160602.81+290048.7 (B) as a candidate and perform an initial analysis. Components A and B are 3.″36 apart and they display blue during photometric observations. The redshift values of components A and B are 0.2% different, their Gaia_g values are 1.3% different, and their ugriz values are 1.0% or less different. For the spectra covering from 3690 to 9100 Å, the emission lines of C ii, Mg, H γ, O iii, and H β are present for both components A and B and the ratio of flux(B) to flux(A) from LAMOST is basically a constant, around 2.2. However, no galaxies have been found between components A and B. Inada et al. identified them as binary quasars. But we accidentally find a galaxy group near components A and B. If the center of dark matter in the galaxy group is at the center between components A and B, components A and B are probably gravitationally lensed QSOs. We estimate that the Einstein mass is 1.46 × 1011 M ⊙ and the total mass of the lens is 1.34 × 1013 M ⊙. The deflection angle is 1.″97 at positions A and B and the velocity dispersion is 261 km s−1. Theoretically, this candidate could be a pair of fold images of a strong lensing system by a galaxy group, and we will investigate the possibility when the redshifts of nearby galaxies are available.

A MUSE view of the massive merging galaxy cluster ACT-CL J0102−4915 (El Gordo) at z = 0.87

We present a detailed strong lensing analysis of the massive and distant (z = 0.870) galaxy cluster ACT-CL J0102−4915 (ACT0102, also known as El Gordo), taking advantage of new spectroscopic data from the Multi Unit Spectroscopic Explorer (MUSE) on the Very Large Telescope and archival imaging from the Hubble Space Telescope. Thanks to the MUSE data, we were able to measure secure redshifts for 374 single objects, including 23 multiply lensed galaxies, and 167 cluster members of ACT0102. We used the observed positions of 56 multiple images, along with their new spectroscopic redshift measurements, as constraints for our strong lensing model. Remarkably, some multiple images are detected out to a large projected distance of ≈1 Mpc from the brightest cluster galaxy, allowing us to estimate a projected total mass value of 1.84−0.04+0.03 × 1015 M⊙ within that radius. We find that we need two extended cluster mass components, the mass contributions from the cluster members and the additional lensing effect of a foreground (z = 0.633) group of galaxies, to predict the positions of all multiple images with a root mean square offset of 0″​​.75. The main cluster-scale mass component is centred very close to the brightest cluster galaxy, and the other extended mass component is located in the north-west region of the cluster. These two mass components have very similar values of mass projected within 300 kpc of their centres, namely 2.29−0.10+0.09 × 1014 M⊙ and 2.10−0.09+0.08 × 1014 M⊙, in agreement with the major merging scenario of ACT0102. We make publicly available the lens model, including the magnification maps and posterior distributions of the model parameter values, as well as the full spectroscopic catalogue containing all redshift measurements obtained with MUSE.

The role of groups in galaxy evolution: compelling evidence of pre-processing out to the turnaround radius of clusters

ABSTRACT We present clear and direct evidence of the pre-processing effect of group galaxies falling into clusters in the local Universe (z ≲ 0.1). We start with a sample of 238 clusters, from which we select 153 with N200 ≥ 20. We considered 1641 groups within the turnaround radius (∼5 × R200) of these 153 clusters. There are 6654 individual cluster galaxies and 4133 group galaxies within this radius. We considered two control samples of galaxies, in isolated groups and in the field. The former comprises 2601 galaxies within 1606 isolated groups, and the latter has 4273 field objects. The fraction of star-forming galaxies in infalling groups has a distinct clustercentric behaviour in comparison to the remaining cluster galaxies. Even at 5 × R200 the group galaxies already show a reduced fraction of star-forming objects. At this radius, the results for the individual cluster galaxies are actually compatible with the field. That is strong evidence that the group environment is effective to quench the star formation prior to the cluster arrival. The group star-forming fraction remains roughly constant inwards, decreasing significantly only within the cluster R200 radius. We have also found that the pre-processing effect depends on the group mass (indicated by the number of members). The effect is larger for more massive groups. However, it is significant even for pairs and triplets. Finally, we find evidence that the time-scale required for morphological transformation is larger than the one for quenching.

RASS-MCMF: a full-sky X-ray selected galaxy cluster catalogue

ABSTRACT We present the RASS-MCMF catalogue of 8449 X-ray selected galaxy clusters over 25 000 deg2 of extragalactic sky. The accumulation of deep multiband optical imaging data, the development of the Multi-Component Matched Filter (MCMF) cluster confirmation algorithm, and the release of the DESI Legacy Survey DR10 catalogue makes it possible – for the first time, more than 30 yr after the launch of the ROSAT X-ray satellite – to identify the majority of the galaxy clusters detected in the second ROSAT All-Sky-Survey (RASS) source catalogue (2RXS). The resulting 90 per cent pure RASS-MCMF catalogue is the largest intracluster medium (ICM)-selected cluster sample to date. RASS-MCMF probes a large dynamic range in cluster mass spanning from galaxy groups to the most massive clusters. The cluster redshift distribution peaks at $z$ ∼ 0.1 and extends to redshifts $z$ ∼ 1. Out to $z$ ∼ 0.4, the RASS-MCMF sample contains more clusters per redshift interval (dN/dz) than any other ICM-selected sample. In addition to the main sample, we present two subsamples with 6912 and 5506 clusters, exhibiting 95 per cent and 99 per cent purity, respectively. We forecast the utility of the sample for a cluster cosmological study, using realistic mock catalogues that incorporate most observational effects, including the X-ray exposure time and background variations, the existence likelihood selection and the impact of the optical cleaning with the algorithm MCMF. Using realistic priors on the observable–mass relation parameters from a DES-based weak lensing analysis, we estimate the constraining power of the RASS-MCMF×DES sample to be of 0.026, 0.033, and 0.15 (1σ) on the parameters Ωm, σ8, and $w$, respectively.