arXiv subjects

Y. E. Bahar

Publications and source records attributed to Y. E. Bahar.

At least 19 recordsLinked to original sources

arXiv2026

The SRG/eROSITA all-sky survey: X-ray scaling relations of galaxy groups and clusters in the western Galactic hemisphere

The soft X-ray telescope on board the SRG mission, eROSITA, has produced the largest sample to date of galaxy groups and clusters detected via their intracluster and intragroup medium emission. Scaling relations between the intrinsic properties of these systems provide valuable insight into their formation and evolution. In this work, we investigated the scaling relations between key physical properties, such as soft band X-ray luminosity, temperature, gas mass, and the low-scatter mass proxy $Y_{\rm X}$, for the galaxy groups and clusters detected in the first eROSITA All-Sky Survey (eRASS1). Our analysis fully accounts for selection effects and the redshift evolution of the observable distributions. We constructed a high-purity sample of $3061$ galaxy groups and clusters spanning the redshift range of $0.05 3$~keV, our inferred $L_{\rm X}-T$ relation is consistent with simulation models that include strong or stronger AGN feedback.

astro-ph.CO↗

arXiv2026

Thermal or Non-thermal? Diffuse emission in the infall region of stacked galaxy groups

The faint infall regions surrounding the virial radius of galaxy groups remain largely unexplored due to their low X-ray surface brightness. Using the large statistical power of SRG/eROSITA survey observations, we present the first spectroscopic measurement of the intragroup medium (IGrM) in the infall regions of a large sample of low-mass galaxy groups ($M_{\rm tot}<1\times10^{14}\,M_{sun}$), extending to $\sim2\,R_{200m}$ (2.2 Mpc). Through spectral stacking of 827 nearby groups from the first eROSITA All-Sky Survey catalog, we detect diffuse emission and measure the thermodynamic properties of gas at densities previously inaccessible to X-ray observations. The stacked spectra are well described by a Gaussian differential emission measure model, yielding a temperature distribution with a mean temperature of $0.96_{-0.04}^{+0.05}$ keV and width of $0.28_{-0.10}^{+0.10}$ keV, and a metal abundance of $0.21_{-0.04}^{+0.06}$ A$_{sun}$, consistent with expectations for group outskirts. The inferred electron densities decrease from $(4.8\pm1.3)\times10^{-5}$cm$^{-3}$ at $(0.7-2)\,R_{500c}$ to $(5.5\pm2.0)\times10^{-6}$ cm$^{-3}$ at $(2-4)\,R_{500c}$, demonstrating eROSITA's ability to probe the low-density outskirts of galaxy groups. Residual emission in the spectra suggests the presence of an additional spectral component. While a secondary thermal interpretation requires an unexpectedly hot, metal-poor plasma, a non-thermal inverse Compton model provides an equally plausible explanation, contributing $\sim30\%$ of the thermal flux. Assuming that the additional component is produced by inverse Compton emission from a common population of relativistic electrons, the inferred magnetic field strength would be in the sub-$μ$G regime.

astro-ph.HE↗

arXiv2026

The impact of strong feedback on galaxy group scaling relations

Feedback from active supermassive black holes alters the distribution of matter in the Universe by injecting energy in the neighbouring hot gaseous medium, which leads to ejection of gas from the halos of galaxy groups and massive galaxies. Recent cosmological simulations such as FLAMINGO calibrate their feedback model on the baryon fractions of galaxy groups to tune the efficiency of gas ejection. However, recent observational constraints from optically selected groups and the kinetic Sunyaev-Zel'dovich effect yield lower baryon fractions than previous studies, which indicates that feedback may be more ejective than previously thought. Here we show that models involving highly ejective feedback are inconsistent with the scaling relations of local galaxy groups in the mass range $10^{13}-10^{14}M_\odot$. We study the X-ray luminosity-temperature relation in a sample of 44 galaxy groups with high-quality XMM-Newton observations. We show that highly ejective models under-predict the luminosity of galaxy groups at fixed mass at high significance ($5.7σ$). This conclusion is robust against selection effects and is obtained from directly measurable and minimally correlated quantities. We point out that turning observable quantities into gas fraction estimates is challenging, especially in the context of stacking large samples of heterogeneous systems. We argue that calibrating feedback models on baryon fractions is prone to systematic uncertainties and that observable scaling relations are better suited for this task.

astro-ph.CO↗

arXiv2026

Bound or blown: the fate of hot gas in galaxy groups

The impact of AGN feedback on the hot gas content of galaxy groups remains a key uncertainty in galaxy formation and its connection to the large scale structure of the Universe. We aim to compare the XMM-Newton Group AGN Project (X-GAP) sample to the hydrodynamical FLAMINGO simulations, which span a wide range of AGN feedback prescriptions. We construct X-GAP analogues by forward-modelling the full selection function, including detection and observational systematics, and generate end-to-end XMM-Newton mock observations analysed consistently with the data. We study multiple observables, including the L--T and Mgas--T relations, number of groups, mean temperature, and velocity dispersion, accounting for their covariance. The forward model accurately recovers input luminosities, gas masses, and core-excised temperatures for regular systems, enabling direct comparison in observable space. The normalisation of the scaling relations is the best discriminator between feedback models, while cosmic variance introduces > 20% fluctuations in the number of detected systems, making counts alone a weak discriminator. Models with intermediate feedback strength provide the best agreement with X-GAP, with the fgas-2sigma model yielding the lowest tension of only 0.8sigma, while the most extreme feedback scenario (fgas-8sigma) is ruled out at > 4sigma. Our results indicate that the thermodynamic properties of galaxy groups favour feedback stronger than the fiducial FLAMINGO calibration, but disfavour the most ejective models. This highlights the importance of combining forward modelling and multi-observable constraints to probe the fate of hot baryons in low-mass haloes.

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arXiv2026

The SRG/eROSITA all-sky survey: Constraints on Ultra-light Axion Dark Matter through Galaxy Cluster Number Counts

Ultralight axions are hypothetical scalar particles that influence the evolution of large-scale structures of the Universe. Depending on their mass, they can potentially be part of the dark matter component of the Universe as candidates commonly referred to as fuzzy dark matter. While strong constraints have been established for pure fuzzy dark matter models, the more general scenario where ultralight axions constitute only a fraction of the dark matter has been limited to only a few observational probes. In this work, we use the galaxy cluster number counts obtained from the first All-Sky Survey (eRASS1) of the SRG/eROSITA mission together with gravitational weak lensing data from the Dark Energy Survey, the Kilo-Degree Survey, and the Hyper Suprime-Cam to constrain the fraction of ultralight axions in the mass range $10^{-32}$ eV to $10^{-24}$ eV. We put upper bounds on the ultralight axion relic density $Ω_\mathrm{a}$ in independent logarithmic axion mass bins by performing a full cosmological parameter inference. We find an exclusion region in the intermediate ultralight axion mass regime with the tightest bounds reported so far in the mass bins around $m_\text{a} = 10^{-27}$ eV with $Ω_\text{a} < 0.0035$ and $m_\text{a} = 10^{-26}$ eV with $Ω_\text{a} < 0.0079$ (95% C.L.). When combined with cosmic microwave background probes, these bounds are tightened to $Ω_\text{a} < 0.0030$ in the $m_\text{a} = 10^{-27}$ eV mass bin and $Ω_\text{a} < 0.0058$ in the $m_\mathrm{a} = 10^{-26}$ eV mass bin (95% C.L.). This is the first time that constraints on ultralight axions have been obtained using the growth of structure measured by galaxy cluster number counts. These results pave the way for large surveys, which can be utilized to obtain tight constraints on the mass and relic density of ultralight axions with better theoretical modeling of the abundance of halos.

astro-ph.CO↗

arXiv2026

The SRG/eROSITA All-Sky Survey. Detection of shock-heated gas beyond the halo boundary into the accretion region

The hot gas in the outskirts of galaxy cluster-sized halos, extending around and beyond the virial radius into nearby accretion regions, remains among one of the least explored baryon components of the large-scale cosmic structure. We present a stacking analysis of 680 galaxy clusters located in the western Galactic hemisphere, using data from the first two years of the SRG/eROSITA All-Sky Survey. The stacked X-ray surface brightness (SB) profile reveals a statistically significant signal extending out to 2 r_200m (~4.5 Mpc). The best-fit SB profile is well described by a combination of terms describing orbiting and infalling gas, with a transition occurring around r_200m. At this radius, the gas number density corresponds to a baryon overdensity of about 30. By integrating the density profile out to r_200m, we inferred a gas fraction exceeding the universal baryon fraction, assuming a typical halo concentration. However, correcting for possible clumping effects reduces the baryon fraction by more than 20%. Additionally, we examined the distribution of hot gas in massive clusters in the IllustrisTNG simulations, from the halo center to the accretion region. This analysis reveals differences in radial gas profiles depending on whether the direction points toward voids or toward nearby cosmic filaments. Beyond r_200m, the density profile along the filament direction exceeds that along the void direction. This pattern aligns with the observed transition radius between the one-halo and two-halo terms, suggesting that r_200m is the approximate radius marking the location at which cosmic filaments connect to galaxy clusters. Meanwhile, comparisons of the gas density and gas fraction profiles between the observation and the IllustrisTNG simulation suggest that the feedback processes in the stacking sample are more efficient at distributing gas to large radii than the IllustrisTNG model.

astro-ph.CO↗

arXiv2026

CHEX-MATE: Are we getting cluster thermodynamics right?

Galaxy clusters offer powerful insights into the large-scale structure of the Universe and the physics of baryons in hot state. Their scientific exploitation, however, hinges on our ability to accurately measure key thermodynamic properties. In this work, we aim to assess the reliability of current analysis techniques in reconstructing these properties, with particular focus on samples similar to those observed in the Cluster HEritage project with XMM-Newton (CHEX-MATE). We develop a suite of dedicated end-to-end simulations of CHEX-MATE-like clusters selected from large scale hydrodynamical simulations, and processed through a newly developed realistic XMM-Newton simulator. We apply a full X-ray data analysis pipeline to the mock datasets, including imaging, spectral fitting, and profile reconstruction. The gas density profiles can be robustly recovered across a wide radial range, when using azimuthal mean surface brightness profiles. Our reconstruction techniques are able to reproduce the intrinsic density profile with the correct scatter, with deviations of at most 10% between 0.1 and 1xR500c. The gas mass is reconstructed with better than 1% accuracy. Accurate measurement of temperature profiles is more challenging and possibly subject to biases, particularly in the presence of azimuthal variations and multi-temperature gas along the line of sight, which dominate over projection effects. Our results highlight the need for caution in interpreting cluster temperature measurements and underscore the value of tailored mock observations for understanding observational systematics. These findings also suggest that biases in X-ray temperature measurements may alter the interpretation of the thermodynamical state of the intra-cluster medium, an outlook particularly relevant in light of recent low velocity measurements from the XRISM mission.

astro-ph.CO↗

arXiv2026

The SRG/eROSITA all-sky survey: The morphologies of clusters of galaxies: II. The intrinsic distributions of morphological parameters

X-ray selected surveys of clusters of galaxies have been reported to contain more regular cool core clusters compared to samples selected using the Sunyaev-Zel'dovich (SZ) effect. Morphology population studies on X-ray selected clusters will be biased without taking into account selection, as cool cores are more easily detected at low redshifts, but can be mistaken for point sources at high redshift. eROSITA, aboard SRG, found over 12000 optically-identified clusters in its first survey, eRASS1. Taking account of the selection function obtained from simulations, we obtain using a Bayesian framework the intrinsic distribution of morphological parameters, including the concentration, central density, cuspiness, ellipticity and slosh. We construct scaling relations for the parameters as a function of redshift (z) and luminosity (LX), and study their distribution within z or LX bins. We find that the concentration in a scaled aperture evolves positively with LX, similarly to the central scaled density, and negatively with z. When using a fixed aperture, its evolution with LX is lower, but also dependent on the choice of cluster centre. The mean ellipticity does not significantly evolve with z or LX. eRASS1 clusters show indications of higher concentrations compared to SZ-selected objects, even after taking account the selection; this suggests that if our X-ray selection model is correct SZ-selected clusters may also suffer from morphological selection effects. We compare different parameter distribution models in bins of z and LX. The distribution of concentration and ellipticity is generally consistent with a normal one, but other parameters such as the central density and cuspiness strongly favour more complex distributions. However, modelling of all clusters as a single population generally prefers non-normal distributions. [abridged]

astro-ph.CO↗

arXiv2025

The SRG/eROSITA All-Sky Survey: Constraints on AGN Feedback in Galaxy Groups

We investigate the impact of AGN feedback, on the entropy and characteristic temperature measurements of galaxy groups detected in the SRG/eROSITA's first All-Sky Survey (eRASS1) to shed light on the characteristics of the feedback mechanisms. We analyze deeper eROSITA observations of 1178 galaxy groups detected in eRASS1. We divide the sample into 271 subsamples and extract average thermodynamic properties, including electron density, temperature, and entropy at three characteristic radii along with the integrated temperature by jointly analyzing X-ray images and spectra following a Bayesian approach. We present the tightest constraints on the impact of AGN feedback through our average entropy and characteristic temperature measurements of the largest group sample used in X-ray studies, incorporating major systematics in our analysis. We find that entropy shows an increasing trend with temperature in the form of a power-law-like relation at the higher intra-group medium temperatures, while for the low mass groups, a slight flattening is observed on the average entropy. Overall, the observed entropy measurements agree well with the earlier measurements in the literature. The comparisons with the state-of-the-art cosmological hydrodynamic simulations (MillenniumTNG, Magneticum, OWL simulations) after the applications of the selection function calibrated for our galaxy groups reveal that observed entropy profiles in the cores are below the predictions of simulations. At the mid-region, the entropy measurements agree well with the Magneticum simulations, whereas the predictions of MillenniumTNG and OWL simulations fall below observations. At the outskirts, the overall agreement between the observations and simulations improves, with Magneticum simulations reproducing the observations the best. Our measurements will pave the way for more realistic AGN feedback implementations in simulations.

astro-ph.CO↗

arXiv2025

eROSITA clusters dynamical state and their impact on the BCG luminosity

The Spectrum Roentgen Gamma/eROSITA first public release contains 12,247 clusters and groups. We use the offset between the Brightest Cluster Galaxy (BCG) and the X--ray peak (D$_{\rm BCG-X}$) to classify the cluster dynamical state of 3,946 galaxy clusters and groups. The X--ray peaks come from the eROSITA survey while the BCG positions come from the DECaLS DR10 optical data, which includes the DECam eROSITA Survey optical data. We aim to investigate the evolution of the merger and relaxed cluster distributions with redshift and mass, and their impact on the BCG. We model the distribution of D$_{\rm BCG-X}$ as the sum of two Rayleigh distributions representing the cluster's relaxed and disturbed populations, and explore their evolution with redshift and mass. To explore the impact of the cluster's dynamical state on the BCG luminosity, we separate the main sample according to the dynamical state. We define clusters as relaxed if D$_{\rm BCG-X}$ < 0.25$r_{500}$, disturbed if D$_{\rm BCG-X}$>0.5$r_{500}$, and as `diverse' otherwise. We find no evolution of the merging fraction in redshift and mass. We observe that the width of the relaxed distribution to increase with redshift, while the width of the two Rayleigh distributions decreases with mass. The analysis reveals that BCGs in relaxed clusters are brighter than BCGs in both the disturbed and diverse cluster population. The most significant differences are found for high mass clusters at higher redshift. The results suggest that BCGs in low-mass clusters are less centrally bound than those in high-mass systems, irrespective of dynamical state. Over time, BCGs in relaxed clusters progressively align with the potential center. This alignment correlates with their luminosity growth relative to BCGs in dynamically disturbed clusters, underscoring the critical role of the clusters dynamical state in regulating BCG evolution. [Abridged]

astro-ph.CO↗

arXiv2025

The SRG/eROSITA all-sky survey: The morphologies of clusters of galaxies I: A catalogue of morphological parameters

The first SRG/eROSITA all-sky X-ray survey, eRASS1, resulted in a catalogue of over twelve thousand optically-confirmed galaxy groups and clusters in the western Galactic hemisphere. Using the eROSITA images of these objects, we measure and study their morphological properties, including their concentration, central density and slope, ellipticity, power ratios, photon asymmetry, centroid shift and Gini coefficient. We also introduce new forward-modelled parameters which take account of the instrument point spread function (PSF), which are slosh, which measures how asymmetric the surface brightness distribution is, and multipole magnitudes, which are analogues to power ratios. Using simulations, we find some non forward-modelled parameters are strongly biased due to PSF and data quality. For the same clusters, we find similar values of concentration and central density compared to results by ourselves using Chandra and previous results from XMM-Newton. The population as a whole has log concentrations which are typically around 0.3 dex larger than South Pole Telescope or Planck-selected samples and the deeper eFEDS sample. The exposure time, detection likelihood threshold, extension likelihood threshold and number of counts affect the concentration distribution, but generally not enough to reduce the concentration to match the other samples. The concentration of clusters in the survey strongly affects whether they are detected as a function of redshift and luminosity. We introduce a combined disturbance score based on a Gaussian mixture model fit to several of the parameters. For brighter clusters, around 1/4 of objects are classified as disturbed using this score, which may be due to our sensitivity to concentrated objects.

astro-ph.CO↗

arXiv2025

The First SRG/eROSITA All-Sky Survey. Characterization of clusters of galaxies misclassified in the eRASS1 point source catalog

The detection of the extended X-ray-emission of the intracluster medium by the first SRG/eROSITA All-Sky Survey (eRASS1), combined with optical and near-infrared follow-up, resulted in the identification of more than 12000 galaxy clusters, yielding precise constraints on cosmological parameters. However, some clusters of galaxies can be misclassified as point sources by eROSITA's source detection algorithm due to the interplay between the point-spread function, the shallow depth of the survey, compact (cool core) X-ray emission, and bright active galactic nuclei hosted in their centers or their vicinity. To identify such misclassified galaxy clusters and groups, we apply optical follow-up to the eRASS1 X-ray point sources analogously to the treatment of the extent-selected catalog. After rigorous filtering to ensure purity, we find a total of 8347 clusters of galaxies, of which 5819 are novel detections, in a redshift range $0.05 < z \lesssim 1.1$. This corresponds to a 70 % discovery rate, a fraction similar to that of the extent-selected sample. To facilitate finding new exceptional clusters such as the Phoenix cluster (which is recovered in our sample), we divide the clusters into five classes based on the optical properties of likely single-source counterparts to the X-ray emission. We further investigate potential biases in our selection process by analyzing the optical and X-ray data. With this work, we provide a catalog of galaxy clusters and groups in the eRASS1 point source catalog, including their optical and X-ray properties along with a meaningful classification.

astro-ph.GA↗

arXiv2024

The SRG/eROSITA All-Sky Survey : Constraints on the structure growth from cluster number counts

Beyond testing the current cosmological paradigm, cluster number counts can also be utilized to investigate the discrepancies currently affecting current cosmological measurements. In particular, cosmological studies based on cosmic shear and other large-scale structure probes routinely find a value of the amplitude of the fluctuations in the universe S8 smaller than the one inferred from the primary cosmic microwave background. In this work, we investigate this tension by measuring structure evolution across cosmic time as probed by the number counts of the massive halos with the first SRG/eROSITA All-Sky Survey cluster catalog in the Western Galactic Hemisphere complemented with the overlapping Dark Energy Survey Year-3, KiloDegree Survey, and Hyper Suprime-Cam data for weak lensing mass calibration, by implementing two different parameterizations and a model-agnostic method. In the first model, we measure the cosmic linear growth index as γ = 1.19 \pm 0.21, in tension with the standard value of γ = 0.55, but in good statistical agreement with other large-scale structures probes. The second model is a phenomenological scenario in which we rescale the linear matter power spectrum at low redshift to investigate a potential reduction of structure formation, providing similar results. Finally, in a third strategy, we consider a standard ΛCDM cosmology, but we separate the cluster catalog into five redshift bins, measuring the cosmological parameters in each and inferring the evolution of the structure formation, finding hints of a reduction. Interestingly, the S8 value inferred from eRASS1 cluster number counts, when we add a degree of freedom to the matter power spectrum, recovers the value inferred by cosmic shear studies.

astro-ph.CO↗

arXiv2024

The SRG/eROSITA all-sky survey. X-ray emission from the warm-hot phase gas in long cosmic filaments

The properties of the warm-hot intergalactic medium (WHIM) in cosmic filaments are among the least quantified units in modern astrophysics. The Spectrum Roentgen Gamma/eROSITA All Sky Survey ((SRG/eRASS) provides a unique opportunity to study the X-ray emission of the WHIM. We applied both imaging and spectroscopic stacking techniques to the data of the first four eRASS scans to inspect the X-ray emissions from 7817 cosmic filaments identified from Sloan Digital Sky Survey (SDSS) optical galaxy samples. We obtained a $9σ$ significant detection of the total X-ray signal from filaments in the 0.3--1.2~keV band. Here, we introduce a novel method to estimate the contamination fraction from unmasked X-ray halos, active galactic nuclei, and X-ray binaries associated with filament galaxies. We found an approximately 40\% contamination fraction for these unmasked sources, suggesting that the remaining 60\% of the signal could be coming from the WHIM and a $5.4σ$ detection significance of the WHIM. Moreover, we modeled the temperature and baryon density contrast of the detected WHIM by fitting the stacked spectrum and surface brightness profile. The best-fit temperature $\log(T/\mathrm{K})=6.84\pm0.07$, obtained by using a single temperature model, is marginally higher than in the simulation results. This could be due to the fitting of a single temperature model on a multi-temperature spectrum. Assuming a 0.2 solar abundance, the best-fit baryon density contrast $\logΔ_\mathrm{b}=1.88\pm0.18$ is in general agreement with the X-ray emitting phases in the IllustrisTNG simulation. This result suggests that the broadband X-ray emission traces the high end of the temperature and density values that characterize the entire WHIM population.

astro-ph.HE↗

arXiv2024

The SRG/eROSITA All-Sky Survey. Optical identification and properties of galaxy clusters and groups in the western galactic hemisphere

The first SRG/eROSITA All-Sky Survey (eRASS1) provides the largest intracluster medium-selected galaxy cluster and group catalog covering the western galactic hemisphere. Compared to samples selected purely on X-ray extent, the sample purity can be enhanced by identifying cluster candidates using optical and near-infrared data from the DESI Legacy Imaging Surveys. Using the red-sequence-based cluster finder eROMaPPer, we measured individual photometric properties (redshift $z_λ$, richness $λ$, optical center, and BCG position) for 12,000 eRASS1 clusters over a sky area of 13,116 deg$^2$, augmented by 247 cases identified by matching the candidates with known clusters from the literature. The median redshift of the identified eRASS1 sample is $z=0.31$, with 10% of the clusters at $z>0.72$. The photometric redshifts have an accuracy of $δz/(1+z)<0.005$ for $0.05 95% for $z>0.05$. For these and further quality assessments of the eRASS1 identified catalog, we applied our identification method to a collection of galaxy cluster catalogs in the literature, as well as blindly on the full Legacy Surveys covering 24,069 deg$^2$. Using a combination of these cluster samples, we investigated the velocity dispersion-richness relation, finding $\log(λ)=2.401\times\log(σ)-5.074$ with an intrinsic scatter of $0.10\pm0.01$ dex. Our main result is the identified eRASS1 cluster catalog with a high purity and a well-defined X-ray selection process, enabling precise cosmological analyses presented in companion papers.

astro-ph.CO↗

arXiv2024

The SRG/eROSITA all-sky survey: Cosmology constraints from cluster abundances in the western Galactic hemisphere

The cluster mass function traces the growth of linear density perturbations and provides valuable insights into the growth of structures, the nature of dark matter, and the cosmological parameters governing the Universe. The primary science goal of eROSITA, on board the {\it Spectrum Roentgen Gamma (SRG)} mission, launched in 2019, is to constrain cosmology through the evolution of cluster mass function. In this paper, we present the cosmological constraints obtained from 5259 clusters of galaxies detected over an area of 12791~deg$^2$ in the Western Galactic Hemisphere of the eROSITA's first All-Sky Survey (eRASS1). The common footprint region between the eROSITA Survey and DES, KiDS, and HSC surveys is used for calibration of the scaling between X-ray count rate and their total mass through measurements of their weak gravitational lensing signal. eRASS1 cluster abundances constrain the $Λ$CDM parameters, which are the energy density of the total matter to $Ω_{\mathrm{m}}=0.29^{+0.01}_{-0.02}$, and the normalization of the density fluctuations to $σ_8=0.88\pm0.02$ and their combination yields $S_8=σ_8 (Ω_\mathrm{m} / 0.3)^{0.5}=0.86\pm0.01$, consistent and at a similar precision with the state-of-the-art CMB measurements. eRASS1 cosmological experiment places a most stringent upper limit on the summed masses of left-handed light neutrinos to $\sum m_ν< 0.22\mathrm{~eV}$ (95\% confidence interval). Combining eRASS1 cluster abundance measurements with CMB and ground-based neutrino oscillation experiments, we measure the summed neutrino masses to be $\sum m_ν=0.08_{-0.02}^{+0.03}\mathrm{~eV}$ or $\sum m_ν=0.12_{-0.01}^{+0.03}\mathrm{~eV}$ depending on the mass hierarchy scenario for neutrino eigenstates. eRASS1 cluster abundances significantly improve the constraints on the dark energy equation of state parameter to $w=-1.12\pm0.12$. (ABRIDGED)

astro-ph.CO↗

arXiv2024

The SRG/eROSITA All-Sky Survey: SRG/eROSITA cross-calibration with Chandra and XMM-Newton using galaxy cluster gas temperatures

Galaxy cluster gas temperatures ($T$) play a crucial role in many cosmological and astrophysical studies. However, it has been shown that $T$ measurements can vary between different X-ray telescopes. These $T$ biases can propagate to several cluster applications for which $T$ can be used. Thus, it is important to accurately cross-calibrate X-ray instruments to account for systematic biases. In this work, we present the cross-calibration between SRG/eROSITA and Chandra/ACIS, and between SRG/eROSITA and XMM-Newton/EPIC, using for the first time a large sample of galaxy cluster $T$. To do so, we use the first eROSITA All-Sky Survey data and a large X-ray flux-limited cluster catalog. We measure X-ray $T$ for 186 independent cluster regions with both SRG/eROSITA and Chandra/ACIS in a self-consistent way, for three energy bands; 0.7-7 keV (full), 0.5-4 keV (soft), and 1.5-7 keV (hard). We do the same with SRG/eROSITA and XMM-Newton/EPIC for 71 different cluster regions and all three bands. We find that SRG/eROSITA measures systematically lower $T$ than the other two instruments. For the full band, SRG/eROSITA returns 20$\%$ and 14$\%$ lower $T$ than Chandra/ACIS and XMM-Newton/EPIC respectively, when the two latter instruments measure $k_{\text{B}}T\approx 3$ keV each. The discrepancy increases to 38\% and 32\% when Chandra/ACIS and XMM-Newton/EPIC measure $k_{\text{B}}T\approx 10$ keV respectively. For low-$T$ galaxy groups, the discrepancy becomes milder. The soft band shows a marginally lower discrepancy than the full band. In the hard band, the cross-calibration of SRG/eROSITA and the other instruments show stronger differences. We could not identify any possible systematic biases that significantly alleviated the tension. Finally, we provide conversion factors between SRG/eROSITA, Chandra/ACIS, and XMM-Newton/EPIC $T$ which will be beneficial for future cluster studies.

astro-ph.CO↗

arXiv2024

The SRG/eROSITA All-Sky Survey: Exploring halo assembly bias with X-ray selected superclusters

We explore halo assembly bias on cluster scales using large samples of superclusters. Leveraging the largest-ever X-ray galaxy cluster and supercluster samples obtained from the first SRG/eROSITA all-sky survey, we construct two subsamples of galaxy clusters which consist of supercluster members (SC) and isolated clusters (ISO) respectively. After correcting the selection effects on redshift, mass, and survey depth, we compute the excess in the concentration of the intracluster gas of isolated clusters with respect to supercluster members, defined as $δc_{\rm gas} \equiv c_{\rm gas,ISO}/c_{\rm gas,SC}-1$, to investigate the environmental effect on the concentration of clusters, an inference of halo assembly bias on cluster scales. We find that the average gas mass concentration of isolated clusters is a few percent higher than that of supercluster members, with a maximum significance of $2.8σ$. The result on $δc_{\rm gas}$ varies with the overdensity ratio $f$ in supercluster identification, cluster mass proxies, and mass ranges, but remains positive in all the measurements. We measure slightly larger $δc_{\rm gas}$ when adopting a higher $f$ in supercluster identification. $δc_{\rm gas}$ is also larger for low-mass and low-redshift clusters. We perform weak lensing analyses to compare the total mass concentration of the two classes and find a similar trend as obtained from gas mass concentration. Our results are consistent with the prediction of HAB on cluster scales, where halos located in denser environments are less concentrated, and this trend is stronger for halos with lower mass and at lower redshifts. These phenomena can be interpreted by the fact that clusters in denser environments such as superclusters have experienced more mergers than isolated clusters in their assembling history.

astro-ph.CO↗