A New X-ray Selected Sample of Very Extended Galaxy Groups from the ROSAT All-Sky Survey

Some indications for tension have long been identified between cosmological constraints obtained from galaxy clusters and primary CMB measurements. Typically, assuming the matter density and fluctuations, as parameterized with Omega_m and sigma_8, estimated from CMB measurements, many more clusters are expected than those actually observed. One possible explanation could be that certain types of galaxy groups or clusters were missed in samples constructed in previous surveys, resulting in a higher incompleteness than estimated. We aim to determine if a hypothetical class of very extended, low surface brightness, galaxy groups or clusters have been missed in previous X-ray cluster surveys based on the ROSAT All-Sky Survey (RASS). We applied a dedicated source detection algorithm sensitive also to more unusual group or cluster surface brightness distributions. We found many known but also a number of new group candidates, which are not included in any previous X-ray / SZ cluster catalogs. In this paper, we present a pilot sample of 13 very extended groups discovered in the RASS at positions where no X-ray source has been detected previously and with clear optical counterparts. The X-ray fluxes of at least 5 of these are above the nominal flux-limits of previous RASS cluster catalogs. They have low mass ($10^{13} - 10^{14} M_{\odot}$; i.e., galaxy groups), are at low redshift (z<0.08), and exhibit flatter surface brightness distributions than usual. We demonstrate that galaxy groups were missed in previous RASS surveys, possibly due to the flat surface brightness distributions of this potential new population. Analysis of the full sample will show if this might have a significant effect on previous cosmological parameter constraints based on RASS cluster surveys. (This is a shortened version of the abstract - full text in the article)Comment: 18 pages, 7 figures, accepted by A&

High-redshift galaxy groups as seen by Athena/WFI

The first massive galaxy groups in the Universe are predicted to have formed at redshifts well beyond two. Baryonic physics, like stellar and active galactic nuclei (AGN) feedback in this very active epoch, are expected to have left a strong imprint on the thermo-dynamic properties of these early galaxy groups. Therefore, observations of these groups are key to constrain the relative importance of these physical processes. However, current instruments are not sensitive enough to detect them easily and characterize their hot gas content. In this work, we quantify the observing power of the Advanced Telescope for High ENergy Astrophysics (Athena), the future large X-ray observatory of the European Space Agency (ESA), for discovering and characterizing early galaxy groups at high redshifts. We used the SImulation of X-ray TElescopes (SIXTE) simulator to mimic Athena observations, and a custom-made wavelet-based algorithm to detect galaxy groups and clusters in the redshift range $0.5 \le z \le 4$. We performed extensive X-ray spectral fitting in order to characterize their gas temperature and X-ray luminosity. We also investigate how well Athena will constrain different feedback mechanisms. In the deep Wide Field Imager (WFI) survey expected to be carried out during part of Athena's first four years (the nominal mission lifetime) more than 10,000 galaxy groups and clusters at $z\ge 0.5$ will be discovered. We find that Athena can detect $\sim20$ high-redshift galaxy groups with masses of $M_{500}\geq$ $5\times 10^{13}$ $M_{\odot}$ and $z\geq2$, and almost half of them will have a gas temperature determined to a precision of $\Delta T/T \le 25\%$. We demonstrate that high-redshift galaxy groups can be detected very efficiently as extended sources by Athena and that a key parameter determining the total number of such newly discovered sources is the area on the sky surveyed by Athena.Comment: 24 pages, 18 figures, accepted for publication in A&

Toward the low-scatter selection of X-ray clusters : Galaxy cluster detection with eROSITA through cluster outskirts

Context. One key ingredient in using galaxy clusters as a precision cosmological probe in large X-ray surveys is understanding selection effects. The dependence of the X-ray emission on the square of the gas density leads to a predominant role of cool cores in the detection of galaxy clusters. The contribution of cool cores to the X-ray luminosity does not scale with cluster mass and cosmology and therefore affects the use of X-ray clusters in producing cosmological constraints.Aims. One of the main science goals of the extended ROentgen Survey with an Imaging Telescope Array (eROSITA) mission is to constrain cosmology with a wide X-ray survey. We propose an eROSITA galaxy cluster detection scheme that avoids the use of X-ray cluster centers in detection. We calculate theoretical expectations and characterize the performance of this scheme by simulations.Methods. We performed Monte Carlo simulations of the upcoming eROSITA mission, including known foreground and background components. By performing realistic simulations of point sources in survey mode, we searched for spatial scales where the extended signal is not contaminated by the point-source flux. We derive a combination of scales and thresholds, which result in a clean extended source catalog. We designed the output of the cluster detection, which enables calibrating the core-excised luminosity using external mass measurements. We provide a way to incorporate the results of this calibration in producing the final core-excised luminosity.Results. Similarly to other galaxy cluster detection pipelines, we sample the detection space of the flux - cluster core radius of our method and find many similarities with the pipeline used in the 400d survey. Both detection methods require large statistics on compact clusters in order to reduce the contamination from point sources. The benefit of our pipeline consists of the sensitivity to the outer cluster shapes, which are characterized by large core sizes with little cluster to cluster variation at a fixed total mass of the cluster.Conclusions. Galaxy cluster detection through cluster outskirts improves the cluster characterization using eROSITA survey data and is expected to yield well-characterized cluster catalogs with simple selection functions.Peer reviewe

The eROSITA extragalactic CalPV serendipitous catalog

Context. The eROSITA X-ray telescope on board the Spectrum-Roentgen-Gamma (SRG) observatory performed calibration and performance verification (CalPV) observations between September 2019 and December 2019, ahead of the planned 4-yr all-sky surveys. Most of them were deep, pointing-mode observations. Aims. We present here the X-ray catalog detected from the set of extra-galactic CalPV observations released to the public by the German eROSITA consortium, and the multiband counterparts of these X-ray sources. Methods. We developed a source detection method optimized for point-like X-ray sources by including extended X-ray emission in the background measurement. The multiband counterparts were identified using a Bayesian method from the CatWISE catalog. Results. Combining 11 CalPV fields, we present a catalog containing 9515 X-ray sources, whose X-ray fluxes were measured through spectral fitting. CatWISE counterparts are presented for 77% of the sources. Significant variabilities are found in 99 of the sources, which are also presented with this paper. Most of these fields show similar number counts of point sources as typical extragalactic fields, and a few harbor particular stellar populations

Introducing constrained matched filters for improved separation of point sources from galaxy clusters

Matched fillers (MFs) are elegant and widely used tools to detect and measure signals that resemble a known template in noisy data. However, they can perform poorly in the presence of contaminating sources of similar or smaller spatial scale than the desired signal, especially if signal and contaminants are spatially correlated. We introduce new multicomponent MF and matched multifilter (MMF) techniques that allow for optimal reduction of the contamination introduced by sources that can be approximated by templates. The application of these new filters is demonstrated by applying them to microwave and X-ray mock data of galaxy clusters with the aim of reducing contamination by point-like sources, which are well approximated by the instrument beam. Using microwave mock data, we show that our method allows for unbiased photometry of clusters with a central point source but requires sufficient spatial resolution to reach a competitive noise level after filtering. A comparison of various MF and MMF techniques is given by applying them to Planck multifrequency data of the Perseus galaxy cluster, whose brightest cluster galaxy hosts a powerful radio source known as Perseus A. We also give a brief outline how the constrained MF (CMF) introduced in this work can be used to reduce the number of point sources misidentified as clusters in X-ray surveys like the upcoming eROSITA all-sky survey. A PYTHON implementation of the filters is provided by the authors of this manuscript at https://github.com/j-erler/pymf

Catalog of X-ray-selected extended galaxy clusters from the ROSAT All-Sky Survey (RXGCC)

Context. There is a known tension between cosmological parameter constraints obtained from the primary cosmic microwave background and those drawn from galaxy cluster samples. One possible explanation for this discrepancy may be that the incomplete character of detected clusters is higher than estimated and, as a result, certain types of groups or galaxy clusters have been overlooked in the past. Aims. We aim to search for galaxy groups and clusters with particularly extended surface brightness distributions by creating a new X-ray-selected catalog of extended galaxy clusters from the ROSAT All-Sky Survey (RASS), based on a dedicated source detection and characterization algorithm that is optimized for extended sources. Methods. Our state-of-the-art algorithm includes multi-resolution filtering, source detection, and characterization. On the basis of extensive simulations, we investigated the detection efficiency and sample purity. We used previous cluster catalogs in X-ray and other bands, as well as spectroscopic and photometric redshifts of galaxies to identify clusters. Results. We report a catalog of galaxy clusters at high galactic latitude based on the ROSAT All-sky Survey, known as the RASS-based extended X-ray Galaxy Cluster Catalog, which includes 944 groups and clusters. Of this number, 641 clusters have been previously identified based on intra-cluster medium (ICM) emission (Bronze), 154 known optical and infrared clusters are detected as X-ray clusters for the first time (Silver) and 149 are identified as clusters for the first time (Gold). Based on 200 simulations, the contamination ratio of the detections that were identified as clusters by ICM emission and the detections that were identified as optical and infrared clusters in previous work is 0.008 and 0.100, respectively. Compared with the Bronze sample, the Gold+Silver sample is less luminous, less massive, and exhibits a flatter surface brightness profile. Specifically, the median flux in [0.1−2.4] keV band for Gold+Silver and Bronze sample is 2.496 × 10−12 erg s−1 cm−2 and 4.955 × 10−12 erg s−1 cm−2, respectively. The median value of β (the slope of cluster surface brightness profile) is 0.76 and 0.83 for the Gold+Silver and Bronze sample, respectively

The XXL Survey L. AGN contamination in galaxy clusters: detection and cosmological impact

International audienceX-ray observations of galaxy clusters are impacted by the presence of active galactic nuclei (AGN) in a manner that is challenging to quantify, leading to biases in the detection and measurement of cluster properties for both astrophysics and cosmological applications. Using automated X-ray pipeline techniques, we introduce a new automated class for AGN-contaminated (AC) clusters in the XXL source detection software. The majority of these systems are otherwise missed by current X-ray cluster detection methods. The AC selection is also effective at distinguishing AGN and cool core presence using supplementary optical and infrared information. We present 33 AC objects, consisting of 25 clusters in the redshift range, $0.14 \leq z \leq 1.03$, and 8 other sources with significantly peaked central emission based on X-ray observations. Six of these are new confirmed clusters. We compute the missed fraction of the XXL survey, defined as the fraction of genuine clusters that are undetected due to their centrally peaked X-ray profiles. We report seven undetected AC clusters above $z > 0.6$, in the range where X-ray cluster detection efficiency drops significantly. The missed fraction is estimated to be at the level of $5\%$ for the 50 square degree XXL area. The impact on cosmological estimates from missed clusters is negligible for XXL, but produces a $\sim 3\sigma$ tension with the fiducial cosmology when considering larger survey areas. This work demonstrates the first systematic attempt to quantify the percentage of missed clusters in X-ray surveys as a result of central AGN contamination. Looking towards surveys such as eROSITA and Athena, larger areas and increased sensitivity will significantly enhance cluster detection, therefore robust methods for characterising AGN contamination will be crucial for precise cluster cosmology, particularly in the redshift $z > 1$ regime

The XXL Survey L. AGN contamination in galaxy clusters: detection and cosmological impact

International audienceX-ray observations of galaxy clusters are impacted by the presence of active galactic nuclei (AGN) in a manner that is challenging to quantify, leading to biases in the detection and measurement of cluster properties for both astrophysics and cosmological applications. Using automated X-ray pipeline techniques, we introduce a new automated class for AGN-contaminated (AC) clusters in the XXL source detection software. The majority of these systems are otherwise missed by current X-ray cluster detection methods. The AC selection is also effective at distinguishing AGN and cool core presence using supplementary optical and infrared information. We present 33 AC objects, consisting of 25 clusters in the redshift range, $0.14 \leq z \leq 1.03$, and 8 other sources with significantly peaked central emission based on X-ray observations. Six of these are new confirmed clusters. We compute the missed fraction of the XXL survey, defined as the fraction of genuine clusters that are undetected due to their centrally peaked X-ray profiles. We report seven undetected AC clusters above $z > 0.6$, in the range where X-ray cluster detection efficiency drops significantly. The missed fraction is estimated to be at the level of $5\%$ for the 50 square degree XXL area. The impact on cosmological estimates from missed clusters is negligible for XXL, but produces a $\sim 3\sigma$ tension with the fiducial cosmology when considering larger survey areas. This work demonstrates the first systematic attempt to quantify the percentage of missed clusters in X-ray surveys as a result of central AGN contamination. Looking towards surveys such as eROSITA and Athena, larger areas and increased sensitivity will significantly enhance cluster detection, therefore robust methods for characterising AGN contamination will be crucial for precise cluster cosmology, particularly in the redshift $z > 1$ regime

The XXL Survey L. AGN contamination in galaxy clusters: detection and cosmological impact

International audienceX-ray observations of galaxy clusters are impacted by the presence of active galactic nuclei (AGN) in a manner that is challenging to quantify, leading to biases in the detection and measurement of cluster properties for both astrophysics and cosmological applications. Using automated X-ray pipeline techniques, we introduce a new automated class for AGN-contaminated (AC) clusters in the XXL source detection software. The majority of these systems are otherwise missed by current X-ray cluster detection methods. The AC selection is also effective at distinguishing AGN and cool core presence using supplementary optical and infrared information. We present 33 AC objects, consisting of 25 clusters in the redshift range, $0.14 \leq z \leq 1.03$, and 8 other sources with significantly peaked central emission based on X-ray observations. Six of these are new confirmed clusters. We compute the missed fraction of the XXL survey, defined as the fraction of genuine clusters that are undetected due to their centrally peaked X-ray profiles. We report seven undetected AC clusters above $z > 0.6$, in the range where X-ray cluster detection efficiency drops significantly. The missed fraction is estimated to be at the level of $5\%$ for the 50 square degree XXL area. The impact on cosmological estimates from missed clusters is negligible for XXL, but produces a $\sim 3\sigma$ tension with the fiducial cosmology when considering larger survey areas. This work demonstrates the first systematic attempt to quantify the percentage of missed clusters in X-ray surveys as a result of central AGN contamination. Looking towards surveys such as eROSITA and Athena, larger areas and increased sensitivity will significantly enhance cluster detection, therefore robust methods for characterising AGN contamination will be crucial for precise cluster cosmology, particularly in the redshift $z > 1$ regime