29 research outputs found
Studying galaxy clusters through X-rays and the Sunyaev-Zel’dovich effect : simulations, detection and characterisation
Clusters of galaxies are the largest gravitationally-bound objects in the Universe. Thereby clusters are ideal tracers of cosmic expansion and structure formation that allow tight constraints on the average cosmic density of matter and of other fundamental cosmological parameters. Galaxy clusters can be observed over a broad spectral range, in emission from radio wavelengths to X-rays, through their distortion of the cosmic microwave background radiation (the Sunyaev-Zel'dovich effect), or through their gravitational lensing distortion in the optical appearance of background galaxies. These observables allow detailed measurements of the cluster galaxy populations, of the hot intra-cluster gas, and of the gravitationally dominant dark matter. This work studies the detection capability and the accuracy of physical constraints derived from present and future X-ray surveys of galaxy clusters. Such imaging-spectroscopy X-ray surveys produce large samples of clusters that allow accurate measures of cosmic structure formation and cosmological parameters only if selection effects are well understood, and the masses of galaxy clusters can be well quantified out to the most distant, i.e. cosmologically earliest, highest-redshift clusters. In the first part of the thesis, the detection effciency of galaxy clusters for the upcoming eROSITA mission is investigated by means of extensive and dedicated Monte Carlo simulations. Employing a state-of-the-art source detection technique, we determine a cluster detection effciency based on the cluster fluxes and sizes. Using this eROSITA cluster selection function, we found that eROSITA will be able to detect a total of ~1.36x10^5 clusters in the whole sky. This cluster number will allow eROSITA to put stringent constraints on the dark matter and dark energy models. In the second part of the thesis, the galaxy group detection capability of the future ATHENA observatory is investigated. The detection of the thermal X-ray emission from the faint and small galaxy groups (M_500=5x10^13 M_Sun) at high redshifts (z>2.5) will be accessible only to powerful X-ray telescopes like ATHENA. We confirm this assumption by means of detailed simulations. The results confirm that such groups will be detected as extended sources by ATHENA, and that the key instrumental parameters are a large efective area (2.1 m^2 at 1 keV) and a good spatial resolution ( In the third part of this work, the physical differences between galaxy cluster samples selected in different wavebands are investigated. For this research, z>0.8 mid-infrared (SpARCS) and X-ray (XMMLSS) selected cluster samples are compared. We show that the X-ray selected clusters have a more compact distribution of the hot gas and galaxies, compared to the mid-infrared selected clusters. Moreover, we found that a sub-sample of the mid-infrared bright clusters have a lack of X-ray emission, making them consistent with non-virialised systems or filamentary structures projected along the line-of-sight. The final part of the thesis describes a complementary method to probe the baryonic properties of galaxy clusters. The approach consists in constraining the intra-cluster gas pressure model by comparing the predicted thermal Sunyaev-Zel’dovich power spectrum with current Cosmic Microwave Background measurements. We show that a steeper pressure profile in the cluster outskirts, or an evolving gas mass fraction, have mild-to-severe conflicts with the experimental data. It is also shown that the future Sunyaev-Zel’dovich experiments can break the current parameter degeneracies and place simultaneous constraints on the gas pressure profile and its redshift evolution. The different aspects of the statistical studies based on the galaxy cluster population that we explore in this work, revealed possible sources of uncertainties. These uncertainties deserve careful consideration in order to achieve precision cosmology with future galaxy cluster surveys. Possible routes to improve on this systematics are presented
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 (; 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 . 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 will be discovered. We find that
Athena can detect high-redshift galaxy groups with masses of
and , and almost half of
them will have a gas temperature determined to a precision of . 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, , 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 , in the range where X-ray cluster detection efficiency drops significantly. The missed fraction is estimated to be at the level of for the 50 square degree XXL area. The impact on cosmological estimates from missed clusters is negligible for XXL, but produces a 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 regime