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Register a new userStatistical properties of SZ and X-ray cluster detections
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Aims. We calibrate the number density, completeness, reliability and the lower mass limit of galaxy-cluster detections through their thermal SZ signal, and compare them to X-ray cluster detections. Methods. We simulate maps of the thermal SZ effect and the X-ray emission from light cones constructed in a large, hydrodynamical, cosmological simulation volume, including realistic noise contributions. The maps are convolved with linear, optimised, single- and multi-band filters to identify local peaks and their signal-to-noise ratios. The resulting peak catalogues are then compared to the halo population in the simulation volume to identify true and spurious detections. Results. Multi-band filtering improves the statistics of SZ cluster detections considerably compared to single-band filtering. Observations with the characteristics of ACT detect clusters with masses M>6-9e13 M_o/h, quite independent of redshift, reach 50% completeness at ~1e14 M_o/h and 100% completeness at ~2e14 M_o/h. Samples are contaminated by only a few per cent spurious detections. This is broadly comparable to X-ray cluster detections with XMM-Newton with 100 ks exposure time in the soft band, except that the mass limit for X-ray detections increases much more steeply with redshift than for SZ detections. A comparison of true and filtered signals in the SZ and X-ray maps confirms that the filters introduce at most a negligible bias.
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We examine three approaches to the problem of source classification in catalogues. Our goal is to determine the confidence with which the elements in these catalogues can be distinguished in populations on the basis of their spectral energy distribution (SED). Our analysis is based on the projection of the measurements onto a comprehensive SED model of the main signals in the considered range of frequencies. We first first consider likelihood analysis, which half way between supervised and unsupervised methods. Next, we investigate an unsupervised clustering technique. Finally, we consider a supervised classifier based on Artificial Neural Networks. We illustrate the approach and results using catalogues from various surveys. i.e., X-Rays (MCXC), optical (SDSS) and millimetric (Planck Sunyaev-Zeldovich (SZ)). We show that the results from the statistical classifications of the three methods are in very good agreement with each others, although the supervised neural network-based classification shows better performances allowing the best separation into populations of reliable and unreliable sources in catalogues. The latest method was applied to the SZ sources detected by the Planck satellite. It led to a classification assessing and thereby agreeing with the reliability assessment published in the Planck SZ catalogue. Our method could easily be applied to catalogues from future large survey such as SRG/eROSITA and Euclid.
The largest and the deepest super-structure known today is the Shapley super-cluster. This is the sky area with the highest over-density of galaxy clusters and therefore also an ideal region to test the effects of a high density environment on galaxies and on clusters. We performed an X-ray survey of a wide region surrounding the Shapley super-structure. Additionally to previously known super-cluster X-ray members, we identified diffuse X-ray emission from 35 cluster candidates without previous X-ray detection. 21 of them were previously known, optically selected super-cluster members, while the other candidates had not been previously detected in any wavelength range. Optical follow-up observations revealed that at least four of these new candidates also have optical cluster counterparts. The super-cluster shows a slightly flattened and elongated morphology. Clusters outside the central dense core are preferentially located in four perpendicular filaments in a similar way to what is seen in simulations of Large Scale Structure. We measure the cluster number density in the region to be more than one order of magnitude higher than the mean density of rich Abell clusters previously observed at similar Galactic latitudes; this over-density, in the super-cluster outskirts, is mainly due to an excess of low X-ray luminous clusters (with respect to an average population), which leads us to think that the whole region is still accreting low luminosity, small objects from the outskirts. Pushing our total X-ray mass estimate to fainter clusters would drastically increase the total super-cluster mass measure, because of the presence of the rich X-ray low luminosity population.
The interpretation of X-ray detections from Herbig Ae/Be stars is disputed as it is not clear if these intermediate-mass pre-main sequence stars are able to drive a dynamo and ensuing phenomena of magnetic activity. Alternative X-ray production mechanisms, related to stellar winds, star-disk magnetospheres, or unresolved late-type T Tauri star companions have been proposed. In a series of papers we have been investigating high-resolution X-ray Chandra images of Herbig Ae/Be and main-sequence B-type stars to test the T Tauri hypothesis by spatially resolving known visual companions from the primaries. Here we report on six as yet unpublished Chandra exposures from our X-ray survey of Herbig stars. The target list comprises six Herbig stars with known cool companions, and three further A/B-type stars that are serendipitously in the Chandra field-of-view. In this sample we record a detection rate of 100%, i.e. all A/B-type stars display X-ray emission at levels of log(L_x/L_bol) ~ -5...-7. The analysis of hardness ratios confirms that HAeBes have hotter and/or more absorbed X-ray emitting plasma than more evolved B-type stars. Radiative winds are ruled out as exclusive emission mechanism on basis of the high X-ray temperatures. Confirming earlier results, the X-ray properties of Herbig Ae/Be stars are not vastly different from those of their late-type companion stars (if such are known). The diagnostics provided by the presently available data leave open if the hard X-ray emission of Herbig stars is due to young age or indicative of further coronally active low-mass companion stars. In the latter case, our detection statistics imply a high fraction of higher-order multiple systems among Herbig stars.
We aim at an unbiased census of the radio halo population in galaxy clusters and test whether current low number counts of radio halos have arisen from selection biases. We construct near-complete samples based on X-ray and Sunyaev-Zeldovich (SZ) effect cluster catalogues and search for diffuse, extended (Mpc-scale) emission near the cluster centers by analyzing data from the National Radio Astronomy Observatory Very Large Array Sky Survey. We remove compact sources using a matched filtering algorithm and model the diffuse emission using two independent methods. The relation between radio halo power at 1.4 GHz and mass observables is modelled using a power law, allowing for a dropout population of clusters hosting no radio halo emission. An extensive suite of simulations is used to check for biases in our methods. Our findings suggest that the fraction of targets hosting radio halos may have to be revised upwards for clusters selected using the SZ effect: while approximately 60 per cent of the X-ray selected targets are found to contain no extended radio emission, in agreement with previous findings, the corresponding fraction in the SZ selected samples is roughly 20 per cent. We propose a simple explanation for this selection difference based on the distinct time evolution of the SZ and X-ray observables during cluster mergers, and a bias towards relaxed, cool-core clusters in the X-ray selection.
We describe Sunyaev-Zeldovich (SZ) effect measurements and analysis of the intracluster medium (ICM) pressure profiles of a set of 45 massive galaxy clusters imaged using Bolocam at the Caltech Submillimeter Observatory. We have used masses determined from Chandra X-ray observations to scale each clusters profile by the overdensity radius R500 and the mass-and-redshift-dependent normalization factor P500. We deproject the average pressure profile of our sample into 13 logarithmically spaced radial bins between 0.07R500 and 3.5R500. We find that a generalized Navarro, Frenk, and White (gNFW) profile describes our data with sufficient goodness-of-fit and best-fit parameters (C500, alpha, beta, gamma, P0 = 1.18, 0.86, 3.67, 0.67, 4.29). We also use the X-ray data to define cool-core and disturbed subsamples of clusters, and we constrain the average pressure profiles of each of these subsamples. We find that given the precision of our data the average pressure profiles of disturbed and cool-core clusters are consistent with one another at R>~0.15R500, with cool-core systems showing indications of higher pressure at R<~0.15R500. In addition, for the first time, we place simultaneous constraints on the mass scaling of cluster pressure profiles, their ensemble mean profile, and their radius-dependent intrinsic scatter between 0.1R500 and 2.0R500. The scatter among profiles is minimized at radii between ~0.2R500 and ~0.5R500, with a value of ~20%. The best-fit mass scaling has a power-law slope of 0.49, which is shallower than the nominal prediction of 2/3 from self-similar hydrostatic equilibrium models. These results for the intrinsic scatter and mass scaling are largely consistent with previous analyses, most of which have relied heavily on X-ray derived pressures of clusters at significantly lower masses and redshifts compared to our sample.
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