V. Kolokotronis

Searching for cluster substructure using APM and ROSAT data

ABSTRA C T We present a detailed study of the morphological features of 22 rich galaxy clusters. Our sample is constructed from a cross-correlation of optical OAbell 1 APMU data with X-ray (0.1‐2.4 keV) ROSAT pointed observations. We systematically compare cluster images and morphological parameters in an attempt to reliably identify possible substructure in both optical and the X-ray images. To this end, we compute various moments of the optical and X-ray surface-brightness distribution such as the ellipticities, centre-of-mass shifts and ellipsoidal orientations. We assess the significance of our results using Monte Carlo simulations. We find significant correlations between the optical and X-ray morphological parameters, indicating that in both parts of the spectrum it is possible to identify correctly the dynamical state of a cluster. Most of our clusters (17/22) have a good one-to-one correspondence between the optical and the X-ray images, and about 10 appear to have strong indications of substructure. This corresponds to a minimum percentage of order ,45 per cent, which is in very good accordance with other similar analyses. Finally, five out of 22 systems (,22 per cent) seem to have distinct subclumps in the optical which are not verified in the X-ray images, and thus are suspect of being due to optical projection effects. These results will serve as a useful guide in interpreting subsequent analyses of large optical cluster catalogues.

Supercluster properties as a cosmological probe

ABSTRA C T We investigate the supercluster shape properties of the all-sky observed (Abell/ACO) and simulated (Virgo data) cluster catalogues using an approach based on differential geometry. We identify rich superclusters by applying the percolation algorithm to both observed and mock cluster populations, the latter being constructed following the observational selection of the Abell/ACO sample, extended out to zmax # 0:114. We apply a set of shape diagnostics in order to study the morphological features of superclusters with

The Serendipitous XMM-Newton Cluster Athens Survey (SEXCLAS): sample selection and the cluster log N-log S

8 cluster members. Our results demonstrate that filamentarity is the dominant supercluster shape feature. On comparing data and models, we show that the LCDMOVLa 1 2 Vma 0:7U model performs better than tCDM, which is excluded at a relatively high confidence level, in agreement with other recent large-scale structure studies.

The XMM-Newton/2dF survey - III. Comparison between optical and X-ray cluster detection methods

In this paper we serendipitously identify X-ray cluster candidates using XMM-Newton archival observations complemented by five-band optical photometric follow-up observations (r approximate to 23 mag) as part of the X-ray Identification (XID) programme. Our sample covers an area of approximate to 2.1 deg(2) (15 XMM-Newton fields) and comprises a total of 21 (19 serendipitous + two target) extended X-ray sources to the limit f(x) (0.5-2 keV) approximate to 6 x 10(-15) erg s(-1) cm(-2), with a high probability (> 99.9 per cent) of being extended on the XMM-Newton images. Of the 21 X-ray clusters, 14 are detected for the first time while seven are spectroscopically confirmed in the literature. Exploiting the optical data available for these fields we discover that greater than or similar to 68 per cent of the X-ray cluster candidates are associated with optical galaxy overdensities. We also attempt to constrain the redshifts of our cluster candidates using photometric methods. We thus construct the photometric redshift distribution of galaxies in the vicinity of each X-ray selected cluster candidate and search for statistically significant redshift peaks against that of the background distribution of field galaxies. Most of our clusters have photometric or spectroscopic redshifts in the range 0.4 <z <0.6. Comparison of photometric with spectroscopic redshift estimates for the confirmed clusters suggests that our simple method is robust out to z approximate to 0.5. For clusters at higher z, deeper optical data are required to estimate reliable photometric redshifts. Using the sample of the 19 serendipitous X-ray selected cluster candidates, we estimate their surface density down to f(x) (0.5-2 keV) approximate to 6 x 10(-15) erg s(-1) cm(-2) and find it to be in fair agreement with previous and recent studies.

The Needles in the Haystack Survey: searching for X‐ray‐selected normal galaxies

In this work we present combined optical and X-ray cluster detection methods in an area near the North Galactic Pole area, previously covered by the SDSS and 2dF optical surveys. The same area has been covered by shallow (

THE XMM-NEWTON/2DF SURVEY

\sim 1.8

XMM-Newton/2dF survey. II. (Georgakakis+, 2004)

deg

Comparison between optical and X-ray cluster detection methods

^{2}

The XMM-2df Cluster Survey

) XMM-{\em Newton} observations. The optical cluster detection procedure is based on merging two independent selection methods - a smoothing+percolation technique, and a Matched Filter Algorithm. The X-ray cluster detection is based on a wavelet-based algorithm, incorporated in the SAS v.5.2 package. The final optical sample counts 9 candidate clusters with richness of more than 20 galaxies, corresponding roughly to APM richness class. Three, of our optically detected clusters are also detected in our X-ray survey.