I. Georgantopoulos

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.

A deep ROSAT survey – IV. The evolution of X-ray selected QSOs

We report on a new estimate of the QSO X-ray luminosity function and its evolution with redshift based on a sample of 107 QSOs detected at faint X-ray fluxes,

The XMM deep survey in the CDF-S II. A 9–20 keV selection of heavily obscured active galaxies at z > 1.7

S{\rm(0.5-2\,keV)}>4\times10^{-15}\,

ASCA observations of deep ROSAT fields — I. The nature of the X-ray source populations

\ergcms , with the {\it ROSAT} X-ray satellite. For

X-ray selected starbursts in the Great Observatories Origins Deep Survey-North

q_0=0.5

The XMM-Newton/2dF Survey — VI. Clustering and bias of the soft X-ray point sources

, the X-ray evolution of QSOs in this sample is consistent with strong luminosity evolution,

A deep ROSAT survey - XII. The X-ray spectra of faint ROSAT sources

L_{\rm X} ^*(z) \propto (1+z)^{3.25\pm0.1}

The Phoenix Deep Survey: X-ray properties of faint radio sources

, at low redshifts (

A Deep ROSAT Survey — X. X-ray-luminous narrow-emission-line galaxies

z<1.60

A wide search for obscured active galactic nuclei using XMM–Newton and WISE

) and a constant comoving space density at higher redshifts. The derived rate of evolution at low redshifts is thus significantly higher than that obtained previously for the {\it Einstein} Extended Medium Sensitivity Survey (EMSS). Indeed, most luminosity evolution models provide a very poor fit (rejected at the 99 per cent confidence level) when applied to the combined EMSS and {\it ROSAT} samples, although a polynomial evolution model,