Browsing by Author "Valtchanov, I"

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    Batch discovery of nine z ∼ 1 clusters using X-ray and K or R, z′ images
    (2005) Andreon, S; Valtchanov, I; Jones, LR; Altieri, B; Bremer, M; Willis, J; Pierre, M; Quintana, H
    We present the results of an initial search for clusters of galaxies at z similar to 1 and above, using data from 2.9 square degrees of XMM-Newton images. By selecting weak potentially extended X-ray sources with faint or no identifications in deep, ground-based optical imaging, we have constructed a starting sample of 19 high-redshift cluster candidates. Near-IR and R, z' imaging of these fields identified nine of them as high-redshift systems. Six of these were confirmed spectroscopically, three at z similar to 1.0 and the other three in the 0.8 < Z < 0.92 range. The remaining three systems have solid photometric evidence to be at Z(phot) similar to 0-8, 1.0 and 1.3. The present sample significantly increases the number of such clusters. The measured density of z I clusters, after discarding 'low'-redshift systems at z less than or similar to 0.92 is about 1.7 deg(-2) (with 68 per cent confidence interval equal to [1.0, 2.9]) for f(x) greater than or similar to 2.5 10(-15) erg cm(-2) s(-1) ([0.5-2] keV) and this is a lower limit, having, screened not all potential z similar to 1 candidate clusters. Coordinates, X-ray measures and evidence for nine X-ray-selected high-redshift clusters is given.
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    Galaxy evolution in clusters up to z=1.0
    (2004) Andreon, S; Willis, J; Quintana, H; Valtchanov, I; Pierre, M; Pacaud, F
    We present a combined study of the colour-magnitude relation, colour distribution and luminosity function (LF) of a sample of 24 clusters at redshifts 0.3 < z < 1. The sample is largely composed of X-ray selected/detected clusters. Most of the clusters at redshifts z < 0.6 display X-ray luminosity or richness typical of poor clusters or groups, rather than the more typical, massive clusters studied in literature at redshifts z ≳ 0.3. All our clusters, including groups, display a colour-magnitude relation consistent with a passively evolving stellar population formed at a redshift z(f) ≳ 2, in accordance with observed galaxy populations in more massive clusters studied at comparable redshifts. Colours and luminosity functions (LFs) show that the cluster galaxy population is consistent with the presence of at least two components: old systems formed at high redshift that have evolved passively from that epoch, together with a galaxy population displaying more recent star fortnation. The former population forms at 2 ≲ z(f) ≲ 5, the latter at redshifts z < 1. A model in which stars do not evolve is clearly rejected both by the colour of reddest galaxies and by the characteristic luminosity m* measures. All clusters (with one possible exception) are detected independently by an almost three-dimensional optical search employing sky position and colour - this despite the primary X-ray selection and low X-ray flux/optical richness displayed by most of the sample.