arXiv subjects

J. Willis

Publications and source records attributed to J. Willis.

25 records · Page 2Linked to original sources

arXiv2008

Testing the galaxy cluster mass-observable relations at z = 1 with XMM-Newton and Chandra observations of XLSSJ022403.9-041328

We present an analysis of deep XMM-Newton and Chandra observations of the z=1.05 galaxy cluster XLSSJ022403.9-041328 (hereafter XLSSC 029), detected in the XMM-Newton large scale structure survey. Density and temperature profiles of the X-ray emitting gas were used to perform a hydrostatic mass analysis of the system. This allowed us to measure the total mass and gas fraction in the cluster and define overdensity radii R500 and R2500. The global properties of XLSSC 029 were measured within these radii and compared with those of the local population. The gas mass fraction was found to be consistent with local clusters. The mean metal abundance was 0.18 +0.17 -0.15 Zsol, with the cluster core regions excluded, consistent with the predicted and observed evolution. The properties of XLSSC 029 were then used to investigate the position of the cluster on the M-kT, YX-M, and LX-M scaling relations. In all cases the observed properties of XLSSC 029 agreed well with the simple self-similar evolution of the scaling relations. This is the first test of the evolution of these relations at z > 1 and supports the use of the scaling relations in cosmological studies with distant galaxy clusters.

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arXiv2005

Batch discovery of nine z~1 clusters using x-ray and K or R,z' images

We present results of an initial search for clusters of galaxies at z~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~1.0 and the other three in 0.8 ~1 clusters, after discarding "low" redshift systems at z <~ 0.92 is about 1.7 /deg2 (with 68 % confidence interval equal to [1.0,2.9]) for f_X >~2.5 10^{-15} ergs/cm2/s ([0.5-2] keV) and this is a lower limit, having screened not all potential z~1 candidate clusters. Coordinates, x-ray measures and evidence for nine x-ray selected high redshift clusters is given.

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arXiv2004

The XMM-LSS survey. Survey design and first results

We have designed a medium deep large area X-ray survey with XMM - the XMM Large Scale Structure survey, XMM-LSS - with the scope of extending the cosmological tests attempted using ROSAT cluster samples to two redshift bins between 0<z<1 while maintaining the precision of earlier studies. Two main goals have constrained the survey design: the evolutionary study of the cluster-cluster correlation function and of the cluster number density. The results are promising and, so far, in accordance with our predictions as to the survey sensitivity and cluster number density. The feasibility of the programme is demonstrated and further X-ray coverage is awaited in order to proceed with a truly significant statistical analysis. (Abridged)

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arXiv2004

Galaxy evolution in clusters up to z=1.0

We present a combined study of the colour-magnitude relation, colour distribution and luminosity function of a sample of 24 clusters at redshifts 0.3 ~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 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 formation. 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 by 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 the majority of the sample.

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arXiv2004

Colour-detected clusters in the XMM Large Scale Structure Survey

We present first results on the use of a colour space filter for detecting galaxy clusters at cosmological redshifts in the XMM Large Scale Structure Survey. All clusters studied, but one, are successfully colour--detected in spite of their large redshift (0.3<z<1.0), X-ray selection and intrinsic low richness (R=0 or below). We experimentally show that the cluster redshift can be derived, with good accuracy, from the colour of the red sequence, at 0.06<z<1 using a sample of about 160 clusters.

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arXiv2004

Galaxy evolution in clusters from z=1 to z=0

The XMM--LSS project is detecting distant clusters of low mass, quite comparable in mass to the ones in the local universe. This allows a direct comparison of galaxy properties at different redshifts in ``similar'' clusters. We present here first results on the evolution of the reddest galaxies in ~25 clusters/groups at ~0.3< z <~1.0 and for the whole galaxy population in the same clusters. The emerging picture from the current study is that the counterparts of present day clusters tend to show two or more distinct populations : a relatively old (z_f>2-5) population evolving passively together with a younger population, ostensibly responsible for the apparent brightening of the characteristic magnitudes, m*.

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arXiv2004

The XMM-LSS Survey. First high redshift galaxy clusters: relaxed and collapsing systems

We present five newly found galaxy clusters at z>0.6 from the XMM Large-Scale Structure Survey (XMM-LSS). All five objects are extended X-ray sources in the XMM images. For three of them we have sufficient spectroscopically confirmed member galaxies that an estimate of the velocity dispersion is possible: XLSSC 001 at z=0.613 and σ_V=867^{+80}_{-60} km/s, XLSSC 002 at z=0.772 and σ_V=524^{+267}_{-116} km/s and XLSSC 003 at z=0.839 and σ_V=780^{+137}_{-75} km/s. These three clusters have X-ray bolometric luminosities L_X \sim 1-3 \times 10^{44} erg/s and temperatures 2-4 keV, and consequently are less massive than previously known clusters at similar redshifts, but nevertheless they follow the low redshift scaling relations between L_X, T and σ_V, within the limits of the measurement errors. One of the clusters, XLSSC 004, is detected independently as an overdensity of galaxies of a colour R-z'=1.4 that matches the redshift of the central galaxy z=0.87, although it cannot unambiguously be confirmed by the spectroscopic observations alone. The highest redshift candidate cluster pertaining to this paper, XLSSC 005, is most likely a double cluster complex at a redshift around unity, associated with an extended X-ray source with probable substructure.

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