Browsing by Author "Willis, J"

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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.
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    Gemini-South plus FLAMINGOS demonstration science
    (2003) Stern, D; Hall, PB; Barrientos, LF; Bunker, AJ; Elston, R; Ledlow, MJ; Raines, SN; Willis, J
    We report an infrared 1-1.8 mum (J+H-band), low-resolution (R=450) spectrogram of the highest redshift radio-loud quasar currently known, SDSS J083643.85+005453.3, obtained during the spectroscopic commissioning run of the FLAMINGOS multiobject, near-IR spectrograph at the 8 m Gemini-South Observatory. These data show broad emission from both C IV lambda1549 and C III] lambda1909, with strengths comparable to lower redshift quasar composite spectra. The implication is that there is substantial enrichment of the quasar environment, even at times less than a billion years after the big bang. The redshift derived from these features is z=5.774+/-0.0003, more accurate and slightly lower than the z=5.82 reported in the discovery paper based on the partially absorbed Lyalpha emission line. The infrared continuum is significantly redder than lower redshift quasar composites. Fitting the spectrum from 1.0 to 1.7 mum with a power law f(v) proportional to nu(-alpha), the derived power-law index is alpha=1.55 compared to the average continuum spectral index =0.44 derived from the first Sloan Digital Sky Survey composite quasar. Assuming an SMC-like extinction curve, we infer a color excess of E(B-V)=0.09+/-0.01. Only approximate to6% of quasars in the optically selected Sloan Digital Sky Survey show comparable levels of dust reddening.