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 &sigma_V=867⁺⁸⁰_-60 km s^-1, XLSSC 002 at z=0.772 and σ_V=524⁺²⁶⁷_-116 km s^-1 and XLSSC 003 at z=0.839 and σ_V= 780⁺¹³⁷_-75 km s^-1. These three clusters have X-ray bolometric luminosities L_X ∼ 1-3 × 10⁴⁴ erg s^-1 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.

Based on observations obtained with XMM-Newton, CFHT, ESO (program ID: 70.A-0283).