EDP Sciences
Free Access
Issue
A&A
Volume 428, Number 3, December IV 2004
Page(s) 867 - 875
Section Extragalactic astronomy
DOI https://doi.org/10.1051/0004-6361:20041426
Published online 07 December 2004


A&A 428, 867-875 (2004)
DOI: 10.1051/0004-6361:20041426

RX J1821.6+6827: A cool cluster at z = 0.81 from the ROSAT NEP survey

I. M. Gioia1, A. Wolter2, C. R. Mullis3, J. P. Henry4, H. Böhringer5 and U. G. Briel5

1  Istituto di Radioastronomia CNR, via P. Gobetti 101, 40129 Bologna, Italy
    e-mail: gioia@ira.cnr.it
2  INAF - Osservatorio Astronomico di Brera, via Brera 28, 20121 Milano, Italy
    e-mail: anna@brera.mi.astro.it
3  European Southern Observatory, Karl-Schwarzschild-Str. 2, Garching bei München 85740, Germany
    e-mail: cmullis@eso.org
4  Institute for Astronomy, 2680 Woodlawn Drive, Honolulu, HI 96822, USA
    e-mail: henry@ifa.hawaii.edu
5  Max-Planck-Institut fur Extraterrestrische Physik, Giessenbachstrasse, Postfach 1312, Garching 85741, Germany
    e-mail: hxb@xray.mpe.mpg.de,ugb@xray.mpe.mpg.de

(Received 7 June 2004 / Accepted 30 July 2004 )

Abstract
We present an analysis of the properties of the cluster of galaxies RX J1821.6+6827, or NEP 5281, at a redshift $z= 0.816 \pm0.001$. RX J1821.6+6827 was discovered during the optical identification of the X-ray sources in the North Ecliptic Pole (NEP) region of the ROSAT All-Sky Survey and it is the highest redshift cluster of galaxies of the NEP  survey. We have measured spectroscopic redshifts for twenty cluster galaxies using the Keck-I and the Canada-France-Hawai'i (CFH) telescopes. The value for the cluster velocity dispersion is $\sigma_{\rm V}=775^{+182}_{-113}$ km s -1. The cluster was also observed by XMM-Newton. Both the optical and X-ray data are presented in this paper. The cluster has an unabsorbed X-ray flux in the 2-10 keV energy band of $F_{2{-}10~{\rm ~keV}} = 1.24^{+0.16}_{-0.23} \times10^{-13}$ erg cm -2 s -1 and a K-corrected luminosity in the same band of $L_{2{-}10~{\rm ~keV}} = 6.32_{-0.73}^{+0.76} \times10^{44}~h^{-2}_{50}$ erg s -1 (90% confidence level). The cluster X-ray bolometric luminosity is $L_{\rm BOL,X} = 1.35^{+0.08}_{-0.21} \times10^{45}~h_{50}^{-2}$ erg s -1 ( $L_{\rm BOL,X} = 1.17^{+0.13}_{-0.18} \times10^{45}~h_{70}^{-2}$ erg s -1 in the concordance cosmology). The data do not allow fitting both metal abundance and temperature at the same time. The abundance is unconstrained and can vary in the range 0.28-1.42  $Z_{\odot}$ while the best fit X-ray temperature is T=4.7+1.2-0.7 keV. This emission weighted X-ray temperature is a little lower, barely within the uncertainties, than the predicted temperature, T= 6.34+0.13-0.35 keV, from the $L_{\rm X}-T_{\rm X}$ relation of local clusters published in the literature. The optically measured velocity dispersion is consistent with the velocity dispersion expected from the $\sigma_{\rm V}-T_{\rm X}$ relationship. We also examine the point X-ray source RX J1821.9+6818, or NEP 5330, located to the south east of the cluster which was identified as a QSO at $z= 1.692 \pm0.008$ in the ROSAT NEP survey. The X-ray source is well fitted by an absorbed power law model with $N_{\rm H}=4.1_{-3.0}^{+3.1}\times10^{20}$ atoms cm -2 and a photon index $\Gamma=1.67\pm0.12$ typical of an active galactic nucleus.


Key words: galaxies: clusters: general -- galaxies: clusters: individual: RX J1821.6+6827 -- X-rays: general -- X-rays: individuals: RX J1821.6+6827 -- X-rays: galaxies: clusters -- galaxies: intergalactic medium

SIMBAD Objects



© ESO 2004

Current usage metrics show cumulative count of Article Views (full-text article views including HTML views, PDF and ePub downloads, according to the available data) and Abstracts Views on Vision4Press platform.

Data correspond to usage on the plateform after 2015. The current usage metrics is available 48-96 hours after online publication and is updated daily on week days.

Initial download of the metrics may take a while.