Discovery of a cluster of galaxies behind the Milky Way: X-ray and optical observations

¹ Instituto de Astronomia, Geofísica e Ciências Atmosféricas, Universidade de São Paulo, R. do Matão 1226, 05508-900 São Paulo, Brazil e-mail: rlopes@astro.iag.usp.br
² Observatoire Astronomique, UMR 7550 CNRS, 11 rue de l'Université, 67000 Strasbourg, France

Received: 23 May 2006
Accepted: 12 July 2006

Abstract

We report the discovery of Cl 2334+48, a rich cluster of galaxies in the Zone of Avoidance, identified in public images from the XMM-Newton archive. We present the main properties of this cluster using the XMM-Newton X-ray data, along with new optical spectroscopic and photometric observations. Cl 2334+48 is located at ± 0.001, as derived from the optical spectrum of the brightest member galaxy. Such redshift agrees with a determination from the X-ray spectrum (z = 0.263), in which an intense emission line is matched to the rest wavelength of the Fe Kα complex. Its intracluster medium has a plasma temperature of keV, a sub-solar abundance of 0.38 ± , and a bolometric luminosity of 3.2 10⁴⁴ erg s^-1. A density contrast is reached in a radius of Mpc, and the corresponding enclosed mass is 1.5 . Optical images show an enhancement of galaxies around the central galaxy, as expected if these were cluster members. The central object is a luminous E-type galaxy, which is displaced ~ kpc from the cluster X-ray centre. In addition, it has a neighbouring arc-like feature (~ or kpc from it), probably due to strong gravitational lensing. The discovery of Cl 2334+48 emphasises the remarkable capability of the XMM-Newton to reveal new clusters of galaxies in the Zone of Avoidance.