Discovery of a cluster of galaxies behind the milky way: X-ray and optical observations

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 z = 0.271 ± 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 = 0.263-0.010+0.012), 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 4.92 -0.48+0.50 keV, a sub-solar abundance of 0.38 ± 0.12 Z⊙, and a bolometric luminosity of 3.2 × 10 44 erg s-1. A density contrast δ = 2500 is reached in a radius of 0.5 h70-1 Mpc, and the corresponding enclosed mass is 1.5 × 1014 M⊙. Optical images show an enhancement of g′ - i′ > 2.5 galaxies around the central galaxy, as expected if these were cluster members. The central object is a luminous E-type galaxy, which is displaced ∼40 h70-1 kpc from the cluster X-ray centre. In addition, it has a neighbouring arc-like feature (∼22″ or 90 h70-1 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. © ESO 2006.