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^+0.012_-0.010), 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^+0.50_-0.48 keV, a sub-solar abundance of 0.38 ± 0.12~Z_⊙, and a bolometric luminosity of 3.2 × 10⁴⁴ 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 arcsec 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.