Letter to the Editor

Detection of hot gas in the filament connecting the clusters of galaxies Abell 222 and Abell 223

¹ SRON Netherlands Institute for Space Research, Sorbonnelaan 2, 3584 CA Utrecht, The Netherlands e-mail: n.werner@sron.nl
² Max-Planck-Institut für Extraterrestrische Physik, 85748 Garching, Germany
³ Astronomical Institute, Utrecht University, PO Box 80000, 3508 TA Utrecht, The Netherlands
⁴ ESO, Karl-Schwarzschild-Str. 2, 85748 Garching, Germany

Received: 18 February 2008
Accepted: 17 March 2008

Abstract

Context. About half of the baryons in the local Universe are invisible and – according to simulations – their dominant fraction resides in filaments connecting clusters of galaxies in the form of low density gas with temperatures in the range of 10⁵ < T < 10⁷ K. This warm-hot intergalactic medium has never been detected indisputably using X-ray observations.

Aims. We aim to probe the low gas densities expected in the large-scale structure filaments by observing a filament connecting the massive clusters of galaxies A 222 and A 223 (), which has a favorable orientation approximately along our line-of-sight. This filament has been previously detected using weak lensing data and as an over-density of colour-selected galaxies.

Methods. We analyse X-ray images and spectra obtained from a deep observation (144 ks) of A 222/223 with XMM-Newton.

Results. We present observational evidence of X-ray emission from the filament connecting the two clusters. We detect the filament in the wavelet-decomposed soft-band (0.5–2.0 keV) X-ray image with a 5σ significance. Following the emission down to the 3σ significance level, the observed filament is ≈1.2 Mpc wide. The temperature of the gas associated with the filament, determined from the spectra, is kT = 0.91±0.25 keV, and its emission measure corresponds to a baryon density of (3.4±1.3)10^-5(l/15 Mpc)^-1/2 cm^-3, where l is the length of the filament along the line-of-sight. This density corresponds to a baryon over-density of 150. The properties of the gas in the filament are consistent with results of simulations of the densest and hottest parts of the warm-hot intergalactic medium.