Astronomy & Astrophysics

Free access article

A&A 482, L29-L33 (2008)
DOI: 10.1051/0004-6361:200809599

Letter

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

N. Werner¹, A. Finoguenov², J. S. Kaastra^{1, 3}, A. Simionescu², J. P. Dietrich⁴, J. Vink³, and H. Böhringer²

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 (z = 0.21), 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 5 $\sigma$ significance. Following the emission down to the 3 $\sigma$ significance level, the observed filament is $\approx$ 1.2 Mpc wide. The temperature of the gas associated with the filament, determined from the spectra, is kT = 0.91 $\pm$ 0.25 keV, and its emission measure corresponds to a baryon density of (3.4 $\pm$ 1.3) $\times$ 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 $\rho/<\rho_{\mathrm> \approx$ 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.

Key words: cosmology: large-scale structure of Universe -- X-rays: galaxies: clusters -- galaxies: clusters: individual: Abell 222 -- galaxies: clusters: individual: Abell 223

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