A galaxy overdensity at z = 0.401 associated with an X-ray emitting structure of warm-hot intergalactic medium ^*,**

¹ INAF, Istituto di Radioastronomia, Largo E. Fermi 5, 50125 Firenze, Italy e-mail: filippo@arcetri.astro.it
² Università di Siena, Dipartimento di Fisica, via Roma 56, 53100, Siena, Italy
³ Department of Physics and Astronomy, 4129 Frederick Reines Hall, University of California, Irvine, CA 92697-4575, USA
⁴ INAF, Osservatorio Astronomico di Roma, via di Frascati 33, 00040, Monteporzio Catone (RM), Italy
⁵ Fundación Galileo Galilei and Telescopio Nazionale Galileo, PO Box 565, 38700 Santa Cruz de La Palma, Tenerife, Spain

Received: 18 October 2006
Accepted: 5 March 2007

Abstract

We present the results of spectroscopic observations of galaxies associated with the diffuse X-ray emitting structure discovered by Zappacosta et al. (2002, A&A, 394, 7). After measuring the redshifts of 161 galaxies, we confirm an overdensity of galaxies with projected dimensions of at least 2 Mpc, determine its spectroscopic redshift in z = 0.401 ± 0.002, and show that it is spatially coincident with the diffuse X-ray emission. This confirms the original claim that this X-ray emission has an extragalactic nature and is due to the warm-hot intergalactic medium (WHIM). We used this value of the redshift to compute the temperature of the emitting gas. The resulting value depends on the metallicity that is assumed for the IGM, and is constrained to be between 0.3 and 0.6 keV for metallicities between 0.05 and 0.3 solar, in good agreement with the expectations from the WHIM.