A&A 394, 7-15 (2002)
DOI: 10.1051/0004-6361:20021104

Warm-hot intergalactic baryons revealed

L. Zappacosta¹, F. Mannucci², R. Maiolino³, R. Gilli³, A. Ferrara³, A. Finoguenov⁴, N. M. Nagar³ and D. J. Axon<sup>5

1</sup>  Dipartimento di Astronomia e Scienza dello Spazio, Largo E. Fermi 2, 50125 Firenze, Italy
²  Centro per l'Astronomia Infrarossa e lo studio del mezzo interstellare (CAISMI), CNR, Largo E. Fermi 5, 50125 Firenze, Italy
³  Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, 50125 Firenze, Italy
⁴  Max-Planck-Institut für extraterrestrische Physik, Giessenbachstraße, 85748 Garching, Germany
⁵  Department of Physical Sciences, University of Hertfordshire, College Lane, Hatfield, Hertfordshire AL10 9AB, UK

(Received 6 June 2002 / Accepted 29 July 2002)

Abstract
Several popular cosmological models predict that most of the baryonic mass in the local universe is located in filamentary and sheet-like structures associated with galaxy overdensities. This gas is expected to be gravitationally heated to ~ 10⁶ K and therefore emitting in the soft X-rays. We have detected diffuse soft X-ray structures in a high Galactic latitude ROSAT field after point source subtraction and correction for Galactic absorption. These diffuse structures have an X-ray energy distribution that is much softer than expected from clusters, groups or unresolved emission from AGNs, but are consistent with that expected from a diffuse warm intergalactic medium. To discriminate between a Galactic or extragalactic nature of the diffuse gas we have correlated the soft X-map with multiband optical images in this field. We have found a significant overdensity of galaxies in correspondence with the strongest diffuse X-ray structure. The photometric redshift distribution of the galaxies over the X-ray peak has an excess over field galaxies at . This result strongly suggests that the diffuse X-ray flux is due to extragalactic emission by warm gas associated with an overdense galaxy region at .

Key words: large-scale structure of Universe -- X-rays: diffuse background

Offprint request: R. Maiolino, maiolino@arcetri.astro.it

© ESO 2002

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