EDP Sciences
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Volume 484, Number 2, June III 2008
Page(s) 303 - 309
Section Astrophysical processes
DOI http://dx.doi.org/10.1051/0004-6361:20079338
Published online 08 April 2008

A&A 484, 303-309 (2008)
DOI: 10.1051/0004-6361:20079338

The candidate filament close to the 3C 295 galaxy cluster: optical and X-ray spectroscopy

V. D'Elia1, F. Fiore1, S. Mathur2, and F. Cocchia1

1  INAF - Osservatorio Astronomico di Roma via Frascati 33, Monteporzio-Catone (RM) I00040, Italy
    e-mail: delia@mporzio.astro.it
2  The Ohio State University Columbus, OH 43210, USA

Received 28 December 2007 / Accepted 29 February 2008

We present a detailed analysis of the overdensity of X-ray sources close to the 3C 295 galaxy cluster (z=0.46) to assess whether it is associated with a filament of the large-scale structure of the Universe. We obtained optical spectra of the optical counterparts of eleven sources associated with the filament, finding that one is at z=0.474. This is a type 1 AGN at 1.5 arcmin from the cluster center. We found three more sources with a redshift in the range $0.37 \div 0.53$. We extracted the stacked X-ray spectrum of 47 X-ray sources belonging to the putative filament. We found a significant narrow (at the resolution of the Chandra ACIS-I) line at $E \sim4.4$ keV, the energy of the iron K$\alpha$ line at the redshift of the cluster. The detection of this line is confirmed at a confidence level of better than $3\sigma$ and its energy is constrained to be in the range 6.2-6.47 (at a 90% confidence level), excluding an identification with the 6.7 helium-like iron line from the hot cluster ICM at better than 4$\sigma$. We conclude that the detection of the redshifted line is a strong indication that at least several of the excess sources lie at $z\sim0.46$ and that AGNs are efficient tracers of the "filament" connected with the central cluster of galaxies.

Key words: X-rays: galaxies -- galaxies: clusters: individual: 3C 295 -- X-rays: diffuse background -- cosmology: large-scale structure of Universe

© ESO 2008