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

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

Abstract

We present a detailed analysis of the overdensity of X-ray sources close to the 3C 295 galaxy cluster () 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 . 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 . 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  keV, the energy of the iron Kα line at the redshift of the cluster. The detection of this line is confirmed at a confidence level of better than 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 4σ. We conclude that the detection of the redshifted line is a strong indication that at least several of the excess sources lie at and that AGNs are efficient tracers of the “filament” connected with the central cluster of galaxies.