Extended soft X-ray emission in 3CR radio galaxies at z < 0.3: high excitation and broad line galaxies^⋆

¹ INAF – Osservatorio Astrofisico di Torino, Strada Osservatorio 20, 10025 Pino Torinese, Italy
e-mail: balmaverde@oato.inaf.it
² INAF – IASF – Istituto di Astrofisica e Fisica cosmica di Bologna, via P. Gobetti 101, 40129 Bologna, Italy
³ INAF – Istituto di Radioastronomia di Bologna, via Gobetti 101, 40129 Bologna, Italy
⁴ Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA
⁵ Center for Astrophysical Sciences, Johns Hopkins University, 3400 N. Charles Street Baltimore, MD 21218, USA
⁶ Instituto de Astrofísica de Canarias, Calle vía Láctea s/n, 38205 La Laguna, Tenerife, Spain
⁷ European Southern Observatory, Karl-Schwarzschild-Str. 2, 85748 Garching bei Muenchen, Germany
⁸ Dept of Physics, Rochester Institute of Technology, 84 Lomb Memorial Dr., Rochester, NY 14623, USA
⁹ School of Mathematical and Physical Sciences, University of Sussex, Falmer, Brighton, BN2 9BH, UK
¹⁰ Carlson Center for Imaging Science, 84 Lomb Memorial Dr., Rochester, NY 14623, USA
¹¹ Radcliffe Institute for Advanced Study, 10 Garden St. Cambridge, MA 02138, USA
¹² Dipartimento di Astronomia, Universitá di Bologna, via Ranzani 1, 40127 Bologna, Italy
¹³ Harvard Smithsonian Center for Astrophysics, 60 Garden St. Cambridge, MA 02138, USA

Received: 8 May 2012
Accepted: 1 August 2012

Abstract

We analyze Chandra observations of diffuse soft X-ray emission associated with a complete sample of 3CR radio galaxies at z < 0.3. We focus on the properties of the spectroscopic sub-classes of high excitation galaxies (HEGs) and broad line objects (BLOs). Among the 33 HEGs we detect extended (or possibly extended) emission in about 40% of the sources; the fraction is even higher (8/10) when restricting the analysis to the objects with exposure times larger than 10 ks. In the 18 BLOs, extended emission is seen only in two objects; this lower detection rate can be ascribed to the presence of their bright X-ray nuclei that easily outshine any genuine diffuse emission. A very close correspondence between the soft X-ray and optical line morphology emerges. We also find that the ratio between [O III] and extended soft X-ray luminosity is confined within a factor of 2 around a median value of 5. Both results are similar to what is seen in Seyfert galaxies. We discuss different processes that could explain the soft X-ray emission and conclude that the photoionization of extended gas, coincident with the narrow line region, is the favored mechanism.