Alma Mater Studiorum Universit`a degli Studi di Bologna ... - Inaf

Chapter 2
The X-ray environment of radio
galaxies
2.1 Introduction
E
arly Einstein X-ray observations discovered that early-type galaxies, groups and
clusters of galaxies are spatially-extended X-ray sources with luminosities in the range
10 39−43 erg s −1 (e.g. Forman et al. 1979; Kriss et al. 1980; Biermann, Kronberg &
Madore 1982). Subsequent more sensitive and detailed X-ray images provided by the ROSAT,
Asca, XMM-Netwton and Chandra satellites have led to the detection of components on several
scales from several distinct emission mechanisms. These are:
• the end products of stellar evolution (point-like sources);
• emission spatially coincident with the bases of radio jets;
• the hot phase of the interstellar medium (ISM) of the galaxies and
• the hot and diffuse intergalactic medium (IGM) in groups and clusters of galaxies.
It is now well-established that the diffuse X-ray emission arises from hot and dilute plasma
emitting via thermal bremsstrahlung (Sec. 2.2.2).
Radio emission from active galaxies therefore coexists with a complex, hot medium, with
which it interacts in several ways.
• Radio jets can entrain the external medium, which is thought to be able to decelerate the
initially relativistic flows in FR I sources on typical scales of a few kpc (e.g. Laing & Bridle
2002a).
13