Powerful extended radio sources as tools to estimate ambient gas densities, jet luminosities, and other key physical parameters

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Abstract

It would be quite valuable if the radio properties of powerful, extended radio sources could be used to deduce key physical quantities such as the density of the ambient gas in the vicinity of a radio source. It is shown here that radio observations can be used to estimate the ambient gas density, the luminosity in directed kinetic energy, and other key physical parameters relevant to the radio source and its gaseous environment. The methods described are applied to radio galaxies and radio-loud quasars with redshifts from about 0 to 2. The ambient gas density in the vicinity of the radio lobes is estimated by applying the strong shock jump conditions across the forward edge of the radio bridge (referred to as the radio lobe); this requires that the lobe pressure and the lobe propagation velocity be known. The lobe pressure is estimated assuming minimum energy conditions, and the local propagation velocity is estimated from the effects of synchrotron and inverse Compton aging of relativistic electrons on the radio spectrum across the radio bridge. At present this appears to be the only method of estimating the ambient gas density (as a single parameter) in the vicinity of distant powerful radio sources, and this first application of the method indicates that it provides a good rough estimate of the ambient gas density. One interesting result is that galaxies and quasars are found to lie in similar gaseous environments. Another is that the composite density profile is similar to that of gas in clusters of galaxies and is normalized to Cygnus A, which is known to be in a cluster with a hot intracluster medium in place. This suggests that the powerful radio sources considered here are surrounded by extended gaseous halos like those in present-day galaxy clusters, with the radio source interior to the core of the gaseous halo. One interpretation is that the radio sources considered are in the cores of clusters of galaxies with their intracluster media in place, which would suggest that some clusters, or at least cluster cores, exist out to redshifts of about 2. The lobe propagation velocity, the rate at which energy is channeled from the central engine in the form of a collimated outflow (known as the luminosity in directed kinetic energy), and a time-independent characteristic source size, which provides a calibrated yardstick and hence is a useful cosmological tool, are also discussed. The basic assumptions adopted by Daly (1994) in the use of these sources as cosmological probes are empirically tested; it is found that the data are consistent with the assumptions adopted. Relations between different parameters are investigated and discussed in detail. This leads to a wide perspective and good understanding of the environments and nuclear properties of powerful extended radio sources. For example, a maximum value for the energy extraction rate is seen in the present data set and may imply an upper limit to this quantity. And the lobe propagation velocity may also have a maximum value, indicating that there may be an upper limit to the Mach number at which the radio lobe propagates into the ambient medium. The limited data set used here supports the idea that radio galaxies and radio-loud quasars are intrinsically similar but appear different due to different viewing angles, however, they may not be intrinsically identical. Perhaps the two types of active galactic nucleus (AGN) activity observed, highly collimated outflows and radiant energy from the nuclear region of the AGN, should be identified with the two ultimate energy sources associated with massive compact objects: the spin energy of the massive compact object and the gravitational energy of matter falling onto the object, respectively. Then, the three types of AGN observed may not be intrinsically identical: radio-quiet quasars result when only the gravitational energy is being tapped, radio-loud quasars result when both energy sources are being tapped, and radio galaxies result when the spin energy of the massive compact object is being tapped. Since the sources are likely to be anisotropic, such a scheme is likely to be working in parallel with orientation unified models.

Original languageEnglish (US)
Pages (from-to)580-592
Number of pages13
JournalAstrophysical Journal
Volume454
Issue number2
DOIs
StatePublished - Dec 1 1995

All Science Journal Classification (ASJC) codes

  • Astronomy and Astrophysics
  • Space and Planetary Science

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