The bulk kinetic power of radio jets in active galactic nuclei

The formation of highly relativistic jets in active galactic nuclei (AGNs) is one of the unsolved fundamental problems in astrophysics (e.g. Meier et al. 2001). It has been assumed that jets are produced close to the central black hole, involving power extraction from the black hole spin (Blandford & Znajek 1977;Macdonald & Thorne 1982;Thorne & Blandford 1982) and/or from the accretion disk (Blandford & Payne 1982). Although the jet formation remains unclear, the estimate of the jet power is of fundamental physical interest, since it can be used to quantify the power emerging from the central engine of the radio source. Recently, the prescriptions for AGN feedback have been introduced into semi-analytic models of galaxy formation, and both Bower et al. (2006) and Croton et al. (2006) show that this feedback is able to solve the issue of the bright end of the luminosity function, whilst simultaneously solving other problems of galaxy formation models such as why the most massive galaxies are so red. Although the form of the AGN feedback adopted is very different in the two prescriptions, the relativistic ejecta from the AGN is a conceivably important ingredient of AGN feedback. Indeed, in clusters of galaxies containing powerful radio sources, X-ray observations have revealed bubbles and cavities in the hot intracluster medium, evacuated by the expanding radio source (e.g. McNamara et al. 2000;Fabian et al. 2003). Recent studies showed that the mechanical luminosity of radio sources are sufficient to suppress cluster cooling flows (Best et al. 2006;Nusser, Silk & Babul 2006). To understand the interaction between the radio sources and the surrounding medium, it is clearly important to estimate the bulk kinetic power of radio jets, since the expanding radio sources provides a direct way for the AGN output to be coupled to its environment.

The relation between the jets and the accretion processes in active galactic nuclei has been extensively explored by many authors and in different ways. The strong correlations have been found between the low-frequency radio and narrow-line luminosities of 3C radio sources (Baum & Heckman 1989;Rawlings et al. 1989;Saunders et al. 1989), and also between the broad line and extended radio luminosity for radio-loud quasars (e.g. Cao & Jiang 2001). The link between the jets and the accretion processes can also been studied through exploring the relationship between luminosity in line emission and kinetic power of jets in different scales (Rawlings & Saunders 1991;Celotti & Fabian 1993;Falcke, Malkan & Biermann 1995;Wang, Luo & Ho 2004). Rawlings & Saunders (1991) used the narrow-line luminosity as indicative of the accretion power and estimated the power transported by the jet from the energy content and lifetime of the radio lobes, finding a good correlation between the two. Using radio data on very long-baseline interferometry (VLBI) scales and the standard synchrotron self-Compton (SSC) theory, Celotti & Fabian (1993) estimated the jet kinetic power to put constraints on the matter content of jets. It offers some clues to understand the fundamental questions of the mechanisms, such as the collimation and acceleration of jets. Celotti, Padovani & Ghisellini (1997, hereafter C97) explored the relation of luminosity in broad emission lines with the kinetic power of the jets for a sample of radio-loud AGNs. Their estimate of the bulk kinetic power is based on the adoption of the SSC model applied to the radio VLBI data and X-ray (or optical) fluxes. Lacking more accurate information, the minimum Γ for any given δ [i.e. Γ = 0.5(δ + 1/δ)] is used in the derivation of bulk kinetic power for objects with δ > 1, otherwise the Γ is derived from an average δ. They found a suggestive hint of correlation between these two luminosities which is in favour of a link between the accretion process and the jets. However, by re-estimating the luminosity in broad emission lines on the sample of C97, Wang et al. (2004) argued that the jet bulk kinetic power is significantly correlated with the disk luminosity. Maraschi & Tavecchio (2003, hereafter MT03) found that the jet power is linearly proportional to the disk power for a sample of blazars, for which the jet powers were estimated using physical parameters determined from uniformly modeling their spectral energy distributions. However, by studying a sample of quasars from Wang et al. (2004), Punsly & Tingay (2005) argued that the bulk kinetic power and the bolometric luminosity are very weakly correlated in radio-loud quasars that possess blazar cores.

In the framework of the relativistic beaming and the SSC model, the physical quantities in the jets can be estimated using the VLBI observations and the X-ray flux density. Marscher (1987) derived the beaming parameters on the assumption of homogeneous spherical emission plasma. Ghisellini et al. (1993) adopted Marscher’s approach and obtained the Doppler boosting factor δ for 105 sources. More

…(Full text truncated)…

This content is AI-processed based on ArXiv data.