Early on in the discussion about the existence of black holes Lynden-Bell & Rees (1971) suggested that they would be accompanied by compact radio nuclei, detectable by Very Long Baseline Interferometry (VLBI), and predicted such a source for the Galactic Center. Indeed, this source (Sgr A*) was then discovered by Balick & Brown (1974) and it became clear in later years that compact radio cores are indeed good evidence for the existence of active galactic nuclei (AGN) most likely powered by black holes. For luminous radio galaxies and radio-loud quasars the basic nature of these compact radio nuclei has been clarified in the meantime through extensive and detailed VLBI observations (see Zensus 1997 for a review) as being the inner regions of relativistic jets emanating from the nucleus.
Despite this progress, a number of important questions remain when looking back at the initial discussion. First of all, it is unclear whether there indeed is a direct link between compact radio cores and AGN, i.e. whether compact radio cores and jets are just an accidental by-product of black hole activity or a necessary consequence. Secondly, for the lesser studied, low-luminosity AGN the jet nature of compact radio nuclei has not yet been established beyond any doubt, leaving the question open whether in fact a compact radio core in a low-luminosity AGN (LLAGN) is the same as in a high-luminosity AGN, i.e. a quasar.
The latter was exactly the claim we made in an earlier paper (Falcke & Biermann 1995) where we proposed that accretion disks and jets form symbiotic systems and proposed a scaling law which connects high-power and lower-power accretion disks and their associated radio jets (cores). The scaling law was based on the assumption of an equipartition between the energy released and radiated away through dissipation processes in the accretion disk and the power put into the formation of magnetically driven radio jets.
The question whether this scaling law holds all the way down to low-luminosity AGN, as claimed in Falcke & Biermann (1996), also has some very interesting implication for the current discussion of accretion flows. Since the early papers on the observational appearance of black holes (e.g. Shakura & Sunyaev 1973) it was assumed that luminous, thermal emission at optical, ultra-violet (UV), or X-ray wavelengths was the primary sign for the presence of an accreting black hole. It was argued that any matter falling onto the black hole would likely form an accretion disk, if there was any residual angular momentum, and hence would need to dissipate its potential energy into heat by viscous processes allowing it to transport angular momentum outwards while matter is falling inwards (-disk). This idea was used successfully to explain the "big blue bump" in quasars (e.g. Sun & Malkan 1998).
However, the view that the -disk can be extended to much lower powers has been challenged (Narayan & Yi 1994, 1995a&b) and it was argued that accretion disks will turn into advection dominated accretion flows (ADAFs) if the accretion rate onto the black hole is sufficiently sub-Eddington. Narayan et al. (1995 & 1998; see also Rees 1982 and Melia 1994) applied this to the Galactic Center, trying to explain the compact radio source Sgr A* and its faintness at other wavelengths, and Lasota et al. (1996) used the ADAF model to explain the broad-band spectrum of the nearby LLAGN and LINER galaxy NGC 4258 which is famous for its megamaser emission from a molecular disk (Miyoshi et al. 1995). An integral part of these ADAF models is the prediction of very compact radio emission associated with the innermost part of the accretion flow, providing an alternative explanation to the jet model for compact radio nuclei in LLAGN. While initially the predicted, highly inverted radio spectra of the ADAF model, did not fit the observed characteristics of these radio cores very well, Mahadevan (1998) presented a more recent version of this model that was at least able to account for the correct radio spectrum of Sgr A* 1. Still, Di Matteo et al. (1998) found a number of serious constraints for ADAF models-at least for compact radio nuclei in elliptical galaxies.
Hence, the question now is whether indeed the radio emission from compact nuclei in sub-Eddington accretion systems can be used as an argument for the existence and necessity of ADAFs, or whether they are equally well, or even better explained, by a scaled down AGN jet model. The latter will be tested in this paper: firstly, we will use the jet/disk-symbiosis model of Falcke & Biermann (1995) in its most recent version (Falcke 1996b) and present simplified approximate solutions that can be applied easily. Secondly, we will apply those solutions to some specific sources which are of particular interest in this discussion and are observationally well constrained in their parameters. Finally we will discuss our results within the context of the jet/disk-symbiosis model and their implications for ADAFs.
© European Southern Observatory (ESO) 1999
Online publication: December 22, 1998