4.5. Basic differences between FR Is, FR IIs and radio quiet AGNs

4.5.1. Radio loud vs. radio quiet AGNs

What is the difference between E and spiral galaxies that is relevant to making active nuclei produce radio jets often in one, never in the other?

There is a very limited range of inputs that determine the nature of an AGN: the fuel supply, the mass of the BH and its angular momentum. The fuel supply rate is similar for AGNs in radio-loud and radio-quiet objects since both cover the whole range of luminosities; regarding the mass of the BH, the Eddington limit indicates that masses up to 10⁸ M occur in some radio-quiet AGNs; somewhat more massive BHs could occur in radio galaxies, but masses in excess of 10¹⁰ M seem to be ruled out on dynamical grounds, at any rate in the nearest gEs; the difference between E and spiral galaxies should then be the larger angular momentum of the central BH in E galaxies ([46]; [372]).

Blandford (1990) argued that sub-Eddington mass accretion rates produce slowly spinning BHs and that rapidly spinning BHs require high accretion rates. However, in FR Is objects, the accretion rates are certainly small and yet these objects have a radio jet implying a rapidly rotating BH; moreover NLS1s are generally radio quiet spiral galaxies which are believed to contain a small mass BH accreting matter at a high rate. Therefore, the larger angular momentum of the central BH in E galaxies is more probably the result of the merging of two BHs ([372]).

Until about 1975, it was commonly assumed that E galaxies were rotationally flattened and oblate; however, the first accurate measurements of the rotation velocities of E galaxies revealed a significantly lower rotation rate than expected; E galaxies were shown to be ``hot'' stellar systems in which most of the support against gravitational collapse comes from essentially random motions, rather than ``cold'' systems like spiral galaxies in which ordered rotation contributes most of the internal kinetic energy. One model for the formation of E galaxies that has gained many adherents is the merger hypothesis proposed by [415]. According to this hypothesis, most or all E galaxies formed from the coalescence of spiral galaxies. Evidence in favor of the merger hypothesis include the discovery of a number of fine structure features such as ripples, twists, imbedded disks and tidal tails ([291]); many, perhaps all, gE galaxies have galactic cluster systems that show bimodal metallicity distributions generally attributed to merging ([96]; Kissler-Pattig et al. 1998). It seems now clear that E galaxies include some objects formed by fairly recent mergers of disk systems ([20]). The kinematic and photometric properties of luminous IR merging galaxies are consistent with their evolving into E galaxies; gE galaxies, if they are formed through mergers, probably evolve from ULIGs ([380]).

Compact dark masses, probably massive BHs, have been detected in the core of many normal galaxies using stellar dynamics ([232]). The massive BH mass appears to correlate with the galactic bulge luminosity, with the BH being about one per cent of the mass of the spheroidal bulge ([130]; [273]; [350]; [435]). This relation is similar to the one found for QSOs ([244]). The observed relation between BH masses and bulge masses can be reproduced if the mass accreted during a major merger is a fixed fraction of the mass of the gas that has formed stars in the merging galaxies since the last major merger ([79]).

Indirect evidence for binary orbital motion may be present in the wiggles of a milliarcsecond radio jet ([359]). [385], Villata et al. (1998) and [407] showed that the optical light curve of OJ 287 features repeated outbursts at ~ 12 yr intervals suggesting that it contains two supermassive BHs in a binary system (SMBBH) with a (rest-frame) orbital period of 8.9 yr. This prompted [384] to suggest that, in radio quiet objects, there is a single supermassive BH in the nucleus of the host, but in the radio loud objects, there is a SMBBH.

When two galaxies containing a nuclear BH merge, the two BHs form a binary and may eventually coalesce ([31]). For [479], galaxies which have not suffered a major merger event contain a non-rotating or a slowly rotating BH; if the two BHs have roughly equal mass, a rapidly spinning BH will result; such mergers could be the progenitors of powerful radio sources in which the radio jets are powered by the spin energy of the merged hole ([48]). The spindown time due to this loss of energy is much shorter than the Hubble time and consequently the life-time of radio loud AGNs is short and these AGNs should be the result of recent mergers ([290]).

More and more facts support the idea of a dichotomy in E galaxies which can be divided into ``disky'' and ``boxy'' objects from isophotal analysis ([319]); boxy Es are bright and rotate slowly, disky Es are weak and rotate rapidly; radio loud Es have been found to be boxy; disky Es are mostly radio quiet ([33]; [233]). However, the cause of the differences between disky and boxy galaxies is not yet understood. [220] suggested that disky Es have kept their original structure while boxy Es are the result of merger. [112] proposed that disky Es were formed in gas-rich mergers and boxy Es in gas-poor mergers but, for [311] merging of two equal-mass spirals leads to an anisotropic, slowly rotating system with preferentially boxy isophotes, while unequal-mass mergers result in the formation of rotationally supported Es with disky isophotes.