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Radio Loud and Radio Quiet Active Galactic Nuclei

We generated a sample of 409 AGNs for which both the radio luminosity at 5 GHz and the line luminosity in [OIII] 5007 have been measured. The radio luminosity spans a range of ten orders of magnitude, and the [OIII] line luminosity spans a range of eight orders of magnitude --- both considerably larger than the ranges in previous studies. We show that these two quantities are correlated in a similar way for both radio-loud and radio-quiet AGNs. We demonstrate that the observed correlation can be explained in terms of a model in which jets are accelerated and collimated by a vertical magnetic field.


INTRODUCTION
The "unified" scheme of active galactic nuclei (AGNs) has been very successful in explaining a variety of AGN properties on the basis of the viewing angle θ (see e.g., review by Urry & Padovani 1995). It is very clear, however, that more physical parameters are required to construct a truly "unified" model of all AGNs (e.g., Blandford 1990). In particular, it is by now well established that AGNs fall into two families in terms of their radio power (as measured for example by their 5 GHz luminosities), the "radio-loud" and "radio-quiet" AGNs (e.g., Baum & Heckman 1989;Miller, Rawlings & Saunders 1993).
Phenomenologically, radio-louds are always associated with large scale radio jets and lobes while the radio-quiets have very little or weak radio emitting ejecta. Radio-loud AGNs are almost always associated with early type galaxies, while the radio-quiet ones are mostly found in spirals and S0s.
One of the ways to tackle observationally the question of the additional fundamental parameters (other than the viewing angle θ), is to examine the AGN luminosities in a variety of wavebands. In particular, it has been shown that a correlation exists between the radio luminosity (L 5 GHz ) and the [OIII] 5007 narrow line luminosity (L [OIII] ) (e.g., Rawlings & Saunders 1991;Baum & Heckman 1989).
In the present work, we study the L 5 GHz -L [OIII] and related correlations for a much larger data set, covering a very wide range of luminosities. We then use the obtained results in combination with recent theoretical developments in an attempt to place constraints on possible scenarios for the nature of radio-loud and radio-quiet AGNs.
In Section 2 we describe the samples used and the data. The results are presented in Section 3, and discussed in Section 4. A summary and conclusions follow.

SAMPLES AND DATA HANDLING
All the data were compiled from the literature and through the NASA Extragalactic Database (NED). Our original sample included four categories of AGNs: (1) Radio sources (Bennett 1962;Smith & Spinrad 1980;Zirbel & Baum 1995;Condon, Frayer & Broderick 1991), (2) Seyfert galaxies (Lipovetsky, Neizvestung & Neizvestnara 1988;Dahari & Robertis 1988;Whittle 1992), (3) BL Lac objects (Veron-Cetty & Veron 1996;Padovani & Giommi 1995), and (4) Quasars (Schmidt & Green 1983;Brotherton 1996;Boroson & Green 1992). To these we added many sources found individually in the literature, thus generating a sample of about 2,000 AGNs. From this sample we selected all the objects for which measurements of both the radio luminosity at 5 GHz and the luminosity in the [OIII] 5007 forbidden line existed. We have thus generated a sample of 409 sources, including: 162 Seyfert galaxies, 136 quasars, 107 radio galaxies and 4 BL Lac objects. Table 1 gives a list of all the sources used, with the relevant information for each object. Specifically, we list in columns 1-6 respectively the object's IAU name, its catalogue name, its identification (a galaxy (G) or a quasar (Q)), the nature of its activity: Seyfert or Radio Galaxy, its morphology type and its redshift. In columns 7-10 we list respectively: its luminosity in the [OIII] 5007 line, the total radio power at 5 GHz, the core radio power at 5 GHz, the x-ray luminosity in the 2-10 keV band and the corresponding references.
In the last column (column 15) we remark if the object is regarded as radio-loud (L) or radio-quiet (Q) in our study. A more detailed notation is attached at the end of Table 1.
Almost all the data presented on the luminosities represent actual measurements, with very few (e.g., core radio powers from Zirbel & Baum 1995) upper and lower limits. As a rule, if a certain quantity was found to have several different quoted values, the one closest to the mean was taken.
The radio powers at 5 GHz given in Table 1 were either taken directly from the -5literature, or calculated from the given fluxes. A value of the Hubble constant of H 0 = 50 km s −1 Mpc −1 has been assumed throughout. In calculating the radio power of sources for which the spectral index was not available (assuming a power law S ν ∝ ν −α ), a mean index of 0.75 was adopted.
The luminosities in the [OIII] 5007 line were mostly calculated from the fluxes found in the literature (without reddening corrections, since very few of the latter are available, e.g., Koski 1978). In a case in which only the combined fluxes of [OIII] 5007 and 4959 were given (Steiner 1981), the flux in the 5007 line was taken to be 3/4 of the combined flux.
In two cases in which only equivalent widths of [OIII] 5007 were given (Brotherton 1996;Boroson & Green 1992), the continua were determined from the corresponding spectra.
The x-ray luminosities in the 2-10 keV band were calculated from the integrated fluxes.
In a case in which only the HEAO/A-2 count rates was given (Ceca et. al. 1990), the fluxes were calculated assuming a mean energy index of 0.65.
The redshifts were obtained from the literature and cross checked through the NED.
While clearly the use of many sources for the data makes our sample inhomogeneous in terms of the errors involved, this has a very little effect on our conclusions, since the data now span 8 orders of magnitudes in the [OIII] luminosity and 10 orders of magnitude in the radio luminosity.

ANALYSIS AND RESULTS
In Fig. 1, we present the radio luminosity at 5 GHz, L 5 GHz , as a function of the [OIII] 5007 line luminosity, L [OIII] , for all the sources in our sample. The separation into the two families of radio-loud and radio-quiet AGNs with a significant gap between them is immediately apparent. The radio luminosities are different by a factor 10 3 -10 4 between the -6two groups (at a given [OIII] luminosity). We do find a small group (∼ 3% of the sample) of objects which appear to occupy the region between the two main families. These tentative "intermediate" objects have been represented by filled circles in Fig. 1.
Linear fits to the radio-loud and radio-quiet AGNs (excluding the intermediate objects) give: log (L 5 GHz ) = (0.61 ± 0.07) log L [OIII] + (3.7 ± 0.6) radio loud log (L 5 GHz ) = (0.45 ± 0.07) log L [OIII] + (5.6 ± 0.6) radio quiet (1) Here, L 5 GHz is the radio luminosity in Watts Hz −1 Sr −1 , and L [OIII] is the [OIII] line luminosity in Watts. In determining whether an object belongs to the radio-loud, radio-quiet or intermediate group, we first selected out two distinct groups of objects (namely radio-louds and radio-quiets) based on the distribution in the L 5GHz -L [OIII] diagram ( Fig. 1). We then found linear fits to the two groups respectively. We considered the objects whose radio luminosities (at 5 GHz) are higher than the "radio-loud" fitting minus 2.75 σ as radio-loud, those whose radio luminosities are less than the "radio-quiet" fitting plus 2.5 σ as radio-quiet, and those which lie between the two groups as intermediate.
This way of defining radio-loud and radio-quiet is to some extent semi-empirical. The reason we chose slightly different criteria in determining the radio-loud and radio-quiet membership was to achieve the best visual representation of the two groups in Fig. 1.
We note that the samples used by previous authors (e.g., radio quiet quasars by Miller et. al. 1993et. al. , radio-loud objects by Rawlings 1994 represent sub-groups of our sample. These authors found a relation with a somewhat steeper slope (∼ 0.85-1.0) for the subgroups.
In Fig. 2a, we present the same data, now indicating the different classes of AGNs (note that a few of the objects might have been misclassified in the literature). What becomes immediately clear from this figure is that by plotting L 5 GHz vs. L [OIII] only for some individual classes of AGNs (e.g., quasars), one could obtain a distribution of -7points that would have looked like a relation with a rather different slope (see Fig. 2b).
In such a case it would have been difficult to determine whether this indeed represents a different functional dependence of L 5 GHz or L [OIII] , or whether this is merely an artifact of examining, for example, only the rightmost edges of two separate linear relations (as in the case of quasars, Figs. 2a,2b). For the moment we will assume that the latter interpretation is correct, since our basic assumption is one of an underlying "unified" scheme. We do note however that extreme caution should be exercised in attempts to determine the properties of subclasses of AGNs, and that in principle, the former interpretation may be valid (e.g., that radio-loud and radio-quiet QSOs are more fundamentally associated).
In order to examine the question of whether the distinction between the radio-loud and radio-quiet families is mainly a result of the extended radio emission, we plotted in Fig. 3 the 5 GHz core radio luminosity against the [OIII] line luminosity. As we can see from the figure, the gap between the radio-loud and radio-quiet groups is much less pronounced in this case (although instrumental effects introduce uncertainties; see also Nelson &Whittle 1996 andSadler et. al. 1995), but the two groups are still discernible. Interestingly, most of the "intermediate" sources of Fig. 1 become essentially indistinguishable from the radio-loud sources in Fig. 3. This may indicate a close relation between the intermediate sources and the radio louds. A linear fit to the core radio luminosity of the radio-louds gives where L core is the core radio luminosity (at 5 GHz) in Watts Hz −1 Sr −1 .
In order to further clarify the properties of the [OIII] emission, we plot in Fig. 4 the x-ray luminosity in the 2-10 keV band as a function of the [OIII] luminosity. As can be seen from the figure, a clear positive correlation exists, and a linear fit gives log (L HX ) = (1.01 ± 0.08) log(L [OIII] ) + (8.6 ± 0.7) , -8where L HX is the x-ray luminosity (in the 2-10 keV range) in erg s −1 . Individual fits to the radio-loud (plus intermediate) sample and the radio-quiet sample give slopes of 0.89 ± 0.19 and 0.95 ± 0.10 respectively.
It is important to note right away that the radio-louds and radio-quiets share the same L HX -L [OIII] relation, implying probably that the same physical process relates these quantities in the two families (we will return to this point later).
In order to examine the potential effects of the environment, we also plotted the data of Fig. 1 distinguishing among the different types of host galaxies (in cases in which the latter have been identified). This is presented in Fig. 5. This figure confirms that all the radio louds are elliptical or S0 galaxies, while almost all of the spirals are radio quiet.
It is interesting to note though that some of the "intermediate" objects are spirals. We should note, however, that quite a few of the morphologies are ambiguous (in particular the distinction between ellipticals and S0s) and therefore caution should be exercised in attempts to draw conclusions on the basis of morphology.

DISCUSSION
We have found that over a very wide range in luminosities, AGNs separate into two classes of objects, radio-loud and radio-quiet, and that the two classes obey two parallel relations between their radio and [OIII] luminosities, of the form with β ∼ 0.5 ( Fig. 1 and eq. 1). At this point we need to consider whether the observed relationships are true correlations or whether selection effects can be dominating what we are seeing. First, we need to ask, are there really two distinct classes of radio-quiet and radio-loud types of AGN or is the apparent gap region merely an artifact of the selection -9process? In particular, many, though not all, of the radio loud objects come from radio flux density selected samples and many, though not all, of the radio quiet objects come from optically selected samples.
We do not think that selection effects are responsible for the apparent population of the L radio − L [OIII] plane for the following reasons: (i) radio flux density selected samples always include objects of low radio luminosity at low redshift and of progressively higher and higher radio luminosity with redshift. For example, the 3CR cut off in radio flux is about 9 Jy at 178 MHz (Bennett 1962); if we take the spectral index as 0.75 (which is a typical value for AGNs), then the extrapolated radio flux at 5 GHz is 0.74 Jy. This corresponds to L 5GHz ∼ 2.3 × 10 21 W/Hz/Sr at z=0.003 and L 5GHz ∼ 2.56 × 10 22 W/Hz/Sr at z=0.01.
These values lie within the radio-quiet group in our classification ( Fig. 1). Actually, we do find sources from the 3CR catalogue which are indeed radio-quiets (3C71 for example).
Likewise, purely optically selected samples of AGN (e.g., Padovani 1993;Miller et. al. 1990 andKellermann et. al. 1989) also show the radio-quiet-radio-loud dichotomy, with roughly 1 in 10 of optically selected quasars falling into the radio loud category. (ii) Perhaps more importantly, the apparent dichotomy in properties we have found is not a dichotomy of either L radio or L [OIII] , but the ratio of these two quantities. It is clear that selection effects can cause us to find sources with high, on average, radio luminosity when we select radio flux density limited samples; however, there is no a priori reason why such samples should show a limited and specific ratio of radio to line luminosity. The absence of identified sources with high radio luminosity and low line luminosity in radio flux density selected samples has been known for years (e.g., Baum and Heckman, 1989); sources with high radio luminosity invariably have high accompanying line luminosities. The real surprise is then that the converse does not also appear to be true. That is, if one selects AGN of high line luminosity at any given redshift, one finds two distinct classes of objects; those with high radio luminosity and those without. Said in a different way, there are sources with very -10high line luminosity which do not have accompanying very high radio luminosity. This is the radio-quiet radio-loud dichotomy that has been known for many years (e.g., Antonucci 1993 and references therein); and the fundamental paradox: the nuclei of active galaxies can produce copious amounts of line luminosity (and UV through X-ray luminosity) without producing large amounts of radio luminosity, however the nuclei of active galaxies cannot produce large amounts of radio luminosity without producing concomitant large amounts of line and UV/X-ray luminosity.

The Physical Origin Of The L radio − L [OIII] Relation
We will now attempt to understand the origin of this relation in terms of the physical processes involved.
There exists strong observational evidence that suggests that L [OIII] is one of the best orientation independent measures of the intrinsic luminosity of the nuclei of AGNs (e.g., Miller et. al. 1992;Jackson & Browne 1991;Mulchaey et. al. 1994). This fact, in combination with the data presented in Fig. 4, suggests that L [OIII] is proportional to the accretion rate through the accretion disk,Ṁ acc . We will therefore assume that L [OIII] is proportional toṀ acc , L [OIII] ∝Ṁ acc .
We will now attempt to obtain a general relation between the accretion rate through the disk and the mass flux into the jet,Ṁ j (see e.g., Pringle 1993; Tout & Pringle 1996; Livio 1997).
To this goal, we first note that the most promising models for jet acceleration and collimation involve an accretion disk that is threaded by a large scale, vertical magnetic field (e.g., Blandford & Payne 1982;Königl 1989; and see Livio 1997 for a review). We will now make the following simple assumptions: (i) The accretion disk is largely a standard -11 -(geometrically thin), Shakura-Sunyaev (1973) disk. (ii) The jet velocity, V j , is of the order of the Keplerian velocity in the inner disk. (iii) The vertical magnetic field component is of the order of the azimuthal one, B z ∼ B φ . A detailed justification of these assumptions can be found in Livio (1999) and references therein. Realizing that the back pressure from the jet on the disk is given by P jet ∼Ṁ j V j /R 2 , where R is the radius from which the jet originates, and using assumptions (i)-(iii) above, we obtain Here P g is the gas pressure, H is the disk half-thickness and α is the Shakura-Sunyaev (1973) viscosity parameter. If we assume in addition that the disk viscosity is generated by a dynamo, which in turn is powered by MHD turbulence (e.g., Hawley, Gammie & Balbus 1995;Stone et. al. 1996;Brandenburg et. al. 1995), then α ∼ B 2 D /(4π P g ), where B D is the magnetic field in the disk. Substituting this into eq. (5) gives An independent relation between B z and B D can be obtained if we make an assumption about the origin of the large-scale vertical field. In principle, such a field can either be advected inwards by the accreting matter (e.g., Blandford & Payne 1982;Königl 1989; Pelletier & Pudritz 1992), or it can be generated locally by the same dynamo processes which generate the disk viscosity (Tout & Pringle 1996). If we assume the latter to be true, then the large scale field may be obtained through the reconnections of magnetic loops (leading to an inverse cascade process) which have a length distribution of the form In such a case, it can be easily shown that (Tout & Pringle 1996; Livio 1997) Combining eqs. (6) and (7) we obtaiṅ -12 -Observations of accretion disks, jets and outflows in young stellar objects, supersoft x-ray sources and cataclysmic variables and theoretical models suggest that δ is in the range 1.7-3.4 (Livio 1997;Tout & Pringle 1996). Therefore, assuming that the jet formation mechanism is similar in all the classes of objects which produce jets, and noting that H/R is approximately constant in standard disks (Shakura & Sunyaev 1973), we find thatṀ j is roughly proportional toṀ acc ,Ṁ j ∝Ṁ acc .
The final ingredient that is needed to explain the relation obtained in Fig. 1 is a relation between L 5 GHz andṀ j . Since we have shown that L [OIII] ∝Ṁ acc , and thatṀ j ∝Ṁ acc , it is clear that the dependence observed in Fig. 1 (and in Rawlings 1994) would be obtained if L 5 GHz ∝Ṁ β j , with β ∼ 0.5 -1.0. Observations of individual Galactic jets in systems containing black hole accretors (e.g., Hjellming & Rupen 1995;Mirabel & Rodriguez 1994;Tavani et. al. 1996) indeed suggest that the radio luminosity is proportional to some power (of order unity) of the mass flow rate into the jet. Simple models of radio emission from jets also predict radio luminosities which are roughly proportional to the mass flux into the jet (with the constant of proportionality depending on some power of the magnetic field strength and on the age of the source, e.g., Bicknell, Dopita & O'Dea 1997). We therefore conclude that the general correlation in Fig. 1 is entirely consistent with a model in which jets are formed by accretion disks (around supermassive black holes) which are threaded by a vertical magnetic field (e.g., Blandford & Payne 1982;Königl 1989;Ostriker 1997;Matsumoto et. al. 1996; and see Livio 1997 for a review).
A question that needs to be asked at this point is: can there be important selection effects which are skewing the slope of the L 5GHz − L [OIII] correlation to be less than unity?
This could occur, most naturally, if we were missing a class of high radio luminosity, high [OIII] luminosity objects or if we have systematically underestimated the line luminosity of the high luminosity objects. However, there is no evidence that this is the case. A fit -13to the slope for only z≥0.2 radio-loud sources indicates, if anything, a slightly flatter slope (0.37±0.12) than that found either at low (z≤0.2) redshifts or taking the sample as a whole (0.61±0.07), though the differences are not statistically significant. Similarly, the slope of the radio-quiet class is more dominated by low redshift objects, but shows no evidence for a change for redshifts less than (slope 0.44±0.07) or greater than 0.2 (slope 0.45±0.55).
It is important to note that the fact that the L 5 GHz − L [OIII] relation has almost the same slope for both the radio-quiet and radio-loud AGNs probably indicates that the jet formation mechanism is the same in both of these subclasses.

The Black Hole Mass
Another consequence of Fig. 1 that should be pointed out is the following (see also Livio 1997). The mass of the central black hole determines the Eddington luminosity, and therefore the maximum accretion rate which the system can sustain (this translates into: how far to the right, in Fig. 1, the system can be found). Hence, we can expect that the AGNs containing the most massive black holes will occupy the upper right corner of the distribution for each subclass (radio-loud and radio-quiet). Interestingly, we find that the distribution of the radio-louds extends to somewhat larger values of L [OIII] (largerṀ acc ).
In order to further examine the implications of this fact, we show in Fig. 6 the distribution of the sources with respect to their redshifts. We find that the sources at low redshifts (z ≤ 0.2) exhibit the same range inṀ acc (L [OIII] ) in both radio-louds and radio-quiets, but that the radio-loud sources at higher redshifts extend to higher values ofṀ acc . Therefore, if the Eddington luminosity is indeed the limiting factor, then this finding can be regarded as suggestive that the maximum mass of the black holes found in radio-louds is higher than that in radio-quiets. This result would be consistent with the fact that the measured black hole masses appear to correlate with the bulge luminosities (Kormendy & Richstone 1995).

The Distinction Between Radio-Louds And Radio-Quiets
A more difficult and long standing question is what distinguishes the upper group (radio-louds) from the lower one (radio-quiets). Recent discussions of this problem can be found, for example, in Blandford & Levinson (1995), Fabian & Rees (1995), Wilson (1996) and Livio (1997).
Generally, explanations for the existence of these two classes fall into two different categories: (i) the ones that assume that the central engines in radio-louds and radio-quiets are the same, but that either the formation or the propagation of powerful jets is somehow prohibited in radio-quiets by some external circumstances. (ii) Ones in which it is assumed that only the central engines of the radio-louds can produce truly powerful jets.
In recent work, Livio (1997Livio ( , 1999 examined the formation of jets in all the classes of astrophysical objects which are observed to produce jets. On the basis of the assumption that the jet formation mechanism is the same in all the classes of objects, Livio has shown that the following conjecture is consistent with all the available observational data: the formation of powerful jets requires in addition to an accretion disk threaded by a vertical field, an additional energy/wind source like a corona or a source associated with the central object. More recently, Ogilvie & Livio (1998) solved for the local vertical structure of an accretion disk threaded by a poloidal magnetic field. By analyzing the dynamics of the transonic outflow in the disk corona, they showed that a certain potential difference must be overcome even when the inclination angle between the magnetic field and the vertical to the disk surface is larger than 30 • . Thus, the launching of an outflow from an accretion disk indeed requires a hot corona or access to an additional source of energy, in accordance with  (1995). The latter possibility may be supported by the fact that Fig. 5 shows that the high L 5 GHz group contains quite a few S0 galaxies (but no spirals), in which the central environments are generally similar to those in ellipticals.

SUMMARY AND CONCLUSIONS
On the basis of the data collected in the present work and the discussion in §4, we can draw the following (tentative) conclusions: (1) Both radio-quiet and radio-loud AGNs obey a linear log L 5 GHz -log L [OIII] relation, with a nearly identical slope (but with a shift towards higher radio power for the radio-louds, by a factor ∼ 10 3 -10 4 ; see also Rawlings 1994).
(2) The radio-louds and radio-quiets share the same linear correlation between log L x and log L [OIII] (where L x is the X-ray luminosity in the 2-10 keV range).
(3) Consistently with previous studies, we find that radio-loud AGNs are found only in elliptical and S0 galaxies (although the distinction between S0 and elliptical is often ambiguous), while radio-quiets are mostly spirals and S0s. (4) The observationally determined L 5 GHz -L [OIII] -16correlation is consistent with a model in which the radio-emitting jets are formed by an accretion disk which is threaded by a vertical magnetic field. (5) It is still not entirely clear whether the distinction between radio-louds and radio-quiets is a consequence of differences in the central engines of these two classes or whether it merely reflects differences in the environments. Zirbel, E.L., and Baum, S.A., 1995, ApJ, 448, 521 This manuscript was prepared with the AAS L A T E X macros v4.0.