Bicknell (1995) developed the theoretical basis for the dependence of FR-I/II dividing power on the absolute magnitude of the host elliptical (the Owen-Ledlow diagram). The FR-I/II borderline jet energy flux (and hence total radio power) was related to the host optical magnitude through parameters that affected the jet propagation i.e. the central pressure. A given absolute optical magnitude of an elliptical galaxy (or a given mass of elliptical galaxy) has an ambient pressure which corresponds to a unique transition jet flux such that for higher ambient pressures (and hence for more luminous elliptical galaxies) the FR-I/II jet transition occurs at higher jet energy flux. There have been other attempts to relate the jet advance with the ambient medium within the host galaxy (for example, Kawakatu et al. 2009) and also attempts to understand the Owen-Ledlow diagram by redrawing the relation in terms of intrinsic parameters: nuclear photoionizing luminosity versus the black hole mass (Ghisellini & Celotti 2001, Wold et al. 2007).
A point to note about the Owen-Ledlow diagram is the scatter along the Y-axis. One sees that no longer is it that a galaxy of a given optical magnitude and hence mass produces a radio galaxy of a fixed morphology or fixed power. It is exciting to note that galaxies of the same absolute magnitude can host FR-Is as well as FR-IIs and also FR-Is and FR-IIs having a range of different powers. It follows that under different conditions an elliptical galaxy can host an FR-I or FR-II or an FR-I of a different power and an FR-II of a different power.
The Owen-Ledlow diagram is a powerful representation of the story of radio galaxies, of the relation between the large scale radio structures and the galaxies that host them, revealing that for every host galaxy mass there is a threshold jet power which is needed to produce an edge-brightened structure and that if conditions within change the same galaxy can host a radio galaxy of a different power or different morphology in its lifetime. The Owen-Ledlow diagram therefore incorporates within it the possibility of restarting of nuclear activity. We will see below that the simple diagram can also provide a platform for understanding various other findings related to the two morphological types.
The translation of FR classes along the Y-axis was already remarked on by Ledlow & Owen (1996) and Ledlow (1997) who pointed out that this could be suggesting possibility of a transition between the two populations. In their preferred scenario for understanding the two FR classes (Baum et al. 1995) also pointed out that the differences in the black hole spins (with low spin for FR-Is and high spin for FR-IIs) allowed for the possibility of transition from FR-II to FR-I type. Such a transition is not unreasonable to expect since radio galaxies have finite life times and the beam switch-off process occurs more likely over a drawn out period of time rather than abruptly (as also discussed in Saripalli et al. 2012). Saripalli & Subrahmanyan (2009) invoked such an FR-II to FR-I transition in morphology to explain the five, restarted X-shaped sources that on one hand showed main-lobe, edge-darkened structures while exhibiting properties similar to the rest of the X-shaped source population all of which were of FR-II type. Indeed in using Monte-Carlo simulations to test whether observed samples of radio galaxies can be random selections of elliptical galaxies, Scarpa & Urry (2001) found that the two FR classes are hosted by ellipticals extracted from the same population. Also, the Owen-Ledlow diagrams of radio samples in later studies are not found to be as sharply divided between the two classes as originally found and there is non-insignificant number of FR-IIs below the canonical dividing line (Ledlow & Owen 1996, Lin et al. 2010). These below-line FR-IIs, that are also found to be compact in physical size, have been speculated as being FR-IIs that are likely to evolve into FR-Is as their low power jets get frustrated in interactions with the ISM of the host galaxy (Kaiser & Best 2007). While these compact FR-IIs may be low power and young radio galaxies there may also be, as found among the ATLBS-ESS sources (Saripalli et al. 2012), larger FR-II sources that are either relic-type or even restarted FR-II sources (where the outer lobes have reduced in power; Thorat et al, in preparation). The existence of FR-I type sources, the intriguing FR-I quasars, lying above the dividing line has also been reported (Heywood et al. 2007). We discuss this in later in this section.
The growing impression therefore is of flexible physical conditions across both the absolute magnitude axis as well as the integrated (radio) power axis. FR-I/II radio galaxies can both be hosted by elliptical galaxies having a wide range in absolute magnitudes (or masses) within the massive galaxy regime and at the same time not only do both FR types occur in galaxies of the same magnitude but FR-II radio galaxies can have integrated powers that lie below the dividing line. While the transition jet energy flux could be unique to an elliptical galaxy of a given absolute magnitude it remains to identify the physical conditions as well as connection with galaxy mass which will determine the type of extended radio morphology that will result.
7.2. Host galaxy mass and the FR type
With the backdrop of the Owen-Ledlow diagram in mind we sketch the following scenario to understand the differences in the properties of FR-Is and FR-IIs. Massive elliptical galaxies will have enough stellar mass loss to sustain a radio galaxy (Di Matteo et al. 2003, Ho 2009). Massive ellipticals will therefore frequently host radio sources given the regular source of fuel. However because of the higher central gas pressures and more extended interstellar medium of more massive elliptical galaxies, most jet powers generated will find it hard to retain their thrust over long distances through the ISM (Bicknell 1995). Only the more powerful of jets will remain supersonic and form FR-II morphologies. The resulting morphologies will therefore tend to be dominated by FR-I rather than FR-II morphologies. Also high power jets (capable of negotiating successfully the increased ISM) may likely be less often produced than low power jets since such jets would require higher accretion rates than that generated 'in-house' (otherwise most ellipticals will be associated with FR-II sources) and in the case of massive ellipticals will likely need gas-rich mergers.
The inability of massive ellipticals to host FR-II morphologies gets more and more aggravated with increased mass of the galaxies. This and the high frequency of association of massive ellipticals with radio sources is consistent with the finding by Best & Longair (2005) that the fraction of low power radio sources hosted by elliptical galaxies is high (as high as 30%) and this fraction increases with galaxy mass.
It follows that lower the mass of the host galaxy it gets easier to host radio galaxies with FR-II morphologies. Unlike the more massive ellipticals the ISM is less rich and less extended and the threshold jet power is lower. However, while there is still the stellar mass loss that is available as fuel, it is likely to be available at lower rates than in the more massive ellipticals resulting in only weak sources. Overcoming these lower threshold jet powers may not need a large jump in accretion rate however and they may relatively easily be breached by even small ingestions of external fuel. A dependable way for a lower mass elliptical to create radio galaxies having FR-II morphology is if this additional source of fuel comes in, say, through a merger.
Host galaxy mass and its history appear to be key to the type of extended morphologies that result on large scales. Since the galaxy mass correlates with the central black hole mass how do black hole mass differences bear on the two radio galaxy types? Black hole mass has a direct consequence to the Eddington accretion rate. With in-house accreted mass also scaling with the galaxy mass differences in the Eddington ratios could arise more from causes such as e.g. merger opportunities, active star-formation or environments.
7.2.1. Understanding the dust characteristics of FR-Is and FR-IIs
The reasoning given above accounts for several of the properties that are found for FR-I and FR-II radio galaxies. For example, with massive and mostly oblate type ellipticals more likely to be hosting FR-I radio morphologies it explains the large host masses reported for FR-Is. With stellar mass loss and the gas from the hot coronae being the predominant fuel source (Buttiglione et al. 2010) one expects that there is a regular supply of gas and dust. This dust shares the angular momentum of the stars in the galaxy and unless there is a merger it will make its way to the centre of the deep potential well unmolested. With the major axis plane being the equilibrium plane in oblate-type ellipticals the dust will tend to settle there where it can form stable closed orbits (van Dokkum & Franx 1995). The rate of supply of gas and dust is steady and is only as high as the rate at which the stars evolve or the coronal gas is ingested. This decides the upper limit to the disk accretion rate. The relatively low rates ensure low power jets which, given the high galaxy mass, will result in FR-I radio structures. Given the stable nature of the process of accumulation of gas and dust it will settle into a regular disk at the centre of the galaxy. Moreover the dust will get to be observed only when it has accumulated in sufficient amount which happens when it reaches smaller scales near the central regions. The internally originating dust is expected to be generated from regions uniformly distributed over the galaxy without having visible signatures such as clumps or disks or lanes and hence it will not be observed. Hence the correlation of dust morphology with location on small scales and location on the major axis. In such a picture, FR-Is are predominantly seen associated with massive ellipticals and without the need for mergers. We point out that this scale on which the dust is seen is still several orders of magnitude larger than the accretion disk in the vicinity of the black hole. In such a steady state within the galaxy where dust has been collecting at the centre for a long period the black hole would have been re-aligned perpendicular to the dust disk.
As for the FR-II sources, their hosts are more likely to be small and to have swathes of dust of irregular and filamentary structure given the likely merger history that we reasoned was needed to preferentially produce an FR-II. The unsettled dust in FR-IIs is more likely to have been recently acquired given that dust settling time is estimated at 108 yr (van Dokkum & Franx 1995).
Natarajan & Pringle (1998) derived black hole re-alignment timescale for a case where an accreting black hole experiences a reverse torque due to the outer disk of the host galaxy. The re-alignment timescale is derived to be few 105 yr for an AGN radiating at a luminosity which is a tenth of the Eddington luminosity. If we use a more realisitic estimate of the luminosity applicable to FR-I radio galaxies (0.001 or lower; Marchesini et al. 2004, Ho 2009) the black hole will realign over a time scale that is several orders of magnitude larger. Correspondingly the more powerful radio galaxies with their higher luminosities may tend to realign earlier (although still on timescales few orders of magnitude larger than 105 yr). The observation that FR-Is have radio axes often aligned with the minor axes of their hosts suggests that they have been left relatively unperturbed for a long enough time to have their black holes realign with the minor axes. However, for the FR-IIs, such undisturbed conditions may have eluded them and the black hole may have been prevented from realigning with the minor axis.
Dust-radio relationship appears to be stronger than dust-major axis relationship in FR-Is. In several FR-Is the radio axis continues to be orthogonal to the dust even when the dust is not located on the host major axis (see Section 3). We have already noted that FR-Is are more than adequately sustained with the fuel which is in sufficient supply in massive galaxies. The steady supply means that dust has had time to settle at the centre and form a stable disk on the major axis. The dust-radio relation gets established in these calm conditions. With dust-radio relation holding even when dust is not on the major axis (as seen in some FR-Is) it implies that any dust that comes in from outside and is at sufficiently close distance to the central region has a strong effect on the black hole angular momentum and can re-orient it.
At this stage we note that there are some remarkable similarities between the picture we are arriving at in understanding the dust properties of the two FR types and the scenario sketched by Baum et al. (1992) to explain the properties of the extended emission line regions. Already articulated clearly as "gradual, steady" feeding in the case of FR-Is and "impulsive" feeding in the case of FR-IIs as inferred from the distinct extended emission line gas characteristics of the two classes, such a physical picture seems to also emerge from the dust properties. The association of large-extent emission line regions with their large rotational velocities (larger than the stellar rotational velocities) and the large kinematic excursions with FR-IIs (whose hosts also exhibited morphological distortions) strongly suggested that the gas may have been acquired in recent mergers. In contrast the much smaller extents as well as lack of significant large scale motions in the emission line gas in galaxies that were almost all at cluster centres and hosting FR-Is suggested a local origin for the gas.
A potential upset for the model we have been developing for the FR-Is and FR-IIs is that the emission line gas rotation axis and the radio axis in FR-IIs align within about 30 (Baum et al. 1992). However as the authors themselves describe, the emission line gas nebulae in the FR-IIs, "show asymmetric rotation curves, large velocity excursions ± 100 km s-1 from simple rotation, offsets of the kinematic and optical centers, broad lines, and mislignments of the rotation axis and the minor axis of gas distribution". It is indeed difficult to understand how in the midst of a merger and with the gas, "not yet settled into an equilibrium orbit or the activity generated in the galaxy nucleus has disrupted the orderly rotation of the gas" there is the tendency for alignment of the gas rotation axis and radio axis particualrly when little alignment with the dust and radio axis is observed for FR-IIs (with dust and gas assumed to be present together).
The details of the process of fuel accumulation are not clear. At what stage in the fuel accumulation at the centre the activity is triggered is also not clear. However as low power FR-Is may need only a small amount of fuel to trigger them, they can also form in the early stages of ingestion of the externally generated fuel before the full accretion rate is reached or when generated in massive hosts (where there is a continuous supply of fuel which would have settled on the major axis) or in the late stages of an FR-II when the fuel is depleted but enough time has passed and the last vestiges of the dust has settled into a regular disk on the major axis. That the latter possibility cannot be the only way to form FR-Is was already ruled out by de Koff et al. (2000) since there is no difference in the dust masses estimated for large FR-IIs and small size FR-IIs, however it is still a possibility in some cases.
With FR-IIs being preferentially hosted by smaller ellipticals aided by mergers or larger ellipticals also aided by mergers (in the former because internally generated fuel is insufficient and then in the latter because the internal resistance requires higher accretion rates) and with lower mass ellipticals being more numerous given the Schechter luminosity function, we have a situation where in samples of FR-IIs the dust will be seen preferentially on larger scales in a filamentary form rather than as disks at the centre.
What about the general lack of dust-radio perpendicularity in FR-IIs and the lack of relation between the radio axis and the host major axis in FR-IIs? With mergers aiding the hosts in producing jets powerful enough to overcome the ISM and produce FR-II structures this lack of perpendicularity relations may occur if the black hole spin is affected by the merger, whether by the triggered gas inflows or black hole-black hole merger (Hopkins et al. 2011). The likley long re-alignment timescales would ensure that the black hole axes (and hence the radio axes) show no relation with the incoming dust. We note that at the AGN scales there should be the expected perpendicularity between the accretion disk and radio jets but observations only pick out the larger-scale dust. The fueling is more dynamic in FR-IIs where the externally originating dust and gas could come in tranches of possibly differing orientations between them as also with respect to the major axis. The large-scale dust picked up in observations will therefore be of filamentary morphology and distributed with little correspondence with the radio axis. The same can be the case with high power FR-Is. These would be more likely generated in massive ellipticals that have had a merger that brings in more fuel than what the internally generated material brings.
At this stage we bring attention to the fact that in the several examples of restarted FR-II radio galaxies a change in axis between the two activity epochs is rarely observed. This can be taken to infer that the black hole spin axis remains steady between the two epochs. How can we understand this in the light of the reasoning for the observed lack of correlation of radio axes with either the dust or the host major axes in FR-IIs? It is clear that while mergers could have perturbed the black hole axes in FR-II hosts whatever is responsible for the interruption and re-triggering of the AGN, it has been gentler. It is possible for the black hole axis to remain steady within a merger event if we can associate the interruption and restarting of activity with the interruption to the fueling as each tranche of fuel is exhausted. Admittedly timescales are important here, for example the timescale over which the dust (gas) segment gets depleted and the re-alignment timescale. In the context of timescales we point out that (based on the lack of relation between the dust and radio axes in FR-IIs) the AGN may be triggered even as the much of the dust is settling and since there is no trailing radio emission seen, the entire AGN beam activity (including the quiescent phase and renewed activity) may be happening on a timescale small compared to the realignment timescale.
The "gradual, steady" feeding of the central engine in FR-Is with potentially limitless supply of fuel suggests that the fueling can remain active for a long time, perhaps much longer time than in FR-IIs. In these conditions it remains to understand how there can be any interruption to the activity in FR-Is.
There will be cases where the jets are oriented along the host minor axes. With less ISM to propagate through the powerful jets will advance more easily. These minor axis sources - as opposed to the major axis sources - will have a tendency to host a larger fraction of large size radio galaxies. In our earlier work (Saripalli & Subrahmanyan 2009) we reported such a tendency in the 3CR sample and we also reported the tendency for giant radio galaxies to have radio axes oriented along the host minor axes.
The richer environments of FR-Is may be environments that are conducive to creating edge-darkened structures because of the higher resistance that they offer the jets as they propagate out. However for FR-IIs, as we have already reasoned, a reliable means of generating them is via the lower mass ellipticals that have undergone mergers so low mass ellipticals that have had no merger history but reside in rich environments should have the least probability to host jets powerful enough to create FR-II structures. The more frequent mergers at higher redshifts means that there is a higher availability of cold gas and this is conducive to the formation of FR-IIs.
In the steady fuelling conditions inferred to be prevalent in most FR-I hosts the central engine may, in principle, never cease activity or at least may continue to remain active for a long time. However the multiple X-ray cavities observed in several galaxy clusters reveal a different picture that implicates multiple episodes of AGN activity implying an unsteadiness of the AGN activity in these otherwise calm conditions. A missing factor is the feedback effect of the radio lobes which exercise control on cluster scales finally feeding back to the fueling itself (McNamara and Nulsen 2007 and references therein). With nearly every cooling flow cluster hosting multiple cavities the causes of AGN episodicity appear to be a feature of and linked to the specific conditions in a regulatory manner that can itself be a periodic phenomenon subject perhaps to disturbances to the large scale steady flows operating in the cluster.
7.2.2. The classification based on optical spectra
The traditional classification of radio sources based on their large-scale morphology is increasingly being examined in light of the central AGN spectroscopic properties. The radio morphologies may be viewed as consequences of the central AGN properties influenced or mediated by the prevailing conditions and state of the AGN (whether active, or waning or dead) and the environment through which the jet propagates.
As for the optical emission line characteristics of FR-Is and FR-IIs if all high ionization emission line radio galaxies are of FR-II morphologies, all FR-Is are low ionization emission line galaxies and some FR-IIs are low ionization emission line galaxies, then from the strong relation between the radio luminosity and the narrow-line luminosity it appears that the HEG FR-IIs are among the most powerful of the FR-II population (however also see Chiaberge et al. 2002, Buttiglione et al. 2010). We will infer then that there is a high threshold jet power above which HEG characteristics manifest. This threshold power is higher than the dividing power at any optical host luminosity (to exclude FR-Is).
What then of the LEG FR-IIs? How do we understand a LEG FR-II? This was also the question raised and discussed by Laing et al. 1994 and Chiaberge et al. 2002. They gave several reasons to support the view that LEG FR-IIs are an isotropic population with at least a part being the parent population of BL-Lacs. With their close resemblance to FR-Is in their optical nuclear properties (Chiaberge et al. 2002) the FR-Is and LEG FR-IIs share a state of the AGN characterized by a weak central ionizing source (radiatively inefficient disk) and lack of any substantial gas or dust torus. It is tempting to view this class of FR-IIs as inclusive of sources that have waned in AGN activity or even FR-IIs that have restarted at only small accretion rates. The mixed characteristics of FR-IIs (HEG and LEG type) may be reflecting the variable central engine conditions. On the other hand the mixed radio morphologies observed in FR-Is has led to the speculation that at least some lobe-type FR-Is may be dying FR-II radio sources (Saripalli et al. 2012).
In Chiaberge et al. (2000) a small number of FR-IIs were reported clearly showing optical core properties indistinguishable from FR-Is. On examining the radio structures of the five FR-IIs one is of FR-I morphology and three have characteristics that classify them closer to being relic sources: lobes with very low axial ratio and at least one of the lobes a relic lobe and weak hotspots. One other is a classic X-shaped source with bright hotspots in its main lobes. These five sources (one is an FR-I) are also reported to be in cluster environments. It is likely that the three FR-IIs (2 are classified LEG-type and 1 HEG) with nuclear properties similar to FR-Is and radio morphologies that are more non-classic FR-II type are sources where the accretion rate has reduced to a level where it is not high enough to sustain an FR-II morphology and the radio morphology is showing the signatures of a dying FR-II.
On examining the morphologies of the FR-II radio galaxies in Buttiglione et al. (2010) although both classes of FR-IIs (the LEG and HEG FR-IIs) include a mix of source characteristics the LEG FR-IIs are associated with a larger fraction of sources with restarted AGN (3C293, 3C236, 3C388) or relaxed double type (3C310, 3C401). In contrast the HEG FR-II mostly include sources with bright hotspot lobes and X-shaped radio sources (3C136.1, 3C223.1, 3C403). The LEG FR-II group does not include any with X-shaped morphology. If X-shaped structures are a result of deflection of backflows (Saripalli & Subrahmanyan 2009) it is not surprising that these sources are predominantly found to be associated with HEG FR-IIs, sources that are characterized by bright hotspot lobes capable of generating strong backflows.
It is not necessary for all LEG FR-IIs to also experience hot gas accretion for their sustenance like FR-Is. Given the different response times of the sub-galactic narrow line regions and the several hundred kiloparsec-scale regions of synchrotron plasma it is possible that a current slowed-down state of the accretion can reveal an edge-brightened source on extended scales with an associated LEG AGN.
Besides having FR-I like AGN the LEG FR-IIs share more properties with the FR-Is: they have circum-nuclear dust and show little evidence of ongoing starformation (Baldi & Capetti 2008). A compilation of these properties is needed for larger samples of LEG FR-IIs to make stronger inferences.
The HEG FR-IIs on the other hand nicely form an FR-II population that are the unbeamed counterparts of the broad-line radio galaxies and quasars (Chiaberge et al. 2002). The strong ionizing continuum and presence of gas and dust in these sources result in the strong narrow (low and high excitation) lines and broad lines where as the absence or only weak presence of a strong continuum and gas and dust in the FR-Is and LEG FR-IIs prevents emission of strong narrow lines and high excitation lines. The absence of a broad line region in FR-Is has also been linked to the higher gas temperatures (Buttiglione et al. 2010) where dense gas clouds may not form.
We may point to the interesting issue of prevalence of FR-I quasars (Heywood et al. 2007 and reference therein). Having been identified as quasars the hosts must have high ionization as well as broad emission line spectra. This implies that the accretion rates are high and the disks are radiatively efficient. It is possible that (as also speculated by Heywood et al. 2007) the resulting high power beams generated may be encountering high enough resistance in the form of dense gas that makes the powerful beams lossy (as in FR-Is) and not be subject to beaming effects nor create edge-brightened structures where they impinge on the ambient medium.