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8.1. Why Are LLAGNs So Dim?

This review highlights a class of galactic nuclei that are extraordinary for being so ordinary. At their most extreme manifestation, LLAGNs emit a billion times less light than the most powerful known quasars. When quasars were first discovered, the challenge then was to explain their tremendous luminosities. Ironically, more than four decades later, the problem has been reversed: the challenge now is to explain how dead quasars can remain so dormant. The luminosity deficit problem was noted by Fabian & Canizares (1988), who drew attention to the fact that elliptical galaxies, despite being suffused with a ready supply of hot gas capable of undergoing spherical accretion, have very dim nuclei. We can no longer speculate that these systems lack supermassive BHs, for we now know that they are there, at least in galaxies with bulges. And as I have shown in this review, the problem is by no means confined to ellipticals either.

Explanations of the luminosity paradox fall in several categories.

8.2. The Disk-Jet Connection

As the mass accretion rate drops and the radiative efficiency declines, an increasing fraction of the accretion power gets channeled into a relativistic jet whose energy release is mainly kinetic rather than radiative. The principal evidence for the growing importance of jets in LLAGNs comes from the broad-band SEDs, which invariably are prominent in the radio, with the degree of radio-loudness rising systematically (albeit with significant scatter) with decreasing Eddington ratio (Section 5.8; Figure 10b). Where available, VLBI imaging on milliarcsecond scales reveals unresolved cores with nonthermal brightness temperatures and a flat or slightly inverted spectrum - classical signposts of a relativistic jet (Blandford & Königl 1979). Detailed modeling of the SEDs of individual sources often shows that the accretion flow itself does not produce enough radio emission to match the data: that extra "something else" is most plausibly attributed to the jet component (Quataert et al. 1999; Ulvestad & Ho 2001b; Fabbiano et al. 2003; Pellegrini et al. 2003b; Anderson, Ulvestad & Ho 2004; Ptak et al. 2004; Wu & Cao 2005; Nemmen et al. 2006; Wu, Yuan & Cao 2007). Moreover, RIAF models predict radio spectral indices of alpha approx +0.4 (Mahadevan 1997), whereas the observed values more typically fall in the range alpha approx -0.2 to +0.2.

The jet may contribute substantially outside of the radio band, especially in the optical and X-rays. Some advocate that the jet, in fact, accounts for most or even all of the emission across the broad-band SED. For example, Yuan et al. (2002) successfully fitted the multiwavelength data of NGC 4258 with effectively a jet-only model. In their picture, a radiative shock at the base of the jet gives rise to synchrotron emission in the near-IR and optical regions, whose self-Compton component then explains the X-rays; the flat-spectrum radio emission comes from further out in the jet. Similar models have been devised for the Galactic Center source Sgr A* (Falcke & Markoff 2000; Yuan, Markoff & Falcke 2002). The gross similarity between the SEDs of some FR I nuclei and BL Lac objects, which are jet-dominated sources but otherwise also low-accretion rate systems (Wang, Staubert & Ho 2002), has also been noted (e.g., Bower et al. 2000; Capetti et al. 2000; Chiaberge et al. 2003; Meisenheimer et al. 2007).

Statistical samples that are larger but more limited in spectral coverage have come from combining radio data with high-resolution optical or X-ray observations. Studies that specifically target radio galaxies, particularly FR I sources and weak-line FR IIs, report that the core radio power scales tightly with the optical and/or X-ray continuum luminosity, a finding often taken to support a common nonthermal, jet origin for the broad-band emission (Worrall & Birkinshaw 1994; Canosa et al. 1999; Chiaberge, Capetti & Celotti 1999, 2000; Capetti et al. 2002; Verdoes Kleijn et al. 2002; Donato, Sambruna & Gliozzi 2004; Balmaverde & Capetti 2006; Balmaverde, Capetti & Grandi 2006; Evans et al. 2006; for a counterargument, see Rinn, Sambruna & Gliozzi 2005 and Gliozzi et al. 2008). A similar radio-optical correlation, after correcting for Doppler boosting, is also seen among BL Lac objects (Giroletti et al. 2006), strengthening the case that FR I radio galaxies and BL Lac objects are intrinsically the same but misoriented siblings. Many FR II systems, on the other hand, especially those with broad lines, deviate systematically from the baseline FR I correlations, by exhibiting stronger optical and X-ray emission for a given level of radio emission (Chiaberge, Capetti & Celotti 2000, 2002; Varano et al. 2004). In concordance with the frequent detection of X-ray absorption and Fe Kalpha emission (Evans et al. 2006), this suggests that FR IIs have higher accretion rates and a much more dominant accretion flow component, relative to the jet, than FR Is.

Any attempt to explain the broad-band spectrum of LLAGNs with either just a RIAF or just a jet runs the risk of oversimplification. Clearly both are required. The trick is to figure out a reliable way to divvy up the two contributions to the SED. We cannot deny that there is a jet, because we see it directly in the radio at a strength far greater than can be attributed to the RIAF. The jet emission must contribute at some level outside of the radio band. At the same time, the jet cannot exist in isolation; it is anchored to and fed by some kind of accretion flow, of which a promising configuration is a vertically thick RIAF (Livio, Ogilvie & Pringle 1999; Meier 1999). An outstanding problem is that the interpretation of the data is not unique. Because many of the model parameters are poorly constrained and the broad-band data remain largely fragmentary and incomplete, the SEDs often can be fit with pure jet models, pure accretion flow models, or some combination of the two. The recent detection of high levels of polarization in the optical nuclei of FR Is (Capetti et al. 2007) strongly points toward a synchrotron origin in the jet for the optical continuum, but even this observation cannot be considered definitive, because a RIAF can also produce nonthermal flares (e.g., in Sgr A*; Quataert 2003).

The so-called BH fundamental plane - a nonlinear correlation among radio luminosity, X-ray luminosity, and BH mass - offers a promising framework to unify accreting BHs over a wide range in mass and accretion rates. Merloni, Heinz & Di Matteo (2003) first demonstrated that the correlation between Lrad and LX tightens considerably after including MBH as a third variable. Combining observational material for several Galactic stellar BHs and a large sample of nearby LLAGNs, they find that

Equation 3

This empirical correlation agrees well with the theoretical relations between radio flux, BH mass, and accretion rate derived from the scale-invariant disk-jet model of Heinz & Sunyaev (2003). The BH fundamental plane, however, appears to be a very blunt tool. In an independent analysis, Falcke, Körding & Markoff (2004) obtained a similar empirical relation, but unlike Merloni, Heinz & Di Matteo these authors explained the scaling coefficients entirely in terms of a jet-dominated model. Moreover, as emphasized by Körding, Falcke & Corbel (2006), objects with very different emission processes, including luminous quasars and BL Lac objects, sit on the same correlation, albeit with larger scatter.

I illustrate this point in Figure 12a, which includes all Palomar LLAGNs with suitable data, along with the collection of high-luminosity sources from L.C. Ho (in preparation). With the exception of a handful of radio-loud quasars, the vast majority of the objects fall on a well-defined swath spanning ~ 10 orders of magnitude in luminosity. There are no obvious differences among the various subclasses of LLAGNs, except that the type 1 sources appear more tightly correlated. Plotting the residuals of the fundamental plane relation versus the Eddington ratio reveals two interesting points (Figure 12b). First, although the intrinsic scatter of the relation is quite large, it markedly increases for objects with high Eddington ratios, at Lbol / LEdd approx 10-1 ± 1, as already noted by Maccarone, Gallo & Fender (2003) and Merloni, Heinz & Di Matteo (2003). The scatter flares up because the radio-loud quasars lie offset above the relation and the radio-quiet quasars on average lie offset below the relation. At the opposite extreme, sources with Lbol / LEdd ltapprox 10-6.5 may also show a systematic downturn, in possible agreement with the proposal by Yuan & Cui (2005) that below a critical threshold, LX approx 10-5.5 LEdd, both the radio and the X-rays should be dominated by emission from the jet. M31 (Garcia et al. 2005), NGC 821 (Pellegrini et al. 2007), and NGC 4621 and NGC 4697 (Wrobel, Terashima & Ho 2008) seem to conform to Yuan & Cui's prediction, but M32 and especially Sgr A* clearly do not. Additional deep radio and X-ray observations of ultra-low-luminosity nuclei would be very valuable to clarify the situation in this regime.

Figure 12a Figure 12b

Figure 12. (a) Fundamental plane correlation among core radio luminosity, X-ray luminosity, and BH mass. (b) Deviations from the fundamental plane as a function of Eddington ratio.

If, as surmised, the relative proportions between jet and disk output depend on accretion rate, with the bulk of the radiated power, even in the X-rays, originating from the former in the lowest accretion rate systems, two important consequences ensue. With respect to the microphysics of RIAFs, it implies that the radiative efficiencies are even lower than previously inferred under the assumption that the X-rays emanate solely from the accretion flow. On a more global, environmental scale, shifting the emphasis from the disk to the jet changes the balance between kinetic versus radiative output, with important implications for prescriptions of AGN feedback in models of galaxy formation because BHs spend most of their lives in a low-state. From empirical and theoretical considerations (Heinz, Merloni & Schwab 2007; Körding, Jester & Fender 2008), the jet carries a substantial fraction of the accreted rest mass energy: Pjet approx 0.2 eta dot{M} c2 approx 7.2 × 1036 (Lrad / 1030 ergs s-1)12/17 ergs s-1. In fact, the total kinetic energy injected by LLAGN jets is comparable to or perhaps even greater than the contribution from supernovae. At low redshifts, radiative feedback from quasars, which is commonly assumed to operate with an efficiency of ~ 5%, may be less important then jet-driven feedback from LLAGNs (Körding, Jester & Fender 2008).

Figure 13

Figure 13. A cartoon of the central engine of LLAGNs, consisting of three components: an inner, radiatively inefficient accretion flow (RIAF), an outer, truncated thin disk, and a jet or outflow. (Courtesy of S. Ho.)

8.3. The Central Engine of LLAGNs

The preceding sections argue that the weak nuclear activity seen in the majority of nearby galaxies traces low-level BH accretion akin to the more familiar form observed in powerful AGNs. However, multiple lines of evidence indicate that LLAGNs are not simply scaled-down versions of their more luminous cousins. They are qualitatively different. From the somewhat fragmentary clues presented in this review, we can piece together a schematic view of the structure of the central engine in LLAGNs (Ho 2002b, 2003, 2005). As sketched in Figure 13, it has three components.

  1. Radiatively inefficient accretion flow     In the present-day Universe, and especially in the centers of big bulges, the amount of material available for accretion is small, resulting in mass accretion rates that fall far below 10-2 dot{M}Edd. In such a regime, the low-density, tenuous material is optically thin and cannot cool efficiently. Rather than settling into a classical optically thick, geometrically thin, radiatively efficient disk - the normal configuration for luminous AGNs - the accretion flow puffs up into a hot, quasi-spherical, radiatively inefficient distribution, whose dynamics may be dominated by advection, convection, or outflows. This is an area of active ongoing theoretical research. In the interest of brevity, I will gloss over the technical details and simply follow Quataert (2003) by calling these RIAFs. The existence of RIAFs, or conversely the absence of a standard thin disk extending all the way to small radii (a few Schwarzschild radii Rs), is suggested by the feeble luminosities of LLAGNs, by their low Eddington ratios, and especially by their low inferred radiative efficiencies. The great disparity between the available fuel supply and the actually observed accretion luminosity demands that the radiative efficiency of the accretion flow be much less than eta = 0.1 (Section 8.1). Additional support for RIAFs comes from considerations of the SED, particularly the absence of the big blue bump, a classical signature of the thin disk, and the preponderance of intrinsically hard X-ray spectra.

  2. Truncated thin disk     Beyond a transition radius Rtr approx 100-1000 Rs, the RIAF switches to a truncated optically thick, geometrically thin disk. The observational evidence for this component comes in three forms. First, the SEDs of some well-studied LLAGNs require a truncated thin disk to explain the big red bump - the prominent mid-IR peak and the steep fall-off of the spectrum in the optical-UV region (Section 5.8). The thermal disk emission is cool (red) not only because of a low accretion rate (Lawrence 2005) but also because the inner radius of the disk does not extend all the way in to a few Rs as in luminous AGNs. Second, the very same truncated disk structure employed to model the SED simultaneously accounts for the relativistically broadened, double-peaked emission-line profiles observed in some sources (Section 5.5). Indeed, in the case of NGC 1097 (Nemmen et al. 2006), the transition radius derived from modeling the SED (Rtr = 225 Rs) agrees remarkably well with the inner radius of the disk obtained from fitting the double-peaked broad Halpha profile. Ho et al. (2000) suggested that double-peaked broad emission lines are commonplace in LLAGNs. By implication, the truncated disk configuration inferred from this special class of line profiles must be commonplace too. Lastly, the striking absence of broad Fe Kalpha emission in the X-ray spectra of LLAGNs (Section 5.3), a feature commonly attributed to X-ray fluorescence off of a cold accretion disk extending inward to a few Rs in bright Seyfert 1 nuclei (e.g., Nandra et al. 1997b, 2007), strongly suggests that in low-luminosity sources such a structure is either absent or truncated interior to some radius, such that it subtends a significantly smaller solid angle. Similar lines of reasoning have been advanced for broad-line radio galaxies that show weak Fe Kalpha emission and weak Compton reflection (Wozniak et al. 1998; Eracleous, Sambruna & Mushotzky 2000; Lewis et al. 2005), although these characteristics can be mimicked by an ionized but otherwise untruncated disk (Ballantyne, Ross & Fabian 2002).

  3. Jet/outflow     The empirical connection between LLAGNs and jets has been established unequivocally from radio observations. Not only are the SEDs of LLAGNs generically radio-loud, but the strength of the radio emission generally cannot be fit without recourse to a jet component, which in many cases can be seen directly from VLBI-scale radio images. From a theoretical point of view, jets may share a close physical connection with RIAFs. As emphasized by Narayan & Yi (1995) and Blandford & Begelman (1999), RIAFs have a strong tendency to drive bipolar outflows due to the high thermal energy content of the hot gas. Whether such outflows can develop into highly collimated, relativistic ejections remains to be seen, but they at least provide a promising starting point. RIAFs may be additionally conducive to jet formation because its vertically thick structure enhances the large-scale poloidal component of the magnetic field, which plays a critical role in launching jets (Livio, Ogilvie & Pringle 1999; Meier 1999; Ballantyne & Fabian 2005; Ballantyne 2007). It is interesting to recall that the original motivation for ion-supported tori (Rees et al. 1982), an early incarnation of RIAFs, was to explain the low luminosity of radio galaxies. Rees et al. postulated that the puffed-up structure of the ion torus may help facilitate the collimation of the jet.

The above-described three-component structure has been applied to model the broad-band spectrum of a number of LLAGNs, including NGC 4258 (Lasota et al. 1996; Gammie, Narayan & Blandford 1999), M81 and NGC 4579 (Quataert et al. 1999), NGC 3998 (Ptak et al. 2004), and NGC 1097 (Nemmen et al. 2006). For the handful of LLAGNs with available estimates of the transition radii, Rtr seems to scale roughly inversely with Lbol / LEdd (Yuan & Narayan 2004). This trend may be in agreement with models for disk evaporation (Liu & Meyer-Hofmeister 2001). As the latter authors note, however, disks attain their maximum evaporation efficiency at Rtr approx 300 Rs, making sources such as M81 and NGC 4579, both with Rtr approx 100 Rs (Quataert et al. 1999), difficult to explain. At a qualitative level, at least, the general idea that the thin disk recedes to larger and larger radii as the accretion rate drops is probably correct. In an analysis of 33 PG quasars with Fe Kalpha emission detected in XMM-Newton spectra, Inoue, Terashima & Ho (2007) find that the iron line profile varies systematically with Eddington ratio. Specifically, the Fe Kalpha profile becomes narrower with decreasing Lbol / LEdd, a result that can be interpreted as a systematic increase in the inner radius of the accretion disk at low accretion rates.

The basic schematic proposed in Figure 13 is hardly new. To my knowledge, a hybrid model consisting of a RIAF - then called an ion-supported torus - plus a truncated thin disk was most clearly articulated in a prescient paper by Chen & Halpern (1989) in their description of Arp 102B, later elaborated by Eracleous & Halpern (1994) in the general context of double-peaked broad-line radio galaxies. Chen & Halpern identified the 25 µm peak in the SED with the turnover frequency of the synchrotron peak from the RIAF, whose elevated structure illuminates an outer thin disk that emits the double-peaked broad optical lines. The overall weakness of the UV continuum in Arp 102B (Halpern et al. 1996) further corroborates a truncated thin disk structure and potentially provides an explanation for the low-ionization state of the emission-line spectrum. As for the jet component, it was assumed to be present, at least implicitly, insofar as the double-peaked broad-line AGNs were thought to reside preferentially in radio-loud AGNs, and the very concept of ion-supported tori was invented with reference to radio galaxies (Rees et al. 1982).

Recent developments add important refinements and modifications to Chen & Halpern's original picture. First, the mid-IR peak in most objects is dominated by thermal emission from the truncated thin disk rather than by the synchrotron peak of the RIAF. Second, the jet component, which was once regarded as somewhat incidental, has emerged as a natural and perhaps inevitable outgrowth of the inner accretion flow itself. Third, although the original model was invented to explain a small minority of the AGN population (double-peaked radio-loud sources), now we have good reason to believe that similar physical conditions prevail in LINERs as a class (Ho et al. 2000), and, by extension, in the majority of nearby accreting BHs.

The physical picture outlined above for LLAGNs shares strong similarities with that developed for X-ray binaries in their hard state (see Maccarone, Fender & Ho 2005 and references therein), suggesting that the basic architecture of the central engine around accreting BHs - across 10 orders of magnitude in mass - is essentially scale-invariant (Meier 2001; Maccarone, Gallo & Fender 2003; Merloni, Heinz & Di Matteo 2003; Falcke, Körding & Markoff 2004; Ho 2005; Körding, Jester & Fender 2006).

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