I think we now have a fairly good emerging picture of what the BLR is like and what role it plays in the life of an AGN. Interstellar material approaching the nucleus settles into a flattened distribution, the thick torus. Material loses angular momentum because of MRI turbulence and gradually spirals inwards. When the material of the torus gets within the dust sublimation radius, the dust evaporates and we have the BLR. The turbulent BLR continues to spiral inwards towards the black hole where it is eventually accreted. The degree of ionization increases as the gas gets closer in. The optically-thick material, which will tend to be concentrated towards the mid-plane, produces continuum emission; the more optically-thin material produces the BLR. Not all of the BLR is accreted. Some of it is driven off the surface of the BLR/torus in a high-velocity, low-density wind, as is found in all the MHD simulations and as is observed.
Although I believe we are getting a clear overall picture of the BLR, there is still plenty to work on both observationally and theoretically! For a start, the picture discussed above needs to be thoroughly tested to verify that it works for all objects and not just a few well-observed objects such as NGC 5548. More work needs to be done to see whether a disk-wind model (the leading rival to the model presented here) could also explain everything. We know that there is outflowing gas as well as gas accreting onto the black hole. The question is: how much of this is also contributing to the broad-line profiles? (especially to the high-ionization lines). Ilic et al. (2008), for example, have shown that outflows can match some observed BLR profiles, so determining the relative contribution of an outflow to broad-line profiles from line-profile fitting alone is difficult. I think that reverberation mapping and spectropolarimetry (see, for example, Axon et al. 2008) are going to provide the best answers. Kollatschny (2003) found marginal evidence for some outflow in Mrk 110, and Denney et al. (2009b) have found a clearer signature of an apparent outflow component in velocity-resolved reverberation mapping of NGC 3227. Since the Denney et al. (2009b) results are from a single short observing campaign, I do not think that NGC 3227 presents a major problem yet for the general picture presented here. 6 If follow-up observing campaigns confirm the signature of outflow in NGC 3227 then this would be a significant challenge to the model favored here. Nevertheless, the Denney et al. (2009b) result does caution us that AGNs might not all be identical in the relative dominance of inflow and outflow.
Even if we are right about the basic structure of the BLR and torus of AGNs, there are still a lot of interesting and potentially important details in need of further investigation. Although in this review I have been emphasizing the similarities among AGNs and what they imply, there are some significant differences in the BLRs too (see, for example, Marziani et al. 1996). If our basic framework of how an AGN works is correct, then the differences need to be explicable within the framework too. Space here only permits a brief mention of some of these problems, but fortunately many of them are reviewed and discussed elsewhere in these proceedings (see, for example, the reviews by Mike Eracleous and Jack Sulentic).
It has been known for over three decades now that object-to-object differences are correlated with each other, and one of main drivers of the correlated differences is the Eddington ratio (see Sulentic et al. 2000 and these proceedings). Since we now have reliable AGN black holes masses, we also have reliable Eddington ratios, so there is a lot that can be done in investigating the dependence of BLR properties on accretion rate. I think there is a lot that needs explaining here.
The biggest object-to-object difference in optical spectra is optical Fe II emission (Osterbrock 1977). Understanding how the very strong optical Fe II emission seen in AGNs is produced has been a long-standing problem (see Baldwin et al. 2004, Joly et al. 2008, Hu et al. 2008, Kuehn et al. 2008, Verner et al. 2009, Dong et al. 2009 for recent discussions). In the BLR model discussed here, optical Fe II emission arises in the outer part of the BLR just inside the torus (and quite likely overlapping with it), but this does not readily explain why optical Fe II is so much stronger in some objects than others.
Another mystery of the correlated object-to-object differences is the strength of the narrow-line region (NLR) emission. This is the other strong object-to-object difference and it is mysteriously strongly anti-correlated with Fe II emission (Osterbrock 1977, Steiner 1981, Boroson & Oke 1984, Gaskell 1987, Boroson & Green 1992). A complete model of AGNs needs to explain why the NLR and BLR know about each other.
Although I have argued that the basic properties of AGNs with broad disk-like Balmer line profiles are consistent with the picture presented here, these objects, and especially the variability of their profiles, present some special challenges, as is discussed in Mike Eracleous's review. There is also a lot more to be learned with orientation effects.
In summary, I think that although our overall picture of the BLR and the role it plays in the AGN phenomenon is becoming clearer, many mysteries remain, there is still a lot to learn, and there are probably surprises in store.
I am grateful to Luka Popovic, Milan Dimitrijevic, and the other members of the scientific organizing committee of the 7th Serbian Conference on Spectral Line Shapes for inviting me to speak on this topic. I would like to express my appreciation to Dragana Ilic and all the members of the local organizing committee for providing a very pleasant and stimulating experience throughout the conference, both culturally and scientifically. I also have to thank my collaborators and former graduate students for all their contributions and discussions over the years, and the anonymous referee for useful comments. This research has been supported in part by US National Science Foundation grant AST 08-03883.
6 The uncertainties in the red-wind/blue-wing lags can be larger than thought. NGC 5548 provides a good illustration of this. The red-wing/blue-wing lag varies from year to year by more than the formal errors (Welsh et al. 2007), but the NGC 5548 BLR is probably not changing direction at the end of every observing season! A strong reason for believing that the NGC 3227 kinematics are not unusual is that, as Denney et al. (2009b) point out, the mass estimate lies on the M - * relationship. Back