Quasars, Donuts, and the Unified Faith

Six a.m. in a Bavarian Monastery. I am woken again by the bloody bells. Worry not, dear readers, I have not become a monk. The ancient Kloster Seeon has been turned into a hotel and conference centre, and I am here with a hundred astronomers arguing about “Obscured AGN across Cosmic Time”. But in some ways this is a gathering of believers – the Church of the Unified Faith – and its an interesting example of science as a sociological process.

Active Galactic Nuclei (AGN) and their big cousins the Quasars come in two flavours, Artists impression of the torus - courtesy of ESO
imaginatively called Type-1 and Type-2. Since the late 1970s various folk including yours truly have suggested that Type-2s are actually the same as Type-1 but seen edge on through obscuring muck. This is known as the Unified Scheme. Grand Title for a kinda simple idea. In 1985 Antonucci and Miller found particularly convincing evidence that the idea was correct, and in 1988 Krolik and Begelman put forward a theoretical explanation involving a dusty torus surrounding the quasar. Since then every observer compares their data to “the torus model”, and every artists impression of a quasar has this nice neat obscuring torus as well as accretion disc, jets, and so on.

The trouble is, there are at least as many Type-2s as Type-1s, so the obscuring torus has to cover a large fraction of sky as seen from inside the quasar. So the torus is not a swirling disc but a great fat rotating donut. Making a fat rotating thing is hard. Cold rotating things are always thin, like Saturns rings. Stars are fat rotating things, but they’re hot. Various forced attempts have been made to fix this problem. The latest one was put forward this week by Julian Krolik, who argued that the torus is kept puffed up by the radiation pressure from the central quasar light. Nice idea, but not yet clear that its stable, or that it fits the facts well enough.

Another option came from me, arguing that there isn’t really a rotating donut at all, but a severely warped disc – fuel coming from large distances is rotating in a different plane to the quasar disc, and as it tilts and precesses on the way down, it ends up blocking much of the sky. People were interested in the idea, but many were perturbed. In torus land, Ski Antonucci is the Founder of the Faith, and Julian Krolik is the Bishop of Rome; I am I guess one of the Cardinals, seen as responsible for the variant known as the “receding torus model”. People love this, as its basically one line of algebra and so dead easy to compare your data to. People kept asking me “so you don’t believe in the torus anymore ? But you’ve been working on this for years.”

This is is unsettling, not just because of the idea of belief in science, but because you are pressured into buying complete packages. Talk after talk at this meeting claimed that their data “supported the torus model”, to which my reaction was “err.. depends what you mean”. Usually the data supported the general idea of unification of Type-1 and Type-2; sometimes it supported the idea that obscuration is axisymmetric, sometimes that it is geometrically thick ..but showed that there is a rotating molecular donut ? Gimme a break. But once a successful model appears, it swallows all the things that made it up, and is very hard to break apart and re-assemble with some new parts.

This is understandable, not because scientists are conservative, but because science is hard work, and because they have careers to make. This is especially true for young scientists. It is very hard work understanding how your instrument works, collecting the data, reducing it carefully, mastering the background theory, and reading the huge literature. You are desperate to make an impact, to tell a story with your work, and need some organising mental framework. If the world is all scepticism and ambiguity you are sunk. So you cling onto the popular model until the facts make it crack, and then you jump ship to the new fashionable model.

So this of course is just what Thomas Kuhn said in his famous book, “The Structure of Scientific Revolutions”. At heart, the scientific process is sceptical and logical – Conjecture and Refutation, as another famous book by Karl Popper has it. But the realities of human debate, the pressures of the sociological process of science, and the need to think within an organising framework, all lead to resistance to change. This is not conservatism – scientists love to be radical in principle – but inertia.

5 Responses to Quasars, Donuts, and the Unified Faith

  1. David says:


    If we are going to have theological comparisons, I would think you would be more of a “John the Baptist” figure since your 1987 PASP
    paper anticipated so much. At any rate it motivated my thesis research, thanks. Didn’t Jim Pringle a while back also propose a warped disk instead of a torus for AGNs ?

  2. andyxl says:

    John the Baptist ? We have a new PhD student called Salome, so now I am nervous. Not sure who is playing the part of Herod though.

    Anyhoo, thanks for your kind remarks. This is a somewhat old posting. Since then myself and Martin Elvis have written a proper paper on warped disc obscurers which I hope properly credits Jim, Cathie Clarke etc. Jim is even more famous for wrinkly inner discs of course.

  3. David says:

    Thanks for the link, somehow I missed it.
    Who plays the part of Herod ? The funding bodies,
    one often hears “off with their stipends”.

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