He said that they said that he said…
One of the things that really annoys me about the way in which science crops up in the media is the complete lack of disagreement that’s covered. The impression is given that somewhere there is a big book of Scientific Truth from which we all read, thus guaranteeing harmony and steady progress. The truth is that behind every scientific paper lies many months of debate and argument, either clustered around a whiteboard or – in the case of the Galaxy Zoo team – often by email and phone calls. Even when papers are published disagreements remain, and it might be many years before disputes are finally resolved. That’s ok though, because uncertainty is where science can thrive – where we all agree with each other and theory matches observations there is little more to be done. As Isaac Asimov said, the most exciting words in science aren’t ‘Eureka! I’ve got it!’ but rather ‘Now, that’s funny’. When things don’t quite make sense, when there are people to convince then we have work to do.
I’ve been prompted to write this on the acceptance of our paper announcing the discovery of Hanny’s Voorwerp.
Due to the time we took to convince the referee of our case, in the meantime another, friendly team published their paper about their radio observations of this remarkable object. Anyone who has read through both papers will have noticed that there’s something of a disagreement about how to interpret the Voorwerp. While the answer is more observation (and both groups have data that we’re working on), I thought I’d summarize the position.
First, though, a recap. The mystery of the Voorwerp is that the gas is hot – 15,000 K or so (that’s 15000 degrees Centigrade for all practical purposes) – but the Voorwerp itself contains no stars or other sources of high-energy radiation to heat the gas to these temperatures. The obvious solution is to look elsewhere for the culprit, and suspicion naturall fell on IC 2497, the Voorwerp’s neighbouring galaxy. Galaxies the size of IC 2497 (and indeed, the Milky Way) all seem to have black holes at their centres, and when these black holes are feeding they can have profound effects on the surrounding galaxy. At this stage the galaxy is said to have an AGN – an Active Galactic Nucleus.
If IC 2497 showed such activity, then one consequence might be a jet heading out of the disk of the galaxy, hitting the Voorwerp and exciting the gas it contained. This is the explanation put forward by Josza et al. in their paper (in fact, the presence of some familiar names on that author list suggests that several of us are confused within our heads too!). The image above is their figure 1; the red contours show the radio emission they detected, and you can see that instead of being centred only on IC 2497 it extends out in the direction of the Voorwerp. In fact, they conclude that their “observations support the hypothesis that IC 2497 contains an active galactic nucleus with a radio jet emerging in the direction of Hanny’s Voorwerp“. If true, this jet could well be responsible for the excitation we observe in the Voorwerp. (In fact, the presence of such a jet is fascinating – talking to Bill, we can only come up with one other example of such a jet associated with a spiral galaxy).
Case closed? Well, not quite. The jet tells us that the AGN was almost certainly active very recently, but not quite that it’s active now. We’d like to be able to see the influence of the black hole on its surroundings much more directly to be sure. Our first thought was to try detecting x-ray emission we’d expect to come from the material about to fall into the black hole. We looked with the Swift satellite, and found precisely nothing. The only way to reconcile this observation with an active AGN, powerful enough to account for the Voorwerp, is to have more radiation than is typical absorbed.
Such absorption would be expected to show itself in the infrared region of the spectrum. Even if we assume that all the infrared radiation comes from the activity at the nucleus, and not from the galaxy itself, we have an infrared luminosity which is about ten times smaller than the AGN we require. So between the x-ray and the IR, we can pretty well constrain the luminosity of any AGN that is there, and find it just cannot be responsible.
So why the argument? There’s a loophole; if – and I think only if – the material at the centre of
the VoorwerpIC 2497 has some strange arrangement so that it’s thick enough to block almost all radiation in our line of site but allow enough to escape to affect the Voorwerp, then everything fits neatly together. In fact, the gas that we do see near the nucleus is exposed to a much weaker radiation field than the Voorwerp itself, making it even harder to imagine how this geometry might work. This would in itself be fascinating – no object I’ve read about has such an arrangement – and worthy of study. We do know that IC 2497 is quite dusty (watch this space for a new image) which certainly gives us plenty of raw material to sculpt with.
How do we settle the (friendly) debate? The obvious way is more observations – in particular, checking for more energetic x-rays which should punch through even very thick absorption.