The Sudden Death of the Nearest Quasar
When I told Bill Keel the results of the analysis of the X-ray observations by the Suzaku and XMM-Newton space observatories, he summed up the result with a quote from a famous doctor:
“It’s dead, Jim.”
The black hole in IC 2497, that is.
To recap what we know: the Voorwerp is a bit of a giant hydrogen cloud next to the galaxy IC 2497. The supermassive black hole at the heart of IC 2497 has been munching on vast quantities of gas and dust and, since black holes are messy eaters, turned the center of IC 2497 into a super-bright quasar. The Voorwerp is a reflection of the light emitted by this quasar. The only hitch is that we don’t see the quasar. While the team at ASTRON has spotted a weak radio source in the heart, that radio source alone is far too little to power the Voorwerp. It’s like trying to light up a whole sports pitch with a single light bulb – what you really need is a floodlight (quasar).
Now it is possible to hide such a floodlight. You just put a whole bunch of gas and dust in front of it. If there’s enough material, no light even from a powerful floodlight will get through. Imagine pointing it at a solid wall – even the brightest floodlight in the world will be completely blocked by the wall. In the realm of quasars, such a barrier is usually made up of the torus of material (gas and dust) spiralling in towards the black hole and settling into an accretion disk. So you can have quasars that are feeding at enormous rates and being correspondingly enormously bright, but our line of sight is blocked.
So there are two possibilities of what could be going on with IC 2497 and the Voorwerp:
1) The quasar is “on” but hidden by lots of gas and dust, or
2) The quasar switched off recently, but because the Voorwerp is 70,000 light yeas away, the Voorwerp is still seeing the quasar – after all, even light takes a while to travel 70,000 light years. This would make the Voorwerp a “light echo.”
So how do we distinguish between the two possibilities? The best way is to look at a part of the electromagnetic spectrum that generally has no trouble penetrating even thick walls: X-rays!
If the quasar in IC 297 is feeding, then we should see the X-ray light it is emitting even through the thickest barriers. That’s why we asked for observations with Suzaku and XMM-Newton. It took many months to gather and analyze the data before we were ready to write up a paper and submit it to the Astrophysical Journal as a Letter. The referee report was challenging but positive, and the Letter got accepted rapidly. The pre-print is now out on arxiv: http://arxiv.org/abs/1011.0427
So what did we find? We found something, but it isn’t a quasar. With the X-ray data, we can definitely rule out the presence of a quasar in IC 2497 powerful enough to light up the Voorwerp. We do however see some very weak X-ray emission that most likely comes from the black hole feeding at a very low level. Compared to what you need to light up the Voorwerp (the floodlight), the black hole currently puts out 1/10,000 of the required luminosity. That’s like trying to illuminate a sports stadium at night with a candle.
We can therefore conclude that the black hole in IC 2497 dropped in luminosity by a factor of ~10,000 at some point in the last 70,000 years. This implies a number of very exciting things:
1) A mere 70,000 years ago (a blink of an eye, cosmologically speaking), IC 2497 was a powerful quasar. Since it’s at a redshift of only z=0.05, it’s the nearest such quasar to us. Since IC 2497 is so close to us, and the quasar has switched off, it means that images of IC 2496 are the best images of a quasar host galaxy we will ever get.
2) Quasars can just switch off very quickly! We didn’t know they could do this before, and the fact that they can is very exciting.
3) Maybe the quasar didn’t just switch off, but rather switched state, and is now putting out all its energy not as light (i.e. a quasar), but as kinetic energy. That’s an extremely intriguing possibility and something I want to investigate.
We put out a press release via Yale. You can find it here.