For those of you still looking for costume ideas this in from May 6th issue of Nature:
Astronomy: Dust-filled doughnuts in space
Julian Krolik, of Johns Hopkins University, is currently at the Institute of Astronomy, Madingley Road, Cambridge CB3 0HA, UK.
The first images of an extragalactic object to have been captured using infrared interferometry reveal the doughnut-shaped cloud of dust that obscures the heart of a nearby active galaxy.
Active galactic nuclei are among the most exotic objects in the Universe. They radiate as much light as an entire galaxy from a region the size of the Solar System and, unlike stars, spread that light over the entire electromagnetic spectrum, from radio waves to -rays. Thanks to observational advances in the past decade, we now have good evidence that the 'engine' powering each of these objects is a black hole, weighing anywhere from millions to billions of times as much as the Sun. Fortunately for the galaxies that house them (but unfortunately for distant observers like us), active galactic nuclei are often shrouded in opaque gas and dust that block a view of them from most directions. The light from them, intercepted by these dust clouds, is degraded to infrared light that tells us little about the fascinating activity deep inside.
To the further frustration of astronomers, the dust clouds are so close to the active nucleus that their structure cannot easily be made out: rough estimates put their typical size on the sky at 0.01 arcseconds (a few hundred-millionths of a degree), and even the Hubble Space Telescope can't resolve anything smaller than about 0.1 arcseconds. But this barrier is at last being breached. On page 47 of this issue, Jaffe et al.1 present infrared images of an active nucleus that have a resolution of 0.01 arcseconds, achieved through interferometry — the careful combination of images from different telescopes placed some distance apart. The obscuring dust clouds, a scant several light years from a supermassive black hole, have now come into view.
Light can be thought of either as a collection of individual energy packets called photons or as an electromagnetic wave. Interferometry exploits the wave aspect of light in that it hinges on comparing the phases of electromagnetic waves from a single source when they strike different telescopes. Measuring and retaining this phase information is a formidable technical challenge, so interferometry is not a discipline for the faint-hearted. Although the technique has been in regular use for decades at radio wavelengths, the difficulties increase as the wavelength of the light shrinks. The advance into the infrared region of the spectrum has been accomplished only recently2 and, as is usually the case, the first observations were only of nearby, comparatively bright stars3-5.
Jaffe et al.1 have used the interferometer formed by the European Southern Observatory's Very Large Telescope and several smaller telescopes on the same Chilean mountain6. With this apparatus, high-resolution infrared images of faint, distant objects have been obtained for the first time. Indeed, the data shown in Jaffe and colleagues' paper1 are the first to be derived from infrared interferometry on anything outside the Milky Way. And what Jaffe et al. see is as interesting as the gear they built to see it with. Their images show a geometrically thick ring of dust that is extremely close to the central black hole in the nearest powerful active galaxy, NGC 1068 (Fig. 1). The warmest dust is no more than two or three light years from the very centre of the beast.
Figure 1 The active galaxy NGC 1068. Full legend
High resolution image and legend (86k)
This result is a dramatic confirmation of inferences made many years ago, but which were not entirely accepted because they were so hard to understand. Since the late 1980s, many have believed that the dust clouds surrounding active galactic nuclei are not higgledy-piggledy, but are instead arranged more or less like a thick doughnut. As a result, observers whose sight-lines fall near to the equatorial plane of any particular active galactic nucleus (most of the Universe, in fact) are relegated to seats with an obscured view of its centre; the favoured minority close to the axis of the doughnut get to see the full show unobstructed.
But this picture was difficult to accept because, in a gas cool enough to permit dust to survive, the thermal motions would be too slow to keep the doughnut puffed up and thick; the gravity of the central black hole would ensure that the doughnut collapsed to look more like a CD, if the only resistance to it were random thermal motions7. Many speculative suggestions have been made to account for the geometrical thickness of the dust doughnut7-10 but none has gained general credence. Partly to avoid this dynamical conundrum, alternative geometries for the dust (for example, a thin but warped disk11, 12) have been suggested, but these are problematic for other reasons7. Thanks to these new observations1, we now know that, at least in the nearest and brightest example of NGC 1068, a thick doughnut is in fact the right shape.
The fact that most active galactic nuclei cannot be seen clearly has many implications. Most obviously, it means that it is hard to find them, so censuses of massive black holes probably underestimate their numbers by as much as a factor of five. If most active nuclei are visible only in difficult-to-observe infrared light, their total power may actually be so large as to rival the total output of all the stars in the Universe13.
After languishing for a decade largely through lack of data, this field should now see a revival, as it is refreshed by detailed infrared imaging. The dynamical problems guessed at years ago can be brought into clearer focus; with any luck, direct images of these structures may yield clues to their solution.
I often heard the sorrel nag (who always loved me) crying out, ..."Take care of thyself, gentle Yahoo."