Space & Innovation

Supermassive Black Holes are Not Doughnuts!

Conventional thinking suggests that the most massive black holes possess a ringed doughnut-shaped torus of gas and dust trapped in orbit around them. But if we know one thing about black holes, they're anything but conventional.

Conventional thinking suggests that the most massive black holes possess a ringed doughnut-shaped torus of gas and dust trapped in orbit around them. But if we know one thing about black holes, they're anything but conventional.

Now, astronomers have analyzed data from NASA's Wide-field Infrared Survey Explorer (WISE) of thousands of supermassive black holes to find that the "torus model" may be woefully inadequate when explaining what is actually going on.

ANALYSIS: Lasers to Solve the Black Hole Information Paradox?

Most galaxies appear to contain a supermassive black hole in their cores. With masses in the realms of millions to billions of solar masses, these objects truly are the heavyweights of our Universe. With all this mass comes a powerful gravitational field that dominates galactic cores, pulling in any matter - stars, planets, dust, gas, possibly unlucky extraterrestrials - to the black hole's event horizon.

Interactions between infalling matter and the supermassive black holes can generate huge quantities of energy, creating what are known as active galactic nuclei, making the effects of the black hole easy to observe.

In the 1970s, astronomers developed a unified theory that could explain active supermassive black hole observations. The theory arose from the fact that some active black hole emissions could be easily seen by observatories while others seemed obscured by dust. To explain this, astronomers came up with the idea that supermassive black holes must be surrounded by a torus, or ring, of dusty material (as shown in the artistic rendering above).

ANALYSIS: Monster Telescope Combo Spies Feeding Black Hole

Therefore, given their random orientation as observed from Earth, some rings may appear "edge on" (thereby blocking our view of the black hole) or we may be observing the ring from above (revealing the black hole).

Since this unified theory was suggested, it has generally matched observations of black holes and helped us understand how they influence the evolution of their host galaxies.

However, new analyses of WISE data - a space telescope that surveyed the infrared sky twice for a little over a year until its primary mission was complete in February 2011 - has revealed a complication to the unified theory.

ANALYSIS: How to Make a Bigger Black Hole Jet

As expected, after surveying 170,000 galaxies containing supermassive black holes at their cores, the WISE observations showed some black holes that could be seen, whereas others appeared obscured (in line with the torus model), but it also revealed a peculiar pattern. When looking at black holes inside massive galaxies that are clumped together as a part of galactic clusters, more supermassive black holes seemed to be obscured.

This bias toward obscured black holes in large clusters cannot be accounted for if we just consider the unified theory. Why would supermassive black holes inside galaxies that are clumped in clusters be preferentially obscured by their dusty doughnut-shaped rings?

"The main purpose of unification was to put a zoo of different kinds of active nuclei under a single umbrella," said post-doctorate astronomer and lead researcher Emilio Donoso, of the Instituto de Ciencias Astronómicas, de la Tierra y del Espacio in Argentina. "Now, that has become increasingly complex to do as we dig deeper into the WISE data."

ANALYSIS: How Do Supermassive Black Holes Get So Fat?

Donoso and his team's work indicates that some mechanism beyond the unified model is at work and they suggest dark matter may have a part to play.

It is well known that "invisible" dark matter - a type of non-baryonic matter that pervades the entire universe - exerts a strong gravitational influence on galaxies and clusters of galaxies. It is also known that there is a vast, large-scale dark matter structure that forms a huge cosmic "web." At nodes in this dark matter web - known as halos - galaxies form and collect as clusters, apparently anchored in place by the gravitational oomph of dark matter halos. It is also known that some of the most massive supermassive black holes reside in the biggest, most massive clusters that, in turn, is the location of the biggest halos of dense dark matter.

Could dark matter halos have a role in obscuring the clustered supermassive black holes from view? Is dark matter somehow adding more complexity to black hole torus?

For now, we just don't know and further work is needed to understand this bias. But it is fascinating to think that the 'textbook' idea of an active black hole sporting a doughnut-shaped torus may need some reworking.

For more on this fascinating WISE discovery, browse the NASA news release.

Artist's impression of a torus surrounding an active black hole.

Artist's impression of a spinning supermassive black hole with a surrounding accretion disk and relativistic jets.

How to measure the spin of a black hole: This chart illustrates the basic model for determining the spin rates of black holes. The three artist's concepts represent the different types of spin: retrograde rotation, where the disk of matter falling onto the hole, called an accretion disk, moves in the opposite direction of the black hole; no spin; and prograde rotation, where the disk spins in the same direction as the black hole.

Two models of black hole spin: Scientists measure the spin rates of supermassive black holes by spreading the X-ray light into different colors. The light comes from accretion disks that swirl around black holes, as shown in both of the artist's concepts. They use X-ray space telescopes to study these colors, and, in particular, look for a "fingerprint" of iron -- the peak shown in both graphs, or spectra -- to see how sharp it is. Prior to observations with NASA's Spectroscopic Telescope Array, or NuSTAR, and the European Space Agency's XMM- Newton telescope, there were two competing models to explain why this peak might not appear to be sharp. The "rotation" model shown at top held that the iron feature was being spread out by distorting effects caused by the immense gravity of the black hole. If this model were correct, then the amount of distortion seen in the iron feature should reveal the spin rate of the black hole. The alternate model held that obscuring clouds lying near the black hole were making the iron line appear artificially distorted. If this model were correct, the data could not be used to measure black hole spin.

This chart depicts the electromagnetic spectrum, highlighting the X-ray portion. NASA's Nuclear Spectroscopic Telescope Array (NuSTAR) and the European Space Agency's XMM-Newton telescope complement each other by seeing different colors of X-ray light. XMM-Newton sees X-rays with energies between 0.1 and 10 kiloelectron volts (keV), the "red" part of the spectrum, while NuSTAR sees the highest-energy, or "bluest," X- ray light, with energies between 3 and 70 keV.

This image taken by the ultraviolet-light monitoring camera on the European Space Agency's (ESA's) XMM-Newton telescope shows the beautiful spiral arms of the galaxy NGC1365. Copious high-energy X-ray emission is emitted by the host galaxy, and by many background sources. The large regions observed by previous satellites contain so much of this background emission that the radiation from the central black hole is mixed and diluted into it. NuSTAR, NASA's newest X-ray observatory, is able to isolate the emission from the black hole, allowing a far more precise analysis of its properties.

What XMM-Newton saw: The solid lines show two theoretical models that explain the low-energy X-ray emission seen from the galaxy NGC 1365 by the European Space Agency's XMM-Newton. The red line explains the emission using a model where clouds of dust and gas partially block the X-ray light, and the green line represents a model in which the emission is reflected off the inner edge of the accretion disk, very close to the black hole. The blue circles show the measurements from XMM-Newton, which are explained equally well by both models.

Two X-ray observatories are better than one: NASA's Nuclear Spectroscopic Telescope Array, or NuSTAR, has helped to show, for the first time, that the spin rates of black holes can be measured conclusively. It did this, together with the European Space Agency's XMM-Newton, by ruling out the possibility that obscuring clouds were partially blocking X-ray right coming from black holes. The solid lines show two theoretical models that explain low-energy X-ray emission seen previously from the spiral galaxy NGC 1365 by XMM-Newton. The red line explains the emission using a model where clouds of dust and gas partially block the X-ray light, and the green line represents a model in which the emission is reflected off the inner edge of the accretion disk, very close to the black hole. The blue circles show the latest measurements from XMM-Newton, and the yellow circles show the data from NuSTAR. While both models fit the XMM-Newton data equally well, only the disk reflection model fits the NuSTAR data.