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.