Black holes have simple feeding habits
BY DR EMILY BALDWIN
ASTRONOMY NOW
Posted: June 20, 2008

The results of a massive observing campaign of spiral galaxy M81 have shed light on the feeding habits of different sized black holes, and provide a benchmark for predicting the properties of a new class of black hole.

M81 lies about 12 million light years from the Earth and accommodates a black hole at its centre, 70 million times more massive than our Sun. It feeds off the host galaxy, sucking in gas from its central region at high speed. In contrast, stars which have about 10 times more mass than the Sun - stellar mass black holes - acquire their energy by ripping gas from an orbiting companion star.

Composite of the M81 galaxy. Beginning with optical light, see how M81 looks through ultraviolet and infrared eyes before seeing Chandra’s X-ray image. In the centre of M81 is a black hole that is about 70 million times more massive than the Sun, and generates energy and radiation as it pulls gas in the central region of the galaxy inwards. When astronomers compared these data with models for much smaller-scale black holes, they found a similar distribution of optical, radio, and X-ray light. This implies that black holes - either big or little - devour material in the same way. Image: X-ray: NASA/CXC/ Wisconsin/D.Pooley & CfA/A.Zezas; Optical: NASA/ESA/CfA/A.Zezas; UV: NASA/JPL-Caltech/CfA/J.Huchra et al.; IR: NASA/JPL-Caltech/CfA.

Because the bigger and smaller black holes are found in different environments with different sources of material to power them, a question has remained about whether they feed in the same way. To find out, a team of astronomers made simultaneous observations of M81 using the Chandra X-ray telescope, three radio arrays (the Giant Meterwave Radio Telescope, the Very Large Array and the Very Long Baseline Array), two millimetre telescopes (the Plateau de Bure Interferometer and the Submillimeter Array) and the Lick Observatory, to ensure that brightness variations due to changes in feeding rates did not confuse the results.

The results were compared to stellar mass black holes using a detailed theoretical model based on a faint disc of material spinning around the black hole. According to the model, this structure would mainly produce X-rays and optical light, and a region of hot gas around the black hole would be seen largely in ultraviolet and X-ray light. A large contribution to both the radio and X-ray light comes from jets generated by the black hole. Multi-wavelength data from the space and ground based telescopes were able to disentangle these overlapping sources of light, showing that big or little, black holes produce a similar distribution of X-rays, optical and radio light as they dine on their stellar feasts.

The new result also has implications for Einstein’s theory of General Relativity, which says that black holes are simple objects and only their masses and spins determine their effect on space-time. The latest research indicates that this simplicity manifests itself in spite of complicated environmental effects.

"This confirms that the feeding patterns for black holes of different sizes can be very similar," says Sera Markoff of the Astronomical Institute, University of Amsterdam who led the
study. "We thought this was the case, but up until now we haven't been able to nail it."

Optical image of M81, with the central region containing the black hole shown in the inset in a Chandra X-ray image. Image: NASA/ESA/CfA/A.Zezas (optical) and NASA/CXC/Wisconsin/D.Pooley & CfA/A.Zezas (X-ray).

"When we look at the data, it turns out that our model works just as well for the giant black hole in M81 as it does for the smaller guys," adds Michael Nowak from the Massachusetts Institute of Technology. "Everything around this huge black hole looks just the same except it's almost 10 million times bigger."

Among actively feeding black holes the one in M81 is one of the dimmest, meaning that it is "underfed". The research team say that these malnourished black holes are the least fussy eaters of all, sourcing their food from whatever is nearest. An advantage of studying these black holes is that their brightness, or lack of, allows astronomers to see closer to the black hole than for those which have a greater appetite.

The new work will be useful for predicting the properties of a third, unconfirmed class of black holes called intermediate mass black holes, with masses lying between those of stellar and supermassive black holes. Some possible members of this class have been identified, but the evidence is controversial, so specific predictions for the properties of these black holes should be very helpful.