Black holes may be the most energy efficient engines in the universe, astronomers working with NASA's Chandra X-ray Observatory reported on Monday.

If a car was as efficient as these black holes, it could theoretically travel over more than 1.6 billion km on a gallon (3.785 liters) of gasoline, the astronomers said.

These findings were published in an upcoming issue of the Monthly Notices of the Royal Astronomical Society, a British scientific journal. By making the first direct estimate of how efficient black holes are, the study gives insight into how black holes generate energy and affect their environment.

Matter will fall into a super massive black hole when it is attracted, and energy will be released in the form of particle jets that travel away from the black hole at near the speed of light, according to the study.

This is an important step in understanding how such jets can be launched from magnetized disks of gas near the event horizon of a black hole.

"Just as with cars, it's critical to know the fuel efficiency of black holes," said Steve Allen, lead author of the study at Stanford University.

"Without this information, we cannot figure out what is going on under the hood, so to speak, or what the engine can do."

The researchers used Chandra to study nine super massive black holes at the centers of elliptical galaxies. These black holes are relatively old and generate much less radiation than quasars, rapidly growing super massive black holes seen in the early Universe.

Surprisingly, Chandra results showed that these "quiet" black holes are all producing much more energy in jets of high-energy particles than in visible light or X-rays. These jets create huge cavities in the hot gas in the galaxies.

The efficiency of the black hole energy-production was calculated in two steps: first, Chandra images of the inner regions of the galaxies were used to estimate how much energy is available for the black hole; and then Chandra images were used to estimate the power required to produce the cavities.

New details are given about how black hole engines achieve this extreme efficiency. Some of the gas first attracted to the black holes may be blown away by the energetic activity before it gets too near the black hole, but a significant fraction must eventually approach the event horizon where it is used with high efficiency to power the jets.

The study also implies that matter flows toward the black holes at a steady rate for several million years.

"These black holes are very efficient, but it also takes a very long time to refuel them," Allen said in a statement.

The study also shows another role of black holes, according to the researchers.

They found that the energy transferred to the hot gas by the jets should keep hot gas from cooling, and prevent billions of new stars from forming. This will place limits on the growth of the largest galaxies.

Source: Xinhua