The surprise is these high-mileage black holes aren't quasars， which are considered the most energetic and efficient bodies in the universe at converting matter to energy . Instead， astronomers have discovered， they are relatively old and quiet supermassive black holes that somehow can maintain similar efficiencies while expelling much less energy.

　　The astronomers used data from the Chandra X-ray Observatory to study nine supermassive black holes populating very large elliptical galaxies. In all cases， they found the areas around the black holes to be dim in visible light but quite bright in x-ray wavelengths. For the nine objects studied， they calculated that the black holes could convert up to 2.5% of the infalling gas and dust to energy——not quite as good as a quasar， which can average 5% or more， but still about 25 times better than the best nuclear power reactors.

　　The team also found that the jets produced by the supermassives are streaming outward at incredible speeds——in some cases 95% of the speed of light. "The energy in these jets is absolutely huge，" says lead researcher Steven Allen of Stanford University in Palo Alto， California， "about a trillion， trillion， trillion watts." The findings were announced during a media teleconference today and will be published in an upcoming issue of the Monthly Notices of the Royal Astronomical Society.

　　The question is what process converts the energy from the gas streaming in toward the black holes to the enormous energy in the jets. So far， there is only speculation， says co-author Christopher Reynolds of the University of Maryland， College Park. One idea is that the rotational energy of the supermassives powers the engine.

　　"We already knew quasars were enormously efficient at making light，" says Kimberly Weaver， an astrophysicist at NASA's Goddard Space Flight Center in Greenbelt， Maryland. "Now we know black holes in elliptical galaxies are also as efficient at making x-rays." This also could explain why there are few young stars in these galaxies： When the jets collide with the surrounding interstellar gas， they heat it to the point where it cannot condense into new stars.

　　"Just as with cars， it's critical to know the fuel efficiency of black holes，" Allen adds. "Without this information， we cannot figure out what is going on under the hood， so to speak， or what the engine can do."

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