黑洞的能量效率惊人

High-Mileage Black Holes

By Phil Berardelli
ScienceNOW Daily News
24 April 2006

Astronomers¹ have discovered that, deep inside the biggest and brightest galaxies² in the universe, jets are spewing particles from around black holes in an incredibly energy-efficient manner. If an automobile³ engine worked as well as one of these monsters, it could go more than a billion miles on a gallon of gas.

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 relatively⁴ 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 Observatory⁵ 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 wavelengths⁶. 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 reactors⁷.

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 watts⁸." The findings were announced during a media teleconference today and will be published in an upcoming issue of the Monthly Notices of the Royal Astronomical⁹ 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 speculation¹⁰, says co-author Christopher Reynolds of the University of Maryland, College Park. One idea is that the rotational¹¹ energy of the supermassives powers the engine.

"We already knew quasars were enormously efficient at making light," says Kimberly Weaver¹², 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 hood¹³, so to speak, or what the engine can do."