Eye of Sauron
in the constellation Leo
45 million times the mass of the Sun
Diameter equal to the size of the orbit of Saturn
For chemists, a laboratory is a room filled with test tubes and electronic measuring devices. For marine biologists, it's a room filled with water tanks. For astronomers studying the interactions between black holes and their surrounding galaxies, though, the nearest good laboratory is in the heart of NGC 4151 -- more than 40 million light-years away.
NGC 4151 is a large spiral galaxy that we view almost face-on. A bar of stars crosses its center, with faint spiral arms wrapping around the bar.
Several techniques agree that a black hole roughly 45 million times the mass of the Sun sits at the galaxy's center. Astronomers have measured the motions of stars and of gas around the galactic center. Their high speeds indicate they are orbiting a large, dark object, such as a supermassive black hole.
Astronomers have also used NGC 4151 to test another technique for measuring supermassive black holes, known as reverberation mapping. The technique compares the spectra of gas in the large, bright accretion disk around the black hole with thinner clouds of gas at greater distances from the black hole. The accretion disk is so bright that its radiation causes the more distant gas clouds to glow. Comparing the two spectra can reveal the mass of the central black hole. In this case, the technique closely matches the mass determined by the other techniques.
NGC 4151 is considered a good laboratory because it's one of the closest examples of a Seyfert galaxy, which has a bright nucleus and gas swirling around the nucleus at high speeds. This gas produces winds and radiation that interact with the surrounding galaxy. Because NGC 4151 is so close, it offers a good chance for astronomers to study that interaction, known as feedback.
A leading idea says there's a close correlation between the mass of a galaxy's central black hole and the mass of the "bulge" of stars around it. According to this idea, as a galaxy is taking shape, the black hole at its center grows by pulling in vast clouds of gas and dust. As these clouds move toward the black hole they pile up, fragmenting into clumps that give birth to new stars. In this way, the black hole helps give birth to the galaxy's stars.
As the black hole grows bigger, however, it surrounds itself with a larger and larger accretion disk. As the disk gets bigger it gets hotter, so it produces intense radiation that pushes away some of the gas and dust around it. That shuts down the process of star formation, regulating how many stars are found in the galaxy's central bulge.
Astronomers are testing that idea with NGC 4151. Massive plumes of hot gas -- carrying enough material every year to make two stars as massive as the Sun -- are squirting away from the vicinity of the black hole, moving in opposite directions. One of them appears to be slowing down as it slams into clouds of cold carbon monoxide gas on the outskirts of the galaxy's bulge.
In addition, there's a ring around the galaxy's nucleus that produces enormous amounts of X-rays. Astronomers have proposed two explanations for the ring, both involving "outflows" from the accretion disk around the black hole.
The first scenario says that the accretion disk grew so hot that it stripped the electrons from atoms in the surrounding gas clouds. As the electrons linked up with other atoms, they emitted X-rays.
The second scenario says that the X-rays were produced when the accretion disk became so hot that it produced "winds" of charged particles that blew into the surrounding gas clouds. If this scenario is correct, it would be a strong example of feedback, in which the black hole alters the environment around it.
Astronomers are continuing to study this cosmic laboratory to learn more about the link between black holes and their surrounding galaxies.
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This document was last modified: May 11, 2012.