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Black holes at galactic centers set up whirlpools

About 1% of all galaxies have bright, powerful sources of energy in their very centers. Called "Active Galaxies", they include among their number the brightest pbjects in the Universe: the quasars that can be seen shining from billions of light years away.

The power source behind these active galaxies is thought to be a supermassive galactic black hole, with masses ranging from a million to a billion times the mass of our Sun. Such objects are about the size of our solar system, but can shine forth with the light of billions of stars combined, often outshining the 100 to 200 billion stars that make up its host galaxy. Sometimes, in addition to feeding on gas, some of the material is accelerated into jets that can extend for nearly a million light years.

The source of power is the strong gravitational fields of the supermassive black holes. We have recently come to recognize that taken as a whole, active galaxies may account for up to 50% of all the energy emitted by the entire Universe over its history!

Black Hole power is clearly important, but a key problem of understanding it is where the monster in the middle of the galaxy gets its food... To shine as brightly as they do, the central supermassive black hole in an active galaxy must be fed at a prodigious rate. Estimates range from 1/10th to a few times the mass of our Sun per year. That's hungry...

The problem is that Black holes are powerful, but only really close up. From far away, they have no stronger gravitational pull than a massive cluster of stars of comparable mass. The problem of how to feed a central black hole is not a lack of food, galaxies are loaded with gas and stars. Rather, the problem is how to get it from the outer parts of the galaxy into the very center where it can be fed to the black hole.

If it gets its food from its host galaxy, can we see it? To look for the possible food sources in the centers of active galaxies, we need to get pictures taken of as close as we can get to the central black hole. This is very hard to do from the ground, peering up through the Earth's turbulent atmosphere. Professor Richard Pogge and graduate student Paul Martini of Ohio State University are using the Hubble Space Telescope to take a closer look at how active black holes in galaxies might be fed.

How close can we get? In the nearest few dozen active galaxies to the Milky Way, we can see in to a few 10s or 100s of light years. This is still way bigger than the few-light-day sizes of the central black hole region, but it is the most likely place to look for the food reserves that might feed the monster in the middle.

Further, if gas is being fed to the center from the outskirts of the host galaxy, we may be able to see the signatures of this "inflow" on larger scales, up to a few 1000 light years. The process is slow, but if steady enough, it could provide for most of the central black hole's needs for a long time (10s or even 100s of millions of years).

At the lower left is a picture of the nearby active galaxy named "Markarian 573." The central black hole source is what causes the intense brightening in the center.

Pogge and his team have developed a number of image processing tricks to try to strip away the confusing starlight in the centers of active galaxies and reveal the gas and dust in the surrounding regions. As a result, the pictures here show regions of thick gas and dust as dark regions, while gas lit-up by light from the central active black hole appears as filamentary bright streaks. They used images taken through red and blue filters to supress most of the light from the billions of stars in the central regions and see the gas and dust around the central active black hole. The dark spiral-shaped lanes of dust and gas are what they were looking for; this is potential Black Hole Chow... The central active source itself is the bright central spot . Notice the spiral form to the dust and gas, as if it were draining into the central regions...

Pogge and his colleagues have used maps like these to map out the available gas and dust reserves in the centers of about a hundred nearby active galaxies so far. This montage hereshows a sampling of nine of the many active galaxies they have looked at using the Hubble Space Telescope and the new image processing techniques. "The spiral-shaped streaks of dense gas and dust we see in nearly all of these is very striking. We think we are seeing signs of gas and dust from the outskirts of the host galaxy being fed, slowly, in the central regions," said Pogge.

The researchers saw swirling patterns in 20 of the 24 galaxies, indicating to them that a huge cyclone of dust was whirling into supermassive black holes. The other four galaxies just showed a "big pile of dust" with no obvious pattern, Pogge said.

Pogge and Martini theorize that the nuclear spirals represent the mechanism for "feeding" black holes so that they start throwing off radiation. In contrast, the black hole at the Milky Way's center—estimated to be 3 million times as massive as our sun - may be inactive because it isn't receiving "nourishment" from its surroundings.

"Before black holes become active, you have to feed them," Pogge said. The researchers believe that inactive galaxies might be in equilibrium, with material cleared out at a safe distance from the central black hole. Pogge said it would take some kind of disturbance to set off a feeding spiral, perhaps a tidal encounter with another galaxy, or shock waves from a cosmic eruption.

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