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How Black Holes Both Consume and Eject Material

January 5, 2005 ::
Chandra X-ray Image of MS 0735.6+7421
With the announcement of the most powerful eruption ever witnessed in the Universe in the galaxy cluster MS 0735.6+7421, astronomers are seeing that how supermassive black holes eject matter is just as interesting as how they consume it.

This discovery, as is often the case, leads to more questions: How can black holes eject so much energy and material? Have similar eruptions been seen, or is this some sort a cosmic loner? What does it teach us about supermassive black holes and about the galaxies where they reside?

To begin with, it sounds illogical that black holes could even generate massive eruptions. After all, hasn't it always been said that nothing, not even light, can escape a black hole? This remains true, but only when matter passes inside the "event horizon" of a black hole. This is a black hole's point of no return, when nothing is capable of escaping from its intense gravity. In other words, it's what puts the "black" in "black holes."

The Huge Scale of the Cavities
To create the cavities in MS 0735, gas is being pushed away across more than a million light years. This distance is over a billion times greater than the size of the black hole's event horizon. This means that if the cavities were shrunk down in size so that they spread across the continental US, then the black hole's event horizon would be less than a tenth of an inch across!
Matter that gets close to a black hole but remains outside the event horizon can undergo a very different experience and is sometimes expelled in violent jets. Such jets, probably originating from an energetic, magnetized, spinning disk around the supermassive black hole, produced the enormous cavities seen in MS 0735. The creation of these huge structures is a vivid demonstration of the enormous power that can be generated by supermassive black holes.

The Power of Gravity and Supermassive Black Holes
Gravity is a very powerful force, when harnessed by supermassive black holes. The swallowing of material by a black hole is one of the most efficient processes in the Universe for extracting energy from matter. It is much more efficient, for example, than the nuclear fusion that takes place in the Sun or a nuclear weapon. About 10% of the energy (from E=mc2) inherent in a given amount of hydrogen is released when falling into a black hole (more than 10% for a rapidly rotating black hole). For the same amount of hydrogen only about 0.7% of the inherent energy is released by fusion. Super-advanced aliens would generate much more energy from sending a star into a black hole than letting it shine for billions of years!
Scientists can calculate - based on the gigantic amounts of energy being expelled - how much material was consumed by a black hole. In the case of MS 0735, astronomers estimate a mass equivalent to 300 million Suns was consumed by the black hole. This is a gargantuan amount of material, intriguingly as large as another supermassive black hole.

But, knowing the size of the feast does not identify its contents and astronomers are not sure what was swallowed by the black hole in MS 0735. One possibility is that it comes from the enormous reservoir of hot gas visible in the Chandra image. If the hot gas in the inner part of the galaxy cluster was able to cool down quickly enough, it could collapse onto the black hole in vast amounts. In this scenario, as the sinking gas fed the swirling disk around the black hole, energy released through the erupting jets would then reheat the remaining gas in the cluster. As the energy from the eruption dies down, the gas would start cooling again until this whole process repeats itself.

This scenario could help address a mystery in the life cycle of galaxies. That's because astronomers expect, without the black hole as a heat source, the hot gas near the center of the cluster should keep cooling until it eventually forms many, many new stars. The problem is that astronomers are not finding this excess of stars. Therefore, it may turn out that what these black holes eat might be the key to understanding several different fields of astrophysics: the evolution of supermassive black holes and the evolution of galaxies and the clusters they inhabit. The dining habits of black holes may influence all three of these fields.

How does the powerful eruption in MS 0735 compare with events seen in other galaxy clusters? Sometimes the Chandra images show no evidence at all for energetic events. But, recent Chandra observations have shown evidence for extremely powerful eruptions in both Hercules A and Hydra A (with masses of 170 million and 50 million Suns swallowed respectively). The size, energy and power of the cavities in MS 0735 are the largest of the three, but the others are not far behind.

The large size of these cosmic meals comes as a surprise. A recent study suggests that other large black holes have grown very little in the recent past, and that only smaller black holes are still growing quickly. These three black holes have had the ultimate super-sized meals, just when everyone was ready to believe that they are on starvation diets.

Size Comparison of MS 0735.6+7421 & Perseus Cluster
A much less powerful system of cavities is found in the Perseus cluster. This cluster earned widespread recognition because Chandra determined that the black hole is generating the deepest "note" detected in the Universe. More than just being impressive acoustically, these sound waves are more good evidence for the cycle of cooling, black hole-jet eruptions, heating of gas, followed by more cooling, etc.

The cavities in Perseus required large amounts of energy to form, but they are puny compared to the ones in MS 0735. The cavities in MS 0735 are over 10 times larger in physical size and they are expected to get bigger, unlike with Perseus. Even more impressively, about 250 times more energy was required to generate the MS 0735 cavities. All of this is not to say that the supermassive black hole in Perseus is a wimp, just that it has a very different diet: a lot of small, evenly spaced meals instead of occasional feasts.

An obvious follow-up question arises: what do we know about the sound in MS 0735? The sound waves in Perseus are generated when the expanding cavities slow down below the speed of sound and push against the cluster gas. In MS 0735, the cavities are still growing at supersonic speeds, despite their colossal size.
Animation of Eruption from Supermassive Black Hole
Sound waves may eventually be generated when the cavities slow down. If the eruption near the supermassive black hole repeated itself then the resulting "note" would potentially be far deeper than the one in Perseus. However, no tell-tale ripples are visible in the Chandra image.

Because the cavities are so large in MS 0735, they extend almost to the edge of the observed extent of the hot gas cloud in this cluster. Outside the cavities the X-rays are so faint that even extremely long observations with Chandra might never detect any ripples due to sound waves. In a sense the eruption may simply be too powerful to allow a detection of MS 0735's note.

Intriguingly, very little was known about these cavity systems before Chandra was launched. X-ray vision is needed to see the hot gas in these clusters and Chandra's exquisitely detailed images are needed to clearly see the giant cavities and to understand the magnitude of the outbursts required to produce them. With many more clusters and black holes to be observed by Chandra, who knows what exciting discoveries will be found in the future?

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