Galaxy clusters are vast groups of galaxies. Extremely hot, thin gas fills the space between galaxies in a cluster, emitting X-rays. Astronomers have calculated that these X-rays carry away so much energy that the gas should cool and settle into the center of the cluster, growing dense enough to form trillions of new stars. But surprisingly, observations have not detected anything like what had been predicted.
In the 1990s, satellite telescopes discovered two huge cavities or "bubbles" in the hot gas in the center of the Perseus cluster, filled with magnetic fields and energetic particles. These energetic bubbles are expanding and pushing aside the hot gas. Similar bubbles were soon found in several other clusters. Measurements showed that in clusters containing such bubbles, the hot intergalactic gas is not cooling and settling, which suggests that the bubbles keep the gas from forming new stars.
Only one kind of object known to science could generate enough energy to produce these bubbles: a supermassive black hole. A black hole is a kind of gravitational sinkhole that sucks in matter—such as gas—and energy. But the energy the black hole devours also causes it to rotate. The rotation twists the fabric of space around the black hole, catapulting out some of the inflowing gas in two opposing jets. The faster the black hole spins, the greater a proportion of the incoming gas is ejected in these jets.
Each galaxy cluster centers on one especially large galaxy containing a huge black hole. In the Perseus duster, the two vast bubbles in the hot intergalactic gas are aligned with jets of energy emanating from the middle of the large central galaxy. Thus, this galaxy's black hole must be producing the bubbles.
This may be part of a cyclical process, since not all galaxy clusters show such bubbles. As a cluster's gas cools, it falls into the central black hole, making it shoot out jets of energy. These jets create bubbles that heat the gas, diminishing its inflow. With less incoming gas, the black hole spins more slowly, so the energy jets and bubbles dwindle, letting the hot gas cool again to repeat the cycle.
1. According to the passage, in the Perseus cluster A. the energy jets from the central black hole are dwindling
B. the hot intergalactic gas is not continuously cooling
C. intergalactic gas is growing dense enough to form new stars
D. two huge, central black holes are filled with magnetic fields and energetic particles
E. bubbles in the hot intergalactic gas are causing a supermassive black hole to spin rapidly
2. The passage most strongly supports the claim that which of the following processes directly counteracts the heating of a galaxy cluster's intergalactic gas? A. Settling of the gas into the center of the cluster
B. Expansion of huge cavities in the gas
C. Emission of X-rays by the gas
D. Rotation of the gas around a supermassive black hole
E. Ejection of the gas in two opposing jets
3. The passage most strongly suggests that in galaxy clusters that do not contain huge bubbles like those described in the second paragraph A. there is no supermassive black hole in the central galaxy
B. magnetic fields and energetic particles are preventing the hot intergalactic gas from cooling and settling
C. the central black hole is ejecting most of its inflowing gas in two opposing jets
D. cooling gas is forming trillions of new stars
E. intergalactic gas is moving toward the central black hole
4. The main purpose of the passage is to A. speculate about the effects of X-rays emitted by hot, thin intergalactic gas in galaxy clusters
B. offer an explanation of why observations have not detected the many new stars that astronomers calculated should have formed in the centers of galaxy clusters
C. describe how bubbles in the hot gas in the centers of galaxy clusters prevent the gas from cooling and settling
D. argue that the rotation of black holes might create bubbles of hot intergalactic gas in some galaxy clusters
E. discuss how astronomers have measured the cyclical heating and cooling of intergalactic gas in galaxy clusters