Galaxy clusters are vast groups of galaxies. Extremely hot, thin gas

Explanation for Question #1

According to the passage, in the Perseus cluster

Scanning the passage for "in the Perseus cluster," we find that the Perseus cluster is mentioned in the second and fourth paragraphs and that information related to the Perseus cluster also appears in the third and fifth paragraphs. So, to determine whether choices in this question are correct we'll probably first compare them with the information about the Perseus cluster in the second and fourth paragraphs. However, we may also have to consider information in the third and fifth paragraphs and make inferences based on combinations of what the passage says directly about the Perseus cluster and what it says in one or more of the other paragraphs that present related information.

A. the energy jets from the central black hole are dwindling

Scanning the passage for information on energy jets from black holes "dwindling," we find that the last paragraph says that, in general during a portion of a "cycle" that occurs in a cluster, "the black hole spins more slowly, so the energy jets and bubbles dwindle."

At the same time, the passage doesn't say that the Perseus cluster is in the stage of a cycle in which the jets are dwindling. So, the passage doesn't support what this choice says.

Eliminate.

B. the hot intergalactic gas is not continuously cooling

In the second paragraph we see the following:

In the 1990s, satellite telescopes discovered two huge cavities or "bubbles" in the hot gas in the center of the Perseus cluster ... 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.

So, the passage first says that there are "bubbles" in the center of the Perseus cluster. Then, it says that, "in clusters containing such bubbles," meaning in clusters like the Perseus cluster, "the hot intergalactic gas is not cooling."

Thus, we can combine the information about the bubbles in the center of the Perseus cluster and the information on the intergalactic gas "clusters containing such bubbles" to infer that "the hot intergalactic gas is not continuously cooling" in the Perseus cluster.

So, the information in the passage indicates what this choice says.

Keep.

C. intergalactic gas is growing dense enough to form new stars

The second paragraph says the following:

In the 1990s, satellite telescopes discovered two huge cavities or "bubbles" in the hot gas in the center of the Perseus cluster ... 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.

We see that, contrary to what this choice says, the passage indicates that information suggests that bubbles in the Perseus cluster keep the gas there from forming new stars.

Eliminate.

D. two huge, central black holes are filled with magnetic fields and energetic particles

This choice is a little tricky because the passage says the following:

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

So, we might get the impression that this choice is correct since the wording 'two huge cavities or 'bubbles" in the hot gas in the center of the Perseus cluster, filled with magnetic fields and energetic particles" is similar to the wording of this choice.

However, the passage does not say that there are two black holes in the Perseus cluster. In fact, in the fourth paragraph, it mentions "this galaxy's black hole" as what's producing the bubbles in the Perseus cluster. So, the passage indicates that the Perseus cluster has one black hole, not two.

Eliminate.

E. bubbles in the hot intergalactic gas are causing a supermassive black hole to spin rapidly

Regarding the black hole spinning, the passage says the following:

But the energy the black hole devours also causes it to rotate.

The faster the black hole spins, the greater a proportion of the incoming gas is ejected in these jets.

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.

We see that the information provided by the passage indicates that the spinning of the black hole is involved in the production of the bubbles, rather than, as this choice says, the bubbles cause the black hole to spin. In other words, the cause and effect mentioned in this choice are basically the opposite of the cause and effect mentioned in the passage.

Eliminate.

The correct answer is (B).

Explanation for Question #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?

The correct answer to this question must present a process that "counteracts," meaning "works against," the heating of a galaxy cluster's intergalactic gas.

We need to be careful to keep things straight to answer this question since the passage mentions that bubbles keep hot gas in a galaxy cluster from cooling and forming stars. So, the bubbles "counteract" cooling and formation of stars by the gas whereas this question is about what "counteracts heating" of the gas, and we have to be careful not to mix up those two ideas.

A. Settling of the gas into the center of the cluster

This choice is tricky because what the passage says about "setting of the gas into the center of the cluster" is basically the opposite of what this question is about. So, if we don't keep straight what the passage says, we can easily fall for this choice.

The correct answer to this question must be something that counteracts heating of the gas. In contrast, the passage indicates that the heating of the gas counteracts "settling of the gas into the center of the cluster," when it says, among other things, the following:

These jets create bubbles that heat the gas, diminishing its inflow.

Since "its inflow" would be the settling of the gas, we see that the passage indicates that heating works against settling.

We need something that works against heating.

Eliminate.

B. Expansion of huge cavities in the gas

Regarding "expansion of huge cavities," the passage says the following:

satellite telescopes discovered two huge cavities or "bubbles" ... These energetic bubbles are expanding and pushing aside the hot gas. ... 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.

We see that, rather than counteract heating, the cavities or "bubbles" counteract cooling and settling.

Eliminate.

C. Emission of X-rays by the gas

Regarding emission of X-rays, we find the following in the first paragraph of the passage:

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

We see that the passage says that the emission of X-rays serves to "carry away energy" and that, because of this carrying away of energy, "the gas should cool."

That information indicates that "Emission of X-rays by the gas" is a process that is working to cool the gas.

In other words, while the effects of energetic bubbles, black holes, and other things mentioned in the passage are keeping the gas from cooling, the emission of X-rays is doing something such that "the gas should cool."

In other words, we have opposing effects: X-rays are carrying away energy, having a cooling effect, while a black hole and its associated "energetic bubbles" and "energy jets" have heating effects.

So, the emission of X-rays has a cooling effect and thus works against, or "counteracts," the heating of the gas by the other things mentioned in the passage.

Thus, the passage indicates that what this choice presents, "emission of X-rays by the gas," counteracts the heating of a galaxy cluster's intergalactic gas.

Keep.

D. Rotation of the gas around a supermassive black hole

Regarding rotation, the passage says, in the third paragraph, that black holes rotate, not the intergalactic gas.

Eliminate.

E. Ejection of the gas in two opposing jets

Regarding "ejection" of gas "in two opposing jets," the passage says the following in the last paragraph:

These jets create bubbles that heat the gas

We see that, contrary to what this choice says, jets of gas heat, rather than counteract heating of, a galaxy cluster's intergalactic gas.

Eliminate.

The correct answer is (C).

Explanation for Question #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

The passage says the following about galaxy clusters:

Each galaxy cluster centers on one especially large galaxy containing a huge black hole.

We see that, if "each galaxy cluster centers on one especially large galaxy containing a huge black hole," then, contrary to what this choice says, all clusters have a black hole in their central galaxies.

Eliminate.

B. magnetic fields and energetic particles are preventing the hot intergalactic gas from cooling and settling

Regarding "magnetic fields and energetic particles," the passage says the following:

"bubbles" in the hot gas in the center of the Perseus cluster, filled with magnetic fields and energetic particles

We see that the passage indicates that the "magnetic fields and energetic particles" are in the huge bubbles. So, contrary to what this choice says, in galaxy clusters that do not contain huge bubbles, there may be no magnetic fields and energetic particles preventing the hot intergalactic gas from cooling and settling.

Eliminate.

C. the central black hole is ejecting most of its inflowing gas in two opposing jets

Regarding a black hole "ejecting most of its inflowing gas in two opposing jets," the passage says the following in the fourth paragraph:

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.

We see that, if the passage says that the black hole "must be producing the bubbles" by ejecting gas in jets, then where there are no bubbles, it makes sense that, contrary to what this choice says, there is not a central black hole ejecting gas in jets.

Eliminate.

D. cooling gas is forming trillions of new stars

Regarding cooling gas forming trillions of new stars, the passage says the following:

surprisingly, observations have not detected anything like what had been predicted

which is that

the gas should cool and settle into the center of the cluster, growing dense enough to form trillions of new stars

So, the passage indicates that "cooling gas forming is forming trillions of new stars," has not been observed in any galaxy clusters, not even ones "that do not contain huge bubbles like those described in the second paragraph."

Eliminate.

E. intergalactic gas is moving toward the central black hole

Regarding "galaxy clusters that do not contain huge bubbles," the passage says the following in the last paragraph:

not all galaxy clusters show such bubbles

So, we see that the last paragraph is discussing "galaxy clusters that do not contain huge bubbles."

Then, the last paragraph says "As a cluster's gas cools, it falls into the central black hole, making it shoot out jets of energy."

So, we see that, the passage is indicating that, when a galaxy cluster does not contain huge bubbles, the cluster's gas can cool and fall into the central black hole.

Honestly, that this choice is correct is not super clear. After all, the passage doesn't really make clear that the reason the gas cools and falls into the central black hole is that the galaxy cluster does not contain huge bubbles.

At the same time, since we can eliminate all the other choices and since there is a way to justify this choice given what the last paragraph says, we can safely choose this choice as correct.

The correct answer is (E).

Explanation for Question #4

The main purpose of the passage is to

To bring the main purpose of the passage into focus, we can consider the important points of the passage.

The passage's first important point is stated in the first paragraph, introduced by the contrast word "but":

But surprisingly, observations have not detected anything like what had been predicted (which is that "the gas should cool and settle into the center of the cluster, growing dense enough to form trillions of new stars").

Then second paragraph presents what observations have detected:

In the 1990s, satellite telescopes discovered two huge cavities or "bubbles"

and

which suggests that the bubbles keep the gas from forming new stars

The third paragraph presents a cause of the "bubbles":

Only one kind of object known to science could generate enough energy to produce these bubbles: a supermassive black hole.

Then, the remainder of the passage provides more detail on how black holes produce the bubbles.

With this information in mind, let's go to the answer choices.

A. speculate about the effects of X-rays emitted by hot, thin intergalactic gas in galaxy clusters

X-rays are mentioned only briefly in the first paragraph. So, this choice is too narrow to capture the main purpose of the passage.

Furthermore, the passage doesn't speculate about the effects of the X-rays.

So, even though X-rays are mentioned in the beginning of the passage, their effects are not the main topic.

Eliminate.

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

As we saw in considering the important points of the passage, the passage begins by stating the point that "surprisingly, observations have not detected anything like what had been predicted," which is that "the gas should cool and settle into the center of the cluster, growing dense enough to form trillions of new stars."

Then, the remainder of the passage discusses the bubbles in the galaxy clusters and the dynamics involving black holes and the bubbles that keep the gas from forming new stars.

So, we can see that this choice captures what the passage does.

Keep.

C. describe how bubbles in the hot gas in the centers of galaxy clusters prevent the gas from cooling and settling

The passage mentions something about "how bubbles in the hot gas in the centers of galaxy clusters prevent the gas from cooling and settling" in only one sentence in the second paragraph:

These energetic bubbles are expanding and pushing aside the hot gas.

So, this choice is too narrow to capture the main purpose of the passage.

Eliminate.

D. argue that the rotation of black holes might create bubbles of hot intergalactic gas in some galaxy clusters

This choice is a Wrong Verb choice.

The passage does suggest that rotation of black holes creates bubbles of hot intergalactic gas in some galaxy clusters, but it does not "argue," in other words, focus on proving, that the rotation of black holes might create bubbles of hot intergalactic gas.

Also, this passage discusses more than the rotation of black holes creating bubbles. So, this choice is too narrow to capture the main purpose of the passage.

Eliminate.

E. discuss how astronomers have measured the cyclical heating and cooling of intergalactic gas in galaxy clusters

The passage never mentions "how astronomers have measured the cyclical heating and cooling of intergalactic gas."

Eliminate.

The correct answer is (B).