2. 外语
  3. In addition to conventional galaxies, the universe contains very dim galaxies that until recently went unnoticed by astronomers.

In addition to conventional galaxies, the universe contains very dim galaxies that until recently went unnoticed by astronomers.

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问题 In addition to conventional galaxies, the universe contains very dim galaxies that until recently went unnoticed by astronomers. Possibly as numerous as conventional galaxies, these galaxies have the same general shape and even the same approximate number of stars as a common type of conventional galaxy, the spiral, but tend to be much larger. Because these galaxies’ mass is spread out over larger areas, they have far fewer stars per unit volume than do conventional galaxies. Apparently these low-surface-brightness galaxies, as they are called, take much longer than conventional galaxies to condense their primordial gas and convert it to stars—that is, they evolve much more slowly.
These galaxies may constitute an answer to the long-standing puzzle of the missing baryonic mass in the universe. Baryons—subatomic particles that are generally protons or neutrons—are the source of stellar, and therefore galactic, luminosity, and so their numbers can be estimated based on how luminous galaxies are. However, the amount of helium in the universe, as measured by spectroscopy, suggests that there are far more baryons in the universe than estimates based on galactic luminosity indicate. Astronomers have long speculated that the missing baryonic mass might eventually be discovered in intergalactic space or as some large population of galaxies that are difficult to detect.
The author implies that low-surface-brightness galaxies could constitute an answer to the puzzle discussed in the second paragraph primarily because
选项 A、they contain baryonic mass that was not taken into account by researchers using galactic luminosity to estimate the number of baryons in the universe
B、they, like conventional galaxies that contain many baryons, have evolved from massive, primordial gas clouds
C、they may contain relatively more helium, and hence more baryons, than do galaxies whose helium content has been studied using spectroscopy
D、they have recently been discovered to contain more baryonic mass than scientists had thought when low-surface-brightness galaxies were first observed
E、they contain stars that are significantly more luminous than would have been predicted on the basis of initial studies of luminosity in low-surface-brightness galaxies
答案A
解析 Inference
This question requires drawing an inference from information given in the passage. The puzzle is that estimates of the baryonic mass of the universe based on luminosity are lower than those based on spectroscopy (lines 21-25).The passage states that astronomers did not notice dim galaxies until recently (lines 2-3) and that these galaxies may help account for the missing baryonic mass in the universe (lines 15-17).The passage also suggests that astronomers measure the luminosity of specific galaxies (lines 19-21). Thus it can be inferred that, prior to their being noticed by astronomers, the luminosity of these dim galaxies was not measured, and their baryonic mass was not taken into account in the estimates of luminosity that led to the longstanding puzzle.
A Correct. The passage states that the missing baryonic mass in the universe may be discovered in the dim galaxies that have only recently been noticed by astronomers.
B The passage does not suggest that dim and conventional galaxies both originating from primordial gas clouds help solve the long-standing puzzle of the missing baryonic mass in the universe.
C The passage does not suggest that dim galaxies might contain more helium than do conventional galaxies or that measures of baryonic mass using spectroscopy do not take some dim galaxies into account.
D The passage does not suggest that dim galaxies contain more baryonic mass than scientists originally believed upon discovering these galaxies.
E The passage suggests that scientists measured the luminosity of galaxies, not of individual stars.
The correct answer is A.
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