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Light from distant quasars—early galaxies that shine with tremendous brightness—has given researchers a new clue to the origin of vast magnetic fields studding today's galaxies: They were running strong when the universe was only a third of its present age.

Astronomers had observed that radio emissions from quasars tend to be angled, or polarized, in such a way that powerful magnetic fields must have twisted them. The greater their distance from Earth, the more polarized their light. But researchers didn't know whether the magnetic fields were part of the quasar or were present in galaxies encountered by quasar light as it made its journey to our telescopes.

So a team led by astronomers from the Swiss Federal Institute of Technology (ETH Zurich) scanned more than 70 of those quasars using the European Southern Observatory's Very Large Telescope in Chile to look for signs of galaxies hiding in front of the quasars. Specifically, they checked for a feature called the magnesium(II) absorption line, a reduction in the intensity of light of a certain wavelength, which is a commonly used indicator that gas from a star-forming galaxy has soaked up that light.

The researchers report that light from quasars showing the magnesium(II) line was more strongly polarized than light from other quasars in the sample. The interpretation: that light did indeed pass through regular galaxies and that it likely acquired its polarization in the process.

They estimated the age of the magnetic galaxies by measuring the red shift of the absorption line—the observed reddening of light that occurs when galaxies move rapidly apart. The typical red shift of the inferred galaxies corresponded to an age of 5.2 billion years, study author Francesco Miniati says. Precision measurements of the cosmic microwave background peg today's universe at 13.7 billion years old.

Which of the following, if true, would undermine the interpretation offered by the researchers in the fourth paragraph?

(A) Another team of astronomers measures the red shifts of magnesium(II) absorption lines and concludes that the galaxies formed when the universe was less than 5.2 billion years old.
(B) A new study determines that quasars that are not near star-forming galaxies have high levels of polarized light.
(C) A group of astronomers publishes a report that suggests the number of known quasars in the universe is higher than previously documented.
(D) A massive galaxy that causes strongly polarized light is found to be situated in front of a quasar.

答案和解析

答案：B

解析：

Choice B is the best answer. The fourth paragraph indicates that the researchers concluded that light from quasars with magnesium(II) absorption lines acquired strong polarization as a result of passing through galaxies. This interpretation would be most weakened by a study that found that light from a quasar was strongly polarized without having passed through a galaxy near the quasar.