每日一题

Critical Reading>Select an Answer

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 choice best characterizes the study described in the passage?

A.Astronomers used technology to confirm the existence and location of a distant object.
B.Astronomers produced computer models depicting a past state.
C.Astronomers demonstrated that a generally accepted theory is flawed.
D.Astronomers drew two conclusions from a study of one phenomenon.

答案和解析

答案：D

解析：

Choice D is correct. By identifying instances where quasar light passes through galaxies, the astronomers were able to both determine the cause of quasar light polarization and measure the red shift of magnesium(II) absorption by galaxies to estimate their distances from Earth. In other words, their study of one phenomenon led to two findings.