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Text 6. Does missing mass hide at edge of Galaxy?
Some of the missing mass in our Galaxy may be in the form of clouds of cold molecular gas between the Sun and the edge of the galactic disc, say astronomers from France and the US. The gas is so cold that it reveals itself only by absorbing radiation from more distant radio sources.
Ronald Allen of the Space Telescope Science Institute in Baltimore found the clouds with a 30-metre radio telescope in Spain operated by the Institute for Radio Astronomy in Grenoble. Last year, Allen’s group discovered similar cold molecular clouds in the Andromeda Galaxy.
The motion of stars around the centres of disc-shaped galaxies such as our own indicates that the galaxies contain much more mass than is visible as stars or glowing interstellar gas. Astronomers have long sought this hidden or “dark” matter, along with other dark matter between galaxies.
Allen says that if the cold clouds are similar to warmer molecular clouds in the inner galaxy, they should account for at least a third of the previously hidden mass thought to be 33 000 to 39 000 light years from the centre of our Galaxy. This is half as much mass as is contained in stars at that distance.
In earlier surveys of interstellar regions, astronomers observed emissions from carbon monoxide, which has a strong spectral line at a wavelength of 2·6 millimetres, making it much easier to detect than the more abundant molecular hydrogen, which only emits in parts of the spectrum difficult to observe. This approach did not find many molecular clouds farther from the galactic centre than the Sun, possibly because it could not detect clouds that were cooler than about 10 Kelvin (-263°C).
To find such cold clouds, Allen and his French colleagues looked for absorption caused by carbon monoxide in the spectra of two radio sources near the galactic plane. They found one absorption line in the spectrum of one source, and four or five in the second, suggesting that there are several gas clouds on the path from the radio source to Earth.
Little is known yet about the clouds, which hover close to the galactic plane. The temperature of one is about 3·5 K, only slightly above the 2·7 K of the cosmic microwave background left over from the big bang, which permeates all of space. “It is especially interesting that some of these clouds are entirely molecular, with virtually no detectable atomic gas”, says Allen.
Most cold clouds appear to be 33 000 to 39 000 light years from the centre of our Galaxy, with one possible cloud at 51 000 light years, just beyond the galactic rim. Allen believes that more detailed observations will reveal yet more cold gas in our Galaxy.
Jeff Hecht. New Scientist, 1994
Vocabulary and Comprehension Exercises
I. Translate these into your own language:
• missing mass
• the edge of the galactic disc
• the centres of disc-shaped galaxies
• much more
• glowing interstellar gas
• Astronomers have long sought .....
• molecular clouds
• inner galaxy
• along with
• the previously hidden mass
• half as much as
• absorption line
• to hover close to smth.
• big bang
• beyond the galactic rim
II. Form nouns from the following verbs and translate them into your own language:
to absorb, to radiate, to operate, to discover, to observe, to emit.
III. Give the situations from the text in which the following expressions are used:
• galactic disc
• interstellar gas
• “dark matter”
• account for
• interstellar region
• the galactic centre
• the galactic plane
• atomic gas
• the galactic rim
IV. Comment on the structure of the following sentences:
• If the cold clouds are similar to warmer molecular clouds in the inner galaxy, they should account for at least a third of the previously hidden mass thought to be 33 000 to 39 000 light years from the centre of our Galaxy.
• Astronomers observed emissions from carbon monoxide, which has a strong spectral line at a wavelength of 26 millimetres, making it much easier to detect than the more abundant molecular hydrogen, which only emits in parts of the spectrum difficult to observe.
V. Arrange the items of the plan in a logical order according to the text.
• Earlier surveys of interstellar regions.
• The probable form of the missing mass in our Galaxy.
• The clouds hovering close to the galactic plane.
• The contents of the galaxies.
• Gas clouds on the path from the radio source to Earth.
VI. Finish the following sentences:
• Molecular gas reveals itself only by ..... .
• The galaxies contain much more mass than ..... .
• Astronomers have long sought ..... .
• Molecular hydrogen emits in ..... .
• There are several gas clouds on the path ..... .
• Little is known about the clouds, which .... .
• Some of the clouds, which hover close to the galactic plane are entirely ..
VII. Answer the questions:
• In what form may some of the missing mass in our Galaxy be?
• How does cold gas reveal itself?
• What did Allen’s group discover?
• What does the motion of stars around the centres of disc-shaped galaxies indicate?
• What have astronomers long sought?
• What should the cold clouds account for?
• What did astronomers observe in earlier surveys of interstellar regions?
• Were any molecular clouds found farther from the galactic centre than the sun?
• What did Allen and his group look for to find cold clouds? What did they find?
• Is it known much about the clouds which hover close to the galactic plane?
• What is the structure of some of these clouds?
VIII. Write a summary in English (or in your own language).
• Give each paragraph a suitable title in English (or in your own language).
• Develop the titles of the paragraphs into topic sentences. Join the topic sentences together.
• Re-read your summary and make sure that the sentences are presented in a logical order.