科学美国人60秒 SSS UV Rays Strip Small Galaxies of Star Stuff（在线收听）

UV Rays Strip Small Galaxies of Star Stuff 紫外线辐射会阻止小星系形成

The sun spews out ultraviolet radiation—that's why you put on sunscreen. But the sun isn't the only UV-producing celestial body. "Stars and supermassive black holes produce a huge amount of UV radiation." Michele Fumagalli, an astrophysicist at Durham University in the U.K. "And some of this radiation can escape a galaxy, and so this radiation builds up this cosmic UV background."

太阳会发出紫外线——这就是为什么需要涂抹防晒霜的原因。但是，太阳并不是唯一的可以产生紫外线的天体。恒星以及超大质量的黑洞也会产生大量的紫外线。Michele Fumagalli是英国杜伦大学的天体物理学家。这些紫外线和星系有着密切的联系，所以，这些紫外线可以建立宇宙紫外线的环境。

That cosmic UV background permeates the universe. But it's diffuse—meaning hard to measure, especially from here on Earth. "And so the way we do this measurement is with a little trick." That is, when UV radiation hits gas, it gives off a red glow. So Fumagalli and his team used what’s called the MUSE instrument at the Very Large Telescope in Chile to stare—for hours—at the edge of a super thin galaxy, until they saw that red glow.

这种紫外线渗透在宇宙各处。但是它是散漫的——这也就意味着很难测量，尤其是从地球上进行测量。“所以，我们测量也是用了一个小技巧”。也就是说，当紫外线照射到空气，就会散发出红光。所以，Fumagalli和他的团队使用了一个在智利的的缪斯望远镜，观测了一个超级稀薄的星系数个小时，直到看到了红光。

And since they knew how much gas was there, they were able to calculate the intensity of the UV radiation hitting the gas—the cosmic UV background. The finding is in the Monthly Notices of the Royal Astronomical Society.

因为他们知道气体的量，所以能够计算出紫外线辐射的强度——宇宙中的紫外线。该研究结果发表在《英国皇家天文学会月刊》上。

That calculation offered clues to another celestial mystery: why we don't see as many small galaxies in the universe as theory would predict. Fumagalli says the UV background radiation might strip away valuable star-forming gas from the small guys.

这种计算方法为另一个天体秘密提供了线索——为什么在宇宙看到的小的星系的数量没有预测的多。Fumagalli紫外线的背景阻止一些可见的小的恒星形成。

"This is what happens to the small galaxies in the universe. They have gas, which is what's needed to form stars. But if that gas is exposed to intense UV radiation, essentially it evaporates. Which means the galaxies lose their supply of fuel to make new stars. Meaning they don't shine and we do not see them."

“小星系在宇宙中发生了什么。恒星的形成需要气体。但如果气体暴露在强烈的紫外线中接受辐射,实际上气体就会蒸发。这意味着星系失去燃料供应形成新的恒星。这意味着它们不发光,我们看不到它们。”

So the small galaxies remain dark. But bigger galaxies have a much smaller surface-to-volume ratio of star-forming gas—imagine a whole ocean of water compared to a tiny glassful. UV radiation can't strip away all the gas. So stars form. Like our sun. Which, when it comes to ultraviolet rays here on Earth, is still the star of the show.

所以，小的星系是黑暗的。但是较大的星系，与形成星系的气体的比率，具有更小的表面——想象一下，一小杯水和整个大海的对比。但是紫外线辐射不会蒸发的所有的气体，所以，恒星就形成了。例如我们的太阳。当涉及到地球上的紫外线时，太阳才是中心焦点。