Researchers measured the intensity of the universe's ultraviolet background radiation, and say it may be strong enough to strip small galaxies of star-forming gas. Christopher Intagliata reports. 

据克里斯托弗·因塔利亚塔（Christopher Intagliata）报道，研究人员测量了宇宙紫外线背景辐射的强度，并指出它的高强度可能破坏形成恒星前期的星云气体

撰文\播音：克里斯托弗·因塔利亚塔（Christopher Intagliata）

翻译：杨枭

审校：丁可含

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."

太阳放射紫外线，这也是你要擦防晒霜的原因。但是太阳并不是唯一产生紫外线的天体。米歇尔·富马加利（Michele Fumagalli）是英国达勒姆大学的天体物理学家，他说：“恒星和超大质量的黑洞会产生大量紫外线辐射。一些辐射会逃离星系，构成了宇宙的紫外线背景。”

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 superthin galaxy, until they saw that red glow.

广阔的紫外线背景渗透到宇宙各处。但因为它是漫反射而很难测量，尤其是从我们地球上。“所以我们用了些小技巧来测量。”那就是，当紫外线射到气体时，会发出红光。因此，富马加利和他的团队在智利的超大型望远镜上使用了仪器缪斯（MUSE）,它日以继日地观测着超级星系的宇宙边缘，直到他们看到红光闪烁。

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. [Michele Fumagalli et al., A measurement of the z = 0 UV background from Hα fluorescence]

而且由于他们知道了有多少气体，就能够计算出紫外线照射气体的强度——宇宙紫外线背景。这个发现发表在了皇家天文学会的月刊。

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. 

这项计算测量给另一个天体谜团提供了线索——我们为什么看不到像理论预测那样多的小星系。富马加利说，紫外线背景辐射可能击破恒星形成气体的小星系。

"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."

“这就是宇宙中小星系发生的情况。小星系中有恒星赖以形成的气体。但是如果气体被强烈的紫外线照射就会消失。那样的话，星系将失去制造新星的燃料供给。这意味着它们不再发光，所以我们就看不到了。”

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.

因此，小星系保持着黑暗。但是较大的星系有较小的气体的表面积体积比——类似于海洋与一杯水的比例。紫外线辐射并不能摧毁全部气体，所以恒星还是会形成。比如我们的太阳。而当紫外线照射到地球这里，它仍然是恒星的秀场。