The Little Galaxies That Couldn't

Ultraviolet (UV) light doesn’t just give you a sunburn! In space, it also illuminates and sculpts giant clouds of gas, creating the iconic star forming regionsmade famous by the Hubble Space Telescope. And in the early Universe, UV radiation may be responsible for preventing the smallest galaxies from taking shape.

UV light is shorter in wavelength and higher in energy than light our eyes can detect. Its energetic nature makes it damaging to skin cells and enables it to erode the clouds of gas from which stars form. Astronomers suspect that UV may also be energetic enough to strip small galaxies of the gas they need to form stars, stunting their growth and leading to the paucity of small galaxies we see nearby. (Big galaxies like our own Milky Way have enough stuff to withstand the UV onslaught.)

An international team of astronomers led by Michele Fumagalli at Durham University has announced a way to measure the UV background of the Universe—the residual UV radiation bouncing around the cosmos making it difficult for little galaxies to grow into big ones.

In the same way UV light causes the aforementioned star-forming regions to glow, it can also cause the tenuous gas around a galaxy to emit a faint glow—to fluoresce. (And yes, as the name suggests, that’s the same process that causes a fluorescent light to glow.) Indeed, as Fumagalli explains, “Massive stars and supermassive black holes produce huge amounts of ultraviolet radiation, and their combined radiation builds up this ultraviolet background. This UV radiation excites the gas in the Universe, causing it to emit red light in a similar way that the gas inside a fluorescent bulb is excited to produce visible light.”

Specifically, the team looked around large galaxies (for example, “superthin” galaxy UGC 7321 seen in the image above) to see how the UV light interacts with the halos of gas surrounding them (with the results indicated by the red outline around UGC 7321). The team measured the temperature of the UV-heated gas—turns out it’s hotter than the surface of the Sun! Which helps explain why smaller galaxies can’t form in this kind of UV environment—they can’t handle the heat!

UGC 7321 is just one galaxy among the hundreds of billions in the observable universe, so the next task is to make more measurements around different galaxies, in hopes of creating a map of the UV background that Fumagalli describes. We can expect the amount of radiation to vary in space and time, so that’s a lot of mapping! But the results can help inform our understanding of the early Universe, when galaxies were still taking shape.

Astronomers use computers to simulate this epoch of galaxy formation—using the laws of physics to describe how the Universe changed over time. Such simulations require many parameters (including the amount of UV radiation) as inputs, and as they say, garbage in, garbage out. So new estimates of the UV background should lead to better computer simulations and a clearer understanding of how the Universe took shape… Including understanding why we see so few small galaxies close to home.

Image: M. Fumagalli/T. Theuns/S. Berry Pictures