According to cosmological simulations, there should be a lot more small galaxies in the universe than those we see. This mystery, known as the dwarf galaxies problem, is difficult to explain, but an international team of researchers suggest that crucial clues might be hiding in ultraviolet light (UV).
In a paper, published in the Monthly Notices of the Royal Astronomical Society, researchers discuss a way to indirectly measure the UV background of the universe to explain what happened to the dwarf galaxies.
Young hot stars and black holes can emit a huge amount of UV light and this light can warm up surrounding gas. If the gas is too warm, it can’t condense and form new stars, and this might have stopped these small galaxies from forming lots of stars. The galaxies out there are simply too dark for us to see.
“Massive stars and supermassive black holes produce huge amounts of ultraviolet radiation, and their combined radiation builds up this ultraviolet background,” lead author Dr Michele Fumagalli of Durham University said in a statement. “Our research means we now have the ability to measure and map this UV radiation which will help us to further refine models of galaxy formation.”
Galaxies, especially young ones, are full of hydrogen gas. The gas absorbs the UV light and then re-emits it in visible light that can be detected from Earth. By measuring this red light, researchers believe that they can estimate the total UV background.
They tested it for galaxy UGC 7321, located 30 million light-years away. They were able to detect a fluorescent glow surrounding the galaxy and establish a value for the UV background. This is consistent with other estimates that were achieved with different methods.
“Ultraviolet radiation heats the cosmic gas to temperatures higher than that of the surface of the Sun," added co-author Professor Tom Theuns, also from Durham. “Such hot gas will not cool to make stars in small galaxies. This explains why there are so few small galaxies in the universe, and also why our Milky Way has so few small satellite galaxies.”
According to simulations, the Milky Way should be surrounded by about 500 small galaxies, but only about 50 have been observed so far.