Universe's ultraviolet background provides clues about missing galaxies

This movie follows the formation of galaxies with cosmic time, illustrating how ultraviolet radiation suppresses the formation of stars inside small galaxies. The left panel shows a simulation that includes such diffuse UV radiation as in the real Universe, where fewer smaller galaxies form. For comparison, the right panel shows what would happen in the absence of such radiation, with more small galaxies forming. (Credit: S. McAlpine/S. Berry)

fluorescence around galaxy

Galaxy UGC 7321 is surrounded by hydrogen gas, and as this gas is irradiated with UV radiation, it emits a diffuse red glow through a process known as fluorescence. This image shows the light emitted by stars inside the galaxy, surrounded by a red ring that represents the fluorescent emission induced by the UV radiation. (Credit: M. Fumagalli/T. Theuns/S. Berry)

Astronomers have developed a new technique to detect the cosmic ultraviolet background radiation that keeps the universe ionized and hot. Their findings provide clues as to why astronomers observe fewer small galaxies than are predicted by models that simulate galaxy formation.

Massive stars and accreting massive black holes in the nuclei of galaxies produce copious amounts of ultraviolet radiation. A fraction of these ultraviolet photons escape from the interstellar medium of the host galaxy into intergalactic space, building up an extragalactic ultraviolet background. This radiation keeps the bulk of ordinary matter in the universe ionized and at temperatures above 10,000 kelvins (17,500 degrees Fahrenheit), and that process regulates star formation in dwarf galaxies.

Until now, however, it has been impossible to obtain an accurate measurement of this invisible radiation. The international team of researchers that solved the problem includes Piero Madau, professor of astronomy and astrophysics at UC Santa Cruz, and was led by former UCSC graduate student Michele Fumagalli (now at Durham University). The researchers found a novel way to measure the ultraviolet background light by using the Multi-Unit Spectroscopic Explorer (MUSE) on the Very Large Telescope (VLT) of the European Southern Observatory (ESO).

Hydrogen gas absorbs a fraction of this ultraviolet light and converts it into visible red light, a process called fluorescence. Fumagalli and his colleagues pointed MUSE toward a nearby spiral galaxy called UGC 7321, which is known to contain plenty hydrogen gas in its outskirts, and detected a faint red fluorescent glow. The inferred ultraviolet light agrees remarkably well with predictions made by Madau, confirming the theory that the ultraviolet background is due to the integrated emission from accreting black holes.

The findings were published in the journal Monthly Notices of the Royal Astronomical Society.

The researchers said their method can be used to measure the evolution of the ultraviolet background through cosmic time, mapping how and when it suppresses the formation of small galaxies. The study could also help produce more accurate computer simulations of the evolution of the universe.

"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," Fumagalli said.