Astronomers have identified a galaxy 2.9 billion light-years away that's vigorously pumping out ultraviolet radiation and resembles the ones that transformed the entire universe shortly after its birth. The finding could help scientists deepen their understanding of how, a few hundred million years after the big bang, similar galaxies busted up hydrogen atoms throughout intergalactic space.
The universe was born hot. It started off as a mix of electrons and positively charged protons and helium nuclei. But as the universe expanded, it cooled so much that, about 380,000 years after the big bang, the electrons and protons paired up, creating neutral hydrogen atoms. All that free-floating hydrogen would have soaked up the extreme ultraviolet light from the earliest stars. Then, a few hundred million years later, this radiation from those multiplying stars and galaxies ripped the electrons from the protons, "reionizing" the hydrogen gas. But no one knows exactly how that happened.
Only extreme ultraviolet photons, whose wavelengths are shorter than 912 angstroms (91.2 nanometers), pack enough energy to rip apart hydrogen atoms. Hot young stars emit this radiation, but neutral hydrogen gas absorbs it, preventing the ionizing radiation from escaping into intergalactic space. For the typical galaxy today, dust blocks most extreme ultraviolet light, and neutral hydrogen gas soaks up all but 1% of the rest. A 1% escape fraction is not enough to reionize the universe.
But astronomer Sanchayeeta Borthakur of Johns Hopkins University in Baltimore, Maryland, and her colleagues have used the Hubble Space Telescope to examine an unusual galaxy called SDSS J0921+4509 in the constellation Ursa Major that's pumping out large amounts of extreme ultraviolet light. By comparing how much radiation this galaxy emits above and below a wavelength of 912 angstroms, the researchers found that the galaxy has an escape fraction of 21%, as they report online today in Science.
"That's quite high," says astronomer Brian Siana of the University of California, Riverside, who was not involved in the work. "This is roughly the fraction that we think all galaxies in the early universe had to have in order to ionize the hydrogen in the intergalactic medium." Michael Rauch, an astronomer at the Carnegie Observatories in Pasadena, California, agrees that the observation "is what people were hoping for."
So what's the secret to how the galaxy pumps out so much extreme ultraviolet light? "Swiss cheese," Borthakur says. The galaxy is a starburst, containing a compact region less than 1000 light-years wide that spawns lots of hot stars whose light and "winds"—souped-up versions of the solar wind—punch holes in the haze of neutral hydrogen gas, giving it the structure of Swiss cheese. "The [extreme ultraviolet] photons escape through the holes," Borthakur says. The discovery suggests the galaxies that reionized the universe were also starbursts, although most were smaller. But those early galaxies had an advantage over this one: They had little light-blocking dust, because dust consists of heavy elements that stars make, and few stars had yet arisen at that early time.
Several years ago, the researchers had used the galaxy's spectrum to predict the large escape fraction they have now observed. "This is the first case where they've actually proven that this works," Rauch says. That's important, he says, because future telescopes will be able to obtain spectra of galaxies that existed during the epoch of reionization but will never detect their extreme ultraviolet light, because neutral hydrogen between them and us blocks it.
Curiously, if the galaxy were one of our nearest neighbors, the project would have failed. Astronomers don't detect the light at the wavelength that it was emitted but rather at the longer wavelength the light has been stretched to by the expansion of the universe. Had the Swiss-cheese galaxy been close by, that stretching would have been so small that neutral hydrogen gas in the Milky Way would absorb the galaxy's extreme ultraviolet light. However, the galaxy is so distant that its radiation has been stretched by 23.5%, enough to glide through the Milky Way's hydrogen unimpeded, making the measurement possible.