Ghostly galaxies in the distant universe are almost certainly the culprits behind a mysterious change in intergalactic gas that allows us to see across the cosmos. Although these galaxies are too faint to be spotted by current telescopes, future instruments could soon reveal their presence.
About 300,000 years after the big bang, the hydrogen that filled the universe cooled and became neutral and opaque, plunging everything into the so-called cosmic dark ages. Any visible wavelengths from early stars were quickly absorbed by the gas, which formed a cosmic fog that persisted for almost a billion years.
Some type of radiation must have broken up the neutral hydrogen atoms into electrons and protons in a process called reionisation, which ultimately made the universe transparent. But whether galaxies would have been numerous and bright enough back then to produce this radiation was uncertain.
Now the latest observations from the Hubble Ultra Deep Field 2012 survey (UDF12), presented this week at a meeting of the American Astronomical Society in Long Beach, California, have suggested that galaxies could, indeed, have turned the universe clear.
“This is the last uncharted piece of cosmic history,” said UDF12 team member Richard Ellis of the California Institute of Technology in Pasadena, California.
Ellis and colleagues used the Hubble Space Telescope to stare at one spot in the sky for 100 hours – twice as long as in previous surveys – and used a filter that made the telescope more sensitive to faint, distant objects. “For the first time with Hubble, we can do this in a systematic way,” Ellis says.
In December the team reported that they had spotted seven new galaxies hailing from when the universe was between 380 million and 600 million years old, right in the middle of the period when reionisation was under way. Since then, the team has analysed the radiation from these galaxies.
Using spectral colour as a yardstick of stellar age, James Dunlop and Alexander Rogers of the Institute for Astronomy in Edinburgh, UK, found that the UDF12 galaxies contain surprisingly old stars. “What we’re seeing are the second generation of stars,” Rogers said at the meeting. “They’re already mature – and must have been around for 100 million years.”
Older stars do not pump out as much ionising radiation as young ones, so these galaxies, at the limit of Hubble’s vision, could not have done the job by themselves.
The team then needed to figure out how many faint galaxies from this era may have gone undetected. Caltech’s Matthew Schenker and colleagues used statistical modelling, based on known galactic populations, to show that there must be exponentially more faint galaxies in the early universe than bright ones – enough to supply the radiation needed.
“We can say confidently that galaxies can do the job, but the faintest galaxies that do most of the work are just below the limits of the UDF12 project,” said team member Brant Robertson of the University of Arizona in Tucson.
“We’re pretty certain it’s galaxies now,” agrees Steven Finkelstein of the University of Texas at Austin, who was not involved in the new work. Other possible candidates for reionisation, such as colliding dark matter particles, had been all but ruled out by earlier observations.
“I think it’s a happy ending,” Ellis says. “Reionisation is a normal process produced by things we can see, and not yet another dark something that we don’t understand.”
The ghost galaxies will probably be detected by Hubble’s successor, the James Webb Space Telescope, which is expected to launch later this decade. If James Webb does not manage to see them, that would present a puzzle, says Ellis. “We’d need an additional source of radiation, whether annihilating particles or whatever else.” But he said he would be very surprised if the faint galaxies did not turn up.