How leaky galaxies brought the universe out of cosmic dark ages
Astronomers say they have found a compact star factory that is shedding its own light on the structure and processes inside the earliest galaxies responsible for illuminating the universe.
A galaxy some 3 billion light-years from Earth is opening a window on the role budding galaxies in the early universe played in lighting up the cosmos after 400 million years of darkness.
Inside the galaxy, known simply as J0921+4509, clouds of cold hydrogen gas are producing massive, young stars at a furious pace in a tightly packed central region – similar to the budding galaxies forming early in the universe's history.
These stars radiate intense levels of extreme-ultraviolet light, which can ionize the hydrogen gas, turning it from an opaque fog into a medium through which light can travel.
Most of this ultraviolet radiation never leaves the galaxy. It's intercepted by dust as well as by dense clouds of cold hydrogen gas between stars, which forms an almost impenetrable barricade.
Almost, but not quite. A team led by Johns Hopkins University astronomer Sanchayeeta Borthakur has found that after adjusting for the effects of dust, about 21 percent of the extreme-ultraviolet light produced in this galaxy's core is escaping through the hydrogen fog. Given a sufficient number of budding galaxies 13.4 billion years ago, that would be enough to begin clearing the dense fog of neutral hydrogen gas that permeated the cosmos after it cooled following the intense release of energy that formed it, known as the Big Bang.
The team's measurements, set to appear in Friday's issue of the journal Science, also appear to have validated an indirect way to find and gauge this leakage of ultraviolet radiation – an approach that that could be useful in identifying leaky galaxies responsible for brightening the cosmos, galaxies too faint and distant to allow direct detection of escaping ultraviolet light.
Such measurements are vital to understanding how the universe went from dark to bright in a period researchers have dubbed re-ionization. Star forming galaxies would have had the collective energy to do the job. But by some estimates, the leakage rates for extreme-uv radiation would have had to have been around 20 percent. By contrast, only about 1 to 2 percent of the Milky Way's ionizing UV escapes the galaxy.
The team's measurements grew out of project that proposed the indirect approach to detect the ultraviolet leaks, explains Dr. Borthakur. That approach, published in 2011, found four relatively close, tightly packed star-burst galaxies that appeared to be leaking, out of eight surveyed. The new results from J0921+4509 represent the first of these to receive closer scrutiny to see if the indirect approach works.
Using the Cosmic Origins Spectrograph bolted to the Hubble Space Telescope to make the extreme-uv measurements, the team pieced together a picture of the process driving the leaks, one that dovetails with theories of how the process might work.
In essence, stars shining in the galaxy's densely packed center – collectively tipping the scales at several billion times the mass of the sun and crammed into a space about 326 light-years across – are generating powerful outflows of charged particles known as stellar winds. This, combined with the pressure of the stars' radiation itself over time, punch holes in the dense molecular clouds, allowing the ionizing radiation to escape.
Borthakur, who was a member of the team that proposed the indirect to check for UV leaks, notes that at the time, the team predicted that about 25 percent of J0921+4509's ionizing radiation was leaking.
When the team made the direct measurements and found the leak to be 21 percent, "we were surprised and delighted," she says. "It's not very often your numbers match what you predicted two years back."
The ability to study relatively nearby analogues to the earliest star-burst galaxies is a big plus in teasing out the mechanisms driving re-ionization, notes Brian Siana, an astronomer at the University of California at Riverside, who is studying more-distant compact star-burst galaxies with an eye toward testing theories of cosmic re-ionization.
But using near-by galaxies as analogues for the earliest galaxies is tricky, he cautions.
"Galaxies have changed a lot over the last 13 billion years," he says. "In some respects, we're pretty confident that this is not like the typical galaxy that re-ionized the universe."
For instance, the estimated annual star formation rate for J0921+4509 makes it 100 times brighter than the typical galaxy re-ionizing the early universe, he says.
Still, he says, a lot can be learned from near-by analogues.
As for the indirect leak-check for galaxies, "it is very powerful. With the next generation of telescopes, we're going to be able to do this in great detail" for far more-distant galaxies.
Indeed, officials broke ground for one of these telescopes Oct. 7 – the Thirty Meter Telescope, a $1.4 billion eye on the sky slated for Hawaii's Mauna Kea in Hawaii.