Still-forming galaxy spotted near the dawn of the universe

Astronomers using a giant radio telescope in the Atacama Desert of Chile have sighted the star-forming hotspot in a lopsided galaxy that was still growing when the universe was just 800 million years old.

The findings, published in Monthly Notices of the Royal Astronomical Society, shed light on a time in the history of the cosmos that has been shrouded in mystery.

The formation of the earliest galaxies is somewhat hidden from view because the early universe was full of a soup of neutral hydrogen gas that absorbed the ultraviolet light being emitted by these young stars. But that stellar radiation also helped to slowly clear away the haze by ionizing it, a process that started only a few hundred million years after the Big Bang, when the universe was born, and lasted until around the billion-year mark. This period is known as the epoch of reionization. Back then, the universe was still a kid; today it’s roughly 13.8 billion years old.  

Scientists want to look back in time and understand this epoch in the universe’s history, because learning how these early galaxies formed could shed light on the evolution of the cosmos. They do this by using very powerful telescopes.

In some ways, a telescope is a little like a time machine, because the images it captures actually allow you to observe the very distant past – the more distant the object you observe, the further back in time you can see. (For example, if you were to take a snapshot of the sun, you’d be looking at the sun as it was eight minutes ago, because it takes the light about 8 minutes to travel roughly 93 million miles to reach Earth.)

Because the universe is expanding, the wavelengths of ultraviolet light get stretched longer and longer until they’re in the infrared. So astronomers used the European Southern Observatory’s Atacama Large Millimeter/submillimeter Array (ALMA) in Chile, which can detect infrared light, to look for signs of these galaxies.

Many researchers have focused on extremely bright objects in the distant past that would be easier to see, such as galaxies with extremely high rates of star formation. But for this paper, the scientists wanted to examine a dimmer, more "normal" galaxy – the kind that would have been far more common and would have played a key role in reionizing the hydrogen-filled universe.

“In contrast to many previous millimeter observations of high redshift galaxies, which have targeted galaxies with extreme star formation rates (100 – 1000 [solar masses per year]),” the study authors wrote, “the galaxies in our sample have [star formation rates of 5 – 15 solar masses per year], more typical of the bulk of galaxies at these early epochs.”

Rather than simply search for starlight, they looked for the glow from an ionized form of carbon called C II, because scientists can use this signal to locate the cold gas out of which new stars begin to condense.

In a galaxy called BDF 3299, researchers found that the C II signal was coming from one end of the galaxy, rather than from the middle of it.

“These results suggest that molecular clouds in the central parts of primordial galaxies are rapidly disrupted by stellar feedback,” the scientists wrote. “As a result, [CII] emission mostly arises from more external accreting/satellite clumps of neutral gas. These findings are in agreement with recent models of galaxy formation.”

The results shed fresh light on the complex internal structure within these dynamic, growing galaxies, where the powerful radiation from these young stars – and the explosions as some of them go supernova – push the star-forming clouds off-center.

 “Our ALMA observations are directly probing such early phases of galaxy formation, in which both stellar feedback and gas accretion are at work,” the authors wrote.

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