Not from around here, are you?: Decoding the signal from the furthest galaxy

Arnab Chakraborty, a postdoctoral researcher at the Trottier Space Institute of McGill University, and Nirupam Roy, an associate professor of physics at the Indian Institute of Science (IISc), Bengaluru, used data from the Giant Metrewave Radio Telescope (GMRT) in Pune to detect a signal from a distant star-forming galaxy 8.8 billion light years away.

Their findings made news around the world, and offer intriguing clues to what the universe was like in its early years. But it might not even have happened, if another galaxy hadn’t come to their aid.

First, a look at their findings. In their report, published in Monthly Notices of the Royal Astronomical Society, the physicists explain that the signal originated from the galaxy (known as SDSSJ0826+5630) when the universe was only 4.9 billion years old. It was emitted, of course, about 8.8 billion years ago.

“We were looking for a signal from this galaxy, so that we could study its gas content, and better understand the properties of other early galaxies,” Chakraborty says.

What they found is a high concentration of atomic hydrogen, which is hydrogen in the form of individual atoms. This is a highly reactive form of the gas, unlike the hydrogen molecule. And atomic hydrogen exists where stars are born; it is the basic fuel for star formation.

Given the high concentration of atomic hydrogen, it is clear that this was a galaxy of intense, immense star-forming activity, Chakraborty says.

Capturing the signal was an adventure in itself, a bit like playing a videogame.

This kind of radiation, known as the 21 cm signal, for its length, is emitted by galaxies when the atomic hydrogen present goes through levels of transition, before the formation of stars. Because of the distance it travels, the wavelength of the signal increases and the frequency decreases (a phenomenon known as redshift), making it almost invisible to most telescopes. This is why the scientists chose GMRT, an array of 30 massive steerable antennae.

“Since we knew the redshift of the galaxy, we could theoretically calculate how that 21 cm line would travel from the source to the telescope. GMRT is the one of only a few radio telescope in the world sensitive to this frequency,” says Chakraborty.

Even so, they might have missed it. Another ancient galaxy came to the rescue.

The scientists were aided by a phenomenon called gravitational lensing, in which light is bent by the presence of another massive body, in this case likely an early elliptical galaxy. This other galaxy was so massive that it served to magnify the signal like a lens.

“In this case, the magnification was by a factor of about 30,” Chakraborty says. “This allowed us to spot it in a universe of redshifted signals.”

What’s next? The obvious question, Chakraborty says, is where did the hydrogen come from. Answer that one, and we are another step closer to understanding the origins of the universe.