If you peer into the deep reaches of time and space to glimpse the universe when it was just a few billion years old, you’ll see an ancient era populated by many massive galaxies. Simulations suggest these galactic behemoths must have been fed by cold gas in dark matter filaments—structures that make up the cosmic web that connects galaxies in the universe—but the nature of these gas infusions has remained murky in the absence of direct observations.
Now, scientists led by Hai Fu, an associate professor of astronomy at the University of Iowa, have spotted what they describe as a “pipeline” gas filament feeding an enormous galaxy that formed when the universe was 2.5 billion years old, about one fifth of its current age.
The discovery, which was years in the making, confirms long-standing models that suggest star-forming material is delivered to huge galaxies via these cosmic filaments, according to a study published on Wednesday in the Astrophysical Journal.
“This is by far the best evidence we have” for the mysterious gas streams, said Fu in an email. While previous studies have detected possible filaments in the past, they weren’t able to capture detailed chemical information “to support their origin as inflows,” he added.
Fu and his colleagues, in contrast, were able to identify the chemical signatures of the gas stream in the galaxy they studied, thanks to the very rare and fortuitous alignment of giant luminous bodies around it. The galaxy, which is known as SMM J0913, is part of a larger cosmic neighborhood that contains two radiant quasars, which are special galactic cores that are among the most brightest phenomena in the universe.
Because the two quasars are located behind SMM J0913 from our perspective on Earth, the brilliant objects backlight the foreground galaxy, enabling Fu’s team to view never-before-seen details of the gas stream that nourishes this growing entity.
“The stream stood out in silhouette against two bright quasars,” Fu explained.
Using spectral information captured by the Atacama Large Millimeter/submillimeter Array (ALMA), currently the largest radio telescope on Earth, the team probed the abundance of chemicals in the filament.
The results showed that the stream lacked heavy elements such as aluminum, carbon, iron, and magnesium. Since the environment inside massive galaxies like SMM J0913 is constantly being enriched with heavy elements, which are spewed out by the explosion of stars, this telltale hint demonstrated that the gas was getting piped in from outside of the galaxy, from a depleted environment.
Cosmic simulations have proposed that these narrow filaments can explain how cold gas gets pumped into galaxies without being disrupted by the hot atmospheric surroundings of such gargantuan entities. But actually detecting these filaments is a tough hurdle to clear; Fu and his colleagues pored over observations of 70,000 galaxies over a period of five years before they tracked down the perfect system that contained SMM J0913.
“The understanding of the system took many years, so it has been a gradual realization synthesizing data from various telescopes,” he said. He added that he was excited when “the final piece of the jigsaw puzzle was in place” and felt “a tremendous relief” when he received the spectral information from ALMA that confirmed SMM J0913 was the exact right type of galaxy needed to illuminate this question.
“It showed that I didn't waste the precious telescope time and we indeed found something interesting,” Fu added.
While the new study represents a breakthrough in our understanding of how massive galaxies formed in the early universe, there’s still plenty of work left to be done. The team only looked at two points along the stream, leaving room for follow-up studies to attempt to “see its full physical extent,” Fu said, which he hopes to attempt to do in March using the Keck Observatory atop Mauna Kea in Hawaii.
“In the long term, we would need to find more streams around other massive galaxies,” he concluded, “and I wonder how we can achieve that efficiently, using existing telescopes.”