Astronomers catch a galaxy being born, fed by a strand of cosmic web

Astronomers catch a galaxy being born, fed by a strand of cosmic web
A filament of the cosmic web (outlined with parallel curved lines) can be seen funneling cold gas onto the protogalaxy (circled). The red side of the disk is rotating away from us, while the blue side is rotating toward us. The velocity of gas within the filament matches the blue side of the rotating disk. (Christopher Martin / Caltech)

It's not every day scientists get to watch a galaxy being born. But thanks in part to the bright light of a quasar, astronomers say they've identified a swirling disk of gas that's a galaxy in the making — as well as the strand of the universe's cosmic web that's been feeding it.

The protogalactic disk, described this week in the journal Nature, gives scientists an unprecedented glimpse into the development of early galaxies, as well as a window onto the structure of the cosmos.

"What it tells us about galaxy formation and the cosmic web is that they're tied together," said Caltech astrophysicist Christopher Martin, the study's lead author. "They're intimately related, and we believe now that this is the first example of gas coming directly from the cosmic web and forming this giant disk."

This galaxy-in-the-making sits roughly 10 billion light-years away in the constellation Cetus. It stretches about 400,000 light-years across, making it about four times as wide as our Milky Way.

The infant galaxy is connected to a long filament that scientists believe is a strand of the cosmic web, the vast network of ordinary matter and dark matter  that gives the universe its structure. In this network, giant clusters of galaxies hang at the nodes where the web's strands intersect.

The emergence of the cosmic web and the birth of galaxies are thought to be interrelated. Understanding one could offer researchers insight into the other.

Scientists have long debated how galaxies began developing out of the intergalactic medium, the diffuse hydrogen that bridges the vast spaces between the galaxies and makes up the cosmic web.

Some researchers say that galaxies formed out of gas that entered halos of dark matter and gradually fell in toward the center. But this would mean slower growth — which doesn't fit with what astronomers have observed in the universe's early days. Within a few billion years of the big bang, many galaxies were going gangbusters, pumping out bright new stars at an eye-popping rate.

Another theory holds that hydrogen was pumped into these protogalaxies by pipelines from the cosmic web. The web's strands act a little like garden hoses, funneling gas into the center of these growing galaxies and even imparting angular momentum, helping them spin. But until now, evidence for this idea has been sorely lacking.

The strand of cosmic web Martin and his colleagues studied provided that much-needed evidence.

This particular strand, located near two brightly shining quasars, was first described in 2014. At the time astronomers wondered why the filament was unusually bright — more than 10 times as bright as a typical strand.

Using the Cosmic Web Imager at Palomar Observatory, Martin and his colleagues were able to get a spectrum, or light fingerprint, from each pixel in their images. That allowed them to tease out more details about the nature of the structure, including its motion.

They soon discovered the reason it was so bright: The cosmic filament was connected to an enormous disk of gas that easily reflected light from one of the nearby quasars.

"The quasar's just helping us see it. Nice of it to do that," Martin said with a laugh.

By looking at the squeezing and stretching of the wavelengths of light coming from the disk, the researchers were able to tell that half of the disk was moving away from us and half was moving toward us — indicating it was spinning, as a galaxy would. And the motion of the gas from the filament appeared to match the speed of the half of the swirling structure that was moving toward us, indicating it was feeding the protogalaxy.

"It's the smoking gun," Martin said. "It's pretty solid evidence."

The findings allow scientists to further probe the nature of galaxy formation in the early epochs of the universe's history, said UC Irvine cosmologist James Bullock, who was not involved in the study but whose theoretical predictions made with a former graduate student were validated by the discovery.

"The fact that they've actually seen it makes me pretty excited," he said.

Follow-up research could shed more light on galaxy formation and on the nature of dark matter, the mysterious stuff that can't be seen or touched but that so outweighs the normal matter in the universe that its gravity essentially defines the structure of the cosmos.

"It's an extraordinary opportunity to really see what's going on when galaxies form," Martin said. "And that almost never happens."

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