Aug. 05--It's not every day you get to watch a galaxy being born. 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 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, and stretches about 400,000 light-years across, making it roughly four times as wide as our Milky Way. It's connected to a long filament that scientists believe is a strand of the cosmic web, the vast network of matter and dark matter that gives the universe its structure. In this network, at the nodes where the web's strands intersect, hang giant clusters of galaxies.
The emergence of the cosmic web and the birth of galaxies are thought to be interrelated -- and 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. Some researchers say that galaxies formed out of gas falling in straight from the intergalactic medium, but this would mean slower growth -- which doesn't fit with what astronomers see happening in the young universe. 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 didn't rain down on these protogalaxies, but instead was pumped straight in 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 to spin them up. But until now, evidence for this idea has been sorely lacking.
The strand of cosmic web, described in 2014 near two brightly shining quasars, provided that much-needed evidence. 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, allowing them to tease out more details on 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 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.... It's pretty solid evidence," Martin 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."
Got a little stardust in your eye? Follow @aminawrite for more interstellar science news.
