Gigantic Protogalaxy in the Cosmic Web

Astronomers have found that a massive filament of gas in the early universe actually seems to be a humongous, galaxy-forming disk.

Galaxy formation is a complicated affair. It can involve big smashups and nonchalant snacking, but as I explain in my feature article in S&T’s September issue, that’s only a slice of the story. One way that galaxies grow — and possibly the predominant way in the early universe — is from cold gas funneled like a pipeline into wells of dark matter.

cold-gas accretion simulation

This snapshot from a computer simulation shows a protogalaxy (central disk) growing by accreting cold gas from cosmic filaments (blue streams) in the early universe. Over time, hot gas (red) chokes off the filamentary inflow. The protogalaxy shown here is only one-fifth as wide as the disk recently discovered in the early universe, but the overall properties (given the scaling up) are similar.
Oscar Agertz

These dark matter wells are dense filaments in the weblike cosmic structure, along which galaxies form. Computer simulations suggest that cold-gas accretion was a big deal in the universe’s first couple billion years. After that, the halo of gas around a forming galaxy generally grew so hot that it choked off the pipeline (although not necessarily entirely). But observing cold-gas accretion in action is tough, because the gas is fairly diffuse and faint.

Last year, astronomers detected a large, bright filament of gas called UM 287 shining at us from about 11 billion years ago. It’s bathed in the glow — well, vicious radiation — of a nearby quasar, whose supermassive black hole spawns an ultraviolet beam that illuminates the filament and makes it fluoresce. At the time, the team estimated that the cosmic filament was about 10 times more massive than expected, given simulation results.

But it turns out the filament isn’t too massive, and for an interesting reason.

Christopher Martin (Caltech) and colleagues took a second look with the Palomar Cosmic Web Imager, a high-tech spectrograph they built and installed on the 5-meter (200-inch) Hale Telescope on Palomar Mountain in California. The spectrograph homed in on a particular wavelength called Lyman-alpha, which comes from cold neutral hydrogen that’s been irradiated by ultraviolet light. Shifts in the Lyman-alpha line toward redder or bluer wavelengths indicate the gas is moving along our line of sight. By analyzing the filament’s spectra, the team discovered that one part of the filament is moving toward us, while another section is moving away from us. In other words, the filament isn’t merely a filament: it’s a fuel line feeding a gigantic disk.

Big Disk in the Early Universe

The disk is huge: it’s about 400,000 light-years across, or three to four times the size of the Milky Way’s spiral disk. The rotational velocity suggests it’s sitting in a halo of 10 trillion solar masses’ worth of dark matter, 10 times larger than the halo our galaxy inhabits, the team reports August 5th in Nature. There’s even a hint of star formation in its center, but the team isn’t sure of that yet.

“Overall, it’s hard to say with certainty that they’re definitely seeing a cold-flow disk — as opposed to some other phenomenon that just happens to look like a cold-flow disk,” says Kyle Stewart (California Baptist University), whose team has simulated the growth of these objects. “But when you look at all the observable properties of cold-flow disks from the simulations to determine what they should look like in the real universe, in my opinion, it’s amazingly similar to what these authors have just observed.”

One notable parallel is the disk’s high rotation speed: about 500 km/s (1 million mph), twice that of the Milky Way’s disk. That supports the idea that the disk is forming from cold gas channeled in via the filament it's grafted onto, and not from hot gas falling in from all sides.

You can think of the difference like two ways of filling a tub with water. The dark matter well is like a colossal, circular tub. The tub is spinning slowly, because the dark matter’s angular momentum was conserved as gravity collapsed it into a big clump. If infalling gas were hot and flowing in from all sides, the gas would spin slowly with the dark matter, like water gently flowing into a big rotating bowl. But if you shoot the water into the tub from a hose, it’ll whisk around the tub’s sides much faster than the tub itself is spinning. Theorists think the cold flow shoots in with a lot more spin to it because the cold stream is concentrated, and thus more susceptible to the dark matter’s tidal forces, Martin explains.

Since it’s so massive so early in the universe, the disk likely grew into an elliptical, Martin suggests. Depending on what coalesced around it, maybe today it’s even sitting at the center of a big galaxy cluster.

 

Reference: D. C. Martin et al. “A giant protogalactic disk linked to the cosmic web.” Nature. Online August 5, 2015.


Want to know more about how galaxies grow? Pick up a copy of Sky & Telescope's September 2015 issue and read our in-depth feature.