Galaxies are pretty well understood on the whole. We know they’re made mostly of dark matter, we know how they move and rotate, and have grouped them into several classes. We even know a bit about the supermassive black holes that reside at their center. But one aspect of galaxies has remained vague: how they formed in the first place.

This is an important question, not only for the obvious reasons—we live in a galaxy, and we tend to be interested in our own origins—but also because the processes that formed galaxies play into the larger scale structure of the Universe. In the “cosmic web,” each fiber is made of many galaxies, and understanding the fine details of what’s going on within those fibers can help us better understand the web’s behavior as a whole.

One hypothesis about the formation of galaxies on this web is known as the “cold flow” model. It holds that galaxies form from flows of matter that are relatively cold, at about 10,000 kelvins. This diffuse material would stream along the web’s dark matter filaments, some of it ultimately getting stuck at the intersections among them. There, the matter would be attracted to dark matter halos and stream into them, forming into spinning disks of gas and dust—baby galaxies.

Tantalizing glimpse

Last year, some potential evidence turned up in support of this hypothesis. A bright cosmic filament was imaged ten billion light-years away and thus we see it as it was 10 billion years ago, early in the Universe’s 13.8 billion year history. It houses a flow of gas and dust into a system with two quasars (bright, shining systems, each fueled by a supermassive black hole). One of the quasars is near enough that its light acts like a flashlight, illuminating the flow for us to see.



The material appears to be flowing into a very bright object, which the researchers suggested might be a protogalactic disk, a galaxy in the process of forming. But the image alone wasn’t enough to confirm if that’s what’s actually happening.

For that, a more detailed analysis was needed, the kind that can be obtained with spectroscopy. Researchers can tell how objects are moving in space by looking at the wavelengths of light coming from them—their spectra. And if the bright object really was a spinning disk, then one side of it should be moving toward us and the other side should be moving away, producing two different Doppler shifts in the light.

Flowing down the web

Intrigued by the tantalizing image, a group of researchers has returned to the same object to perform that analysis on. Using the Palomar Cosmic Web Imager (CWI), the researchers were able to observe the system in hundreds of different wavelengths simultaneously. This observation revealed information not only about the system’s motions but also about the distribution and composition of mass in the system.

Metals The disk isn’t forming stars on its outer edges and hasn’t been for the past 100 million years. Part of this is likely because of the disk’s proximity to the bright quasar that illuminates it. That quasar’s light could be preventing some of the star formation by disrupting the gas that would otherwise collapse to form stars. But that alone isn’t enough by itself to have halted star formation, and the researchers suggest that this means the newborn galaxy has a low metal content (In Astronomy, ‘metal’ just refers to any elements above helium on the Periodic Table). The disk isn’t forming stars on its outer edges and hasn’t been for the past 100 million years. Part of this is likely because of the disk’s proximity to the bright quasar that illuminates it. That quasar’s light could be preventing some of the star formation by disrupting the gas that would otherwise collapse to form stars. But that alone isn’t enough by itself to have halted star formation, and the researchers suggest that this means the newborn galaxy has a low metal content (In Astronomy, ‘metal’ just refers to any elements above helium on the Periodic Table). Metals are usually the result of generations of stars living and dying. Having a low metallicity means that the galaxy is truly young because there haven't been may previous generations of stars to have died. Near the galaxy’s core, however, there is star formation going on. At least 15 solar masses worth of stars form there each year compared with our own Milky Way’s roughly 0.68-1.45 solar masses per year.



The researchers found that the object is just what it looks like—a spinning disk of gas and dust, a galaxy in the process of forming, and currently about four times the Milky Way’s diameter. What’s more, it looks like the galaxy is connected to a thin filament of material trailing.

"This is the first smoking-gun evidence for how galaxies form," said Christopher Martin, professor of physics at Caltech, principal investigator on CWI, and lead author of the new paper. "Even as simulations and theoretical work have increasingly stressed the importance of cold flows, observational evidence of their role in galaxy formation has been lacking."

Additionally, the galaxy’s baryonic (normal, non-dark) matter is spinning a lot faster than its dark matter. This matches simulations of galaxies forming from cold accretion flows. The disk’s wide diameter is also consistent with these models.

"The filament has a more or less constant velocity. It is basically funneling gas into the disk at a fixed rate," said Matt Matuszewski, one of the paper’s co-authors. "Once the gas merges with the disk inside the dark-matter halo, it is pulled around by the rotating gas and dark matter in the halo." This tug accelerates the newly added material.

Significance

For the past decade, the cold flow model has been a controversial alternative to the standard model of galaxy formation. That latter model holds that galaxies form as a result of dark matter halos collapsing, pulling ordinary matter gas with them. But as the ordinary gas collapses, it heats up from friction and would then take a very long time for it to cool down enough to form stars. This model began to run into problems in the late '90s, when a population of galaxies was discovered that had very high star formation rates less than two billion years after the Big Bang—not much time for matter to cool down.

The cold flow model offered a solution to this problem. Rather than having the dark matter halo draw in matter as it collapses, the cold flow model has gas stream directly into galaxies from the cosmic filaments. This could explain how those early galaxies got their fuel so quickly.

According to Phil Hopkins, assistant professor of theoretical astrophysics at Caltech (who was not involved in the study), the new discovery is "very compelling" as evidence for the cold flow model.

"As a proof that a protogalaxy connected to the cosmic web exists and that we can detect it, this is really exciting," he says. "Of course, now you want to know a million things about what the gas falling into galaxies is actually doing, so I'm sure there is going to be more follow up."

One possible avenue of follow up research could be to identify more galaxies forming in the same way. And this seems like an especially promising possibility, as the study’s authors say they’ve already identified two additional disks that appear to be receiving gas from their filaments in the same way.

The finding is an important one in the understanding of galaxies and of the Universe as a whole. Galaxies play an important role in the Cosmic Web, as there have been hints that galaxies may be tied into their filaments by some unknown mechanism. So this finding could help researchers understand how the Universe works at the largest scales.

Nature, 2015. DOI: 10.1038/nature14616 (About DOIs)