Astronomers are using an imager deployed on the Hale telescope at Palomar Observatory to obtain the first 3D view of the diffuse gas that connects galaxies throughout the universe.
This gas is known as the intergalactic medium (IGM). Until now, its structure has mostly been a matter for theoretical speculation.
“I’ve been thinking about the intergalactic medium since I was a graduate student,” says Christopher Martin, a physics professor at the California Institute of Technology (Caltech), who conceived and developed the Cosmic Web Imager. “Not only does it comprise most of the normal matter in the universe, it is also the medium in which galaxies form and grow.”
Since the late 1980s and early 1990s, theoreticians have predicted that primordial gas from the Big Bang is not spread uniformly throughout space, but is instead distributed in channels that span galaxies and flow between them.
This “cosmic web”—the IGM—is a network of smaller and larger filaments crisscrossing one another across the vastness of space and back through time to an era when galaxies were first forming and stars were being produced at a rapid rate.
Dim vs. bright matter
Martin describes the diffuse gas of the IGM as “dim matter,” to distinguish it from the bright matter of stars and galaxies, and the dark matter and energy that compose most of the universe.
Though you might not think so on a bright sunny day or even a starlit night, fully 96 percent of the mass and energy in the universe is dark energy and dark matter, whose existence we know of only due to its effects on the remaining 4 percent that we can see: normal matter.
Of this 4 percent that is normal matter, only one-quarter is made up of stars and galaxies, the bright objects that light our night sky. The remainder, which amounts to only about 3 percent of everything in the universe, is the IGM.
As Martin’s name for the IGM suggests, “dim matter” is hard to see.
A better view
Prior to the development of the Cosmic Web Imager, the IGM was observed primarily via foreground absorption of light-indicating the presence of matter-occurring between Earth and a distant object such as a quasar (the nucleus of a young galaxy).
“When you look at the gas between us and a quasar, you have only one line of sight,” explains Martin. “You know that there’s some gas farther away, there’s some gas closer in, and there’s some gas in the middle, but there’s no information about how that gas is distributed across three dimensions.”
Matt Matuszewski, a former graduate student at Caltech who helped to build the Cosmic Web Imager and is now an instrument scientist at Caltech, likens this line-of-sight view to observing a complex cityscape through a few narrow slits in a wall: “All you would know is that there is some concrete, windows, metal, pavement, maybe an occasional flash of color. Only by opening the slit can you see that there are buildings and skyscrapers and roads and bridges and cars and people walking the streets.
“Only by taking a picture can you understand how all these components fit together, and know that you are looking at a city.”
Martin and his team have now seen the first glimpse of the city of dim matter. It is not full of skyscrapers and bridges, but it is both visually and scientifically exciting.
Filament flowing into the blob
The first cosmic filaments observed by the Cosmic Web Imager are in the vicinity of two very bright objects: a quasar labeled QSO 1549+19 and a so-called Lyman alpha blob in an emerging galaxy cluster known as SSA22. These objects were chosen by Martin for initial observations because they are bright, lighting up the surrounding IGM and boosting its detectable signal.
Observations show a narrow filament, one million light-years long, flowing into the quasar, perhaps fueling the growth of the galaxy that hosts the quasar. Meanwhile, there are three filaments surrounding the Lyman alpha blob, with a measured spin that shows that the gas from these filaments is flowing into the blob and affecting its dynamics.
The Cosmic Web Imager is a spectrographic imager, taking pictures at many different wavelengths simultaneously. This is a powerful technique for investigating astronomical objects, as it makes it possible to not only see these objects but to learn about their composition, mass, and velocity.
The objects the Cosmic Web Imager has observed date to approximately 2 billion years after the Big Bang, a time of rapid star formation in galaxies. “In the case of the Lyman alpha blob,” says Martin, “I think we’re looking at a giant protogalactic disk. It’s almost 300,000 light-years in diameter, three times the size of the Milky Way.”
Grants from the National Science Foundation and Caltech supported the project. Two papers describing the initial data from the Cosmic Web Imager have been published in the Astrophysical Journal (first paper and second paper).