Cosmic Web Imager: Observes 'dim matter'

This is a comparison of the Lyman alpha blob observed with the Cosmic Web Imager and a simulation of the cosmic web based on theoretical predictions. 

Credit: Christopher Martin, Robert Hurt

Caltech astronomers have taken unprecedented images of the intergalactic medium (IGM), the diffuse gas that connects galaxies throughout the universe, with the Cosmic Web Imager, an instrument designed and built at Caltech.

Until now, the structure of the IGM has mostly been a matter for theoretical speculation.

However, with observations from the Cosmic Web Imager, deployed on the Hale 200-inch telescope at Palomar Observatory, astronomers are obtaining our first three-dimensional pictures of the IGM.

The Cosmic Web Imager will make possible a new understanding of galactic and intergalactic dynamics, and it has already detected one possible spiral-galaxy-in-the-making that is three times the size of our Milky Way.

Chris Martin

The Cosmic Web Imager was conceived and developed by Caltech professor of physics Christopher Martin.

"I've been thinking about the intergalactic medium since I was a graduate student," says Martin. "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"(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.

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 (first inferred by Caltech's Fritz Zwicky in the 1930s), 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. 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."

Observation of quasar (QSO 1549+19) taken with Caltech's Cosmic Web Imager. Blue shows hydrogen gas surrounding and inflowing to quasar. 

Credit: Christopher Martin, Robert Hurt

The integral field unit of the Cosmic Web Imager. 

Credit: Matt Matuszewski

More information: Paper 1. resolver.caltech.edu/CaltechAUTHORS:20140303-152428640 Paper 2. resolver.caltech.edu/CaltechAUTHORS:20140303-145821259