What Lies Between Galaxies?

People often think of space as "empty" or a "vacuum", meaning that there's absolutely nothing there. The term "void of space" often refers to that emptiness. However, it turns out that the space between planets is actually occupied with asteroids and comets and space dust. The voids between stars in our galaxy can be filled with tenuous clouds of gas and other molecules. But, what about the regions between galaxies? Are they empty, or do they have "stuff" in them?

The answer that everyone expects, "an empty vacuum", is not true, either. Just as the rest of space has some "stuff" in it, so does intergalactic space. In fact, the word "void" is now normally used for giant regions where NO galaxies exist, but apparently still contain some kind of matter.

 What lies between our galaxy and others in the universe, such as the Sombrero, shown here in a Hubble Space Telescope view?. NASA/STScI

So, what IS between galaxies? In some cases, there are clouds of hot gas given off as galaxies interact and collide. That material gets "ripped away" from the galaxies by the force of gravity, and often enough it collides with other material. That gives off radiation called x-rays and can be detected with such instruments as the Chandra X-Ray Observatory. But, not everything between galaxies is hot. Some of it is fairly dim and difficult to detect, and is often thought of as cold gases and dust.

Finding Dim Matter Between Galaxies

Thanks to images and data taken with a specialized instrument called the Cosmic Web Imager at Palomar Observatory on the 200-inch Hale telescope, astronomers now know that there is a lot of material in the vast stretches of space around galaxies. They call it "dim matter" because it isn't bright like stars or nebulae, but it's not so dark it can't be detected. The Cosmic Web Imager l(along with other instruments in space) looks for this matter in the intergalactic medium (IGM) and charts where it is most abundant and where it's not.

Observing the Intergalactic Medium 

How do astronomers "see" what's out there? The regions between galaxies are dark, obviously, since there are few or no stars out there to light up the darkness. That makes those regions difficult to study in optical light (the light we see with our eyes). So, astronomers look at light that streams through the intergalactic reaches and study how it is affected by its trip.

The Cosmic Web Imager, for example, is specifically equipped to look at the light coming from distant galaxies and quasars as it streams through this intergalactic medium. As that light travels through, some of it gets absorbed by the gases in the IGM. Those absorptions show up as "bar-graph" black lines in the spectra the Imager produces. They tell astronomers the makeup of the gases "out there." Certain gases absorb certain wavelengths, so if the "graph" shows gaps in certain places, then that tells them what gases exist out there that are doing the absorbing.

Interestingly, they also tell a tale of conditions in the early universe, about the objects that existed then and what they were doing. Spectra can reveal star formation, the flow of gases from one region to another, the deaths of stars, how fast objects are moving, their temperatures, and much more. The Imager "takes pictures" of the IGM as well as distant objects, at many different wavelengths. Not only does it let astronomers see these objects but they can use the data they obtain to learn about a distant object's composition, mass, and velocity.

Probing the Cosmic Web

Astronomers are interested in the cosmic "web" of material that streams between galaxies and clusters. They ask where it's coming from, where it's headed, how warm it is, and how much there is of it.

They look mainly for hydrogen since it is the main element in space and emits light at a specific ultraviolet wavelength called Lyman-alpha. Earth's atmosphere blocks light at ultraviolet wavelengths, so Lyman-alpha is most easily observed from space. That means most instruments that observe it are above Earth's atmosphere. They're either aboard high-altitude balloons or on orbiting spacecraft. But, the light from the very distant universe that travels through the IGM has its wavelengths stretched by the expansion of the universe; that is, the light arrives "red-shifted", which allows astronomers to detect the fingerprint of the Lyman-alpha signal in the light they get through the Cosmic Web Imager and other ground-based instruments.

Astronomers have focused in on light from objects that were active way back when the galaxy was only 2 billion years old. In cosmic terms, that's like looking at the universe when it was an infant. At that time, the first galaxies were ablaze with star formation. Some galaxies were just starting to form, colliding with each other to create larger and larger stellar cities. Many "blobs" out there turn out to be these just-starting-to-pull-themselves-together proto-galaxies. At least one that astronomers have studied turns out to be quite huge, three times larger than the Milky Way Galaxy (which itself is about 100,000 light-years in diameter). The Imager has also studied distant quasars, like the one shown above, to track their environments and activities. Quasars are very active "engines" in the hearts of galaxies. They're likely powered by black holes, which gobble up superheated material that's giving off strong radiation as it spirals into the black hole. 

Duplicating Success

The study of intergalactic stuff continues to unfold much like a detective novel. There are a lot of clues about what's out there, some definite evidence to prove the existence of some gases and dust, and a lot more evidence to gather. Instruments like the Cosmic Web Imager use what they see to uncover evidence of long-ago events and objects in the light streaming from the most distant things in the universe. The next step is to follow that evidence to figure out exactly what's in the IGM and detect even more distant objects whose light will illuminate it. That's an important part of determining what happened in the early universe, billions of years before our planet and star even existed.