What Lies Between Galaxies?

Exploring the Intergalactic Medium

material between galaxies
An x-ray view of hot gases between two colliding galaxies, as seen by the Chandra X-Ray Observatory. NASA/Chandra

We 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 can be filled with tenuous clouds of gas and other molecules.

What is there out between the galaxies? The answer we expect: "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. So, what IS between galaxies?  In some cases, there are clouds of hot gas given off as galaxies interact and collide. It 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. 

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, and that makes them difficult to study in optical light (the light we see with our eyes). The Cosmic Web Imager is specially equipped to look at the light coming from distant galaxies and quasars as it streams through the IGM. As that light travels through whatever is out there between galaxies, 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." 

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

In particular, the astronomers are interested in the cosmic "web" of material that streams between galaxies and clusters. They look mainly at 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 story of intergalactic stuff is like a detective novel. Instruments like the Cosmic Web Imager see 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 the 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.