The Stuff Between the Galaxies

What is It?

This computer simulation by Matt Hall (NCSA) and Eric Hallman (Univ. Colorado) shows the warm-hot intergalactic medium (blue) that runs along filaments between galaxies and galaxy clusters. (Click to biggify.)

Living in a galaxy is like living in a big city.  There’s always something going on, there’s always somebody around. Even if you live out in the country, you’re still not that far from the nearest neighbor or town or burg or hamlet. But, let’s say you lived out in the desert, hundreds of kilometers from anything. You’d be surrounded by nothing, right?

Well, not exactly. It depends on how you define “nothing.”  If you think that a lack of towns or neighbors means there’s nothing out there, that’s one way to think of it. But, you’d still have sand and plants and animals surrounding you. They’re not in your social set, but that doesn’t  mean they don’t exist.

The same thing goes for galaxies and the space between them.  That space may look empty, but it’s  not.  Our galaxy is part of a cluster of galaxies called the Local Group.  The space between our galaxy and the ones next door is filled with material even though (to us with visible-light eyes) it looks like it’s empty.

While you may have heard that there’s a dark matter halo out there surrounding the Milky Way, there’s also regular old baryonic matter.

How do we know this? Bruce Dorminey, who writes for ScienceNow Daily News, sent me an article he wrote about a detection of regular matter between galaxies that was done using x-rays.

How so?  As light travels through the intergalactic medium, it encounters “stuff” — atoms of gases in clouds.  As light from more distant objects runs through those clouds, some of it is absorbed by the material.  We can actually see the fingerprints of this absorption when we look at that light through a spectrograph.

It turns out that these clouds are likely absorbing x-rays (which are also part of the electromagnetic spectrum). David Buote of the University of California at Irvine and a group of astronomers used the Chandra X-ray observatory and the European Space Agency’s XMM-Newton observatory to look at a portion of an object called the Sculptor Wall, part of a large collection of galaxies that lie about 400 million light-years away.  They were specifically looking for the fingerprints of O VII — oxygen that has been stripped of five of its eight electrons.  This O VII is part of what astronomers call the “warm-hot intergalactic medium” — a sort of rarefied plasma that absorbs various wavelengths of light, including x-rays.  Buote and his colleagues are saying that there’s an excellent chance that their discovery will hold up and that they have found another way to probe the matter that exists in the intergalactic medium. Their research will be published in the April 20th issue of Astrophysical Journal.

Astronomers have long known that the intergalactic void wasn’t completely empty, but this new work shows us what’s filling it in some places. It has pretty important implications for how we understand what astronomers call the “large-scale structure of the universe.”  This is because we are still trying to understand just how matter is distributed — whether it’s regular matter or dark matter.  In the long term, astronomers use studies like this to model just how galaxies are formed and how the universe has evolved since it began 13.7 billion years ago.