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Jai Ganesh

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Intergalactic space

Intergalactic space

Gist

Intergalactic space is the vast, extremely low-density region between galaxies. While it is nearly a vacuum, it contains the intergalactic medium (IGM), a tenuous, hot plasma of ionized hydrogen, along with occasional stars and dark matter, all organized in a cosmic filamentary structure. This gas, though sparse, makes up most of the matter in the universe, and is heated enough by events like galactic mergers and active galactic nuclei to be detectable through the X-rays and UV light it emits or absorbs.

Intergalactic space is the vast expanse between galaxies, which is extremely low in density and close to a perfect vacuum. While it may seem empty, it contains a very thin gas called the intergalactic medium (IGM), consisting mostly of hot, ionized hydrogen, along with heavier elements and a small number of stray stars. This matter is hot enough for electrons to be stripped from the hydrogen atoms.

Summary

The space between stars is known as interstellar space, and so the space between galaxies is called intergalactic space. These are the vast empty spaces that sit between galaxies. For example, if you wanted to travel from the Milky Way to the Andromeda galaxy, you would need to cross 2.5 million light-years of intergalactic space.

Intergalactic space is as close as you can get to an absolute vacuum. There's very little dust and debris, and scientists have calculated that there's probably only one hydrogen atom per cubic meter. The density of material is higher near galaxies, and lower in the midpoint between galaxies.

Galaxies are connected by a rarefied plasma that is thought to posses a cosmic filamentary structure, which is slightly denser than the average density of the Universe. This material is known as the intergalactic medium, and it's mostly made up of ionized hydrogen. Astronomers think that the intergalactic medium is about 10 to 100 times denser than the average density of the Universe.

This intergalactic medium can actually be seen by our telescopes here on Earth because it's heated up to tens of thousands, or even millions of degrees. This is hot enough for electrons to escape from hydrogen nuclei during collisions. We can detect the energy released from these collisions in the X-ray spectrum. NASA's Chandra X-Ray Observatory - a space telescope designed to search for X-rays - has detected vast clouds of hot intergalactic medium in regions where galaxies are colliding together in clusters.

Details

Intergalactic space is the physical space between galaxies. Generally free of dust and debris, intergalactic space is very close to a vacuum. The average density of the Universe is less than one atom per cubic meter. The density of the Universe, however, is clearly not uniform; it ranges from relatively high density in galaxies (including very high density in structures within galaxies, such as planets, stars, and black holes) to extremely rarefied conditions in vast voids that have lower density than the Universe's average.

Surrounding and stretching between galaxies, there is a rarefied gas that is thought to possess a cosmic filamentary structure and that is slightly denser than the average density in the Universe. This material is called the intergalactic medium (IGM) and is mostly ionized hydrogen (i.e. a plasma) consisting of equal numbers of electrons and protons. The IGM is thought to exist at a density of 10 to 100 times the average density of the Universe (10 to 100 hydrogen atoms per cubic meter). It reaches densities as high as 1000 times the average density of the Universe in rich clusters of galaxies.

The reason the IGM is thought to be mostly ionized gas is that its temperature is thought to be quite high by terrestrial standards (though some parts of it are only "warm" by astrophysical standards). As gas falls into the Intergalactic Medium from the voids, it heats up to temperatures of {10}^{5} to {10}^{7} K, which is too hot for hydrogen nuclei to retain their electrons. At these temperatures, it is called the Warm-Hot Intergalactic Medium (WHIM). Computer simulations indicate that on the order of half the atomic matter in the universe might exist in this warm-hot, rarefied state. When gas falls from the filamentary structures of the WHIM into the galaxy clusters at the intersections of the cosmic filaments, it can heat up even more, reaching temperatures of {10}^{8} K or more.

Additional Information

The vast voids between galaxies can stretch millions of light-years across and may appear empty. But these spaces actually contain more matter than the galaxies themselves.

"If you took a cubic meter, there would be less than one atom in it," Michael Shull, an astronomer at the University of Colorado Boulder, told Live Science. "But when you add it all up, it's somewhere between 50 and 80% of all the ordinary matter out there."

So, where did all this matter come from? And what's it up to?

The matter between galaxies — often called the intergalactic medium, or IGM for short — is mostly hot, ionized hydrogen (hydrogen that has lost its electron) with bits of heavier elements such as carbon, oxygen and silicon thrown in. While these elements typically don't glow bright enough to be seen directly, scientists know they're there because of the signature they leave on light that passes by.

In the 1960s, astronomers first discovered quasars — incredibly bright and active galaxies in the distant universe — and shortly thereafter, they noticed that the light from the quasars had missing pieces. These pieces had been absorbed by something in between the quasar and the astronomers' telescopes — this was the gas of the IGM. In the decades since, astronomers have discovered vast webs and filaments of gas and heavy elements that collectively contain more matter than all the galaxies combined. Some of this gas was likely left over from the Big Bang, but the heavier elements hint that some of it comes from old stardust, spewed out by galaxies.

While the most-remote regions of the IGM will be eternally isolated from neighboring galaxies as the universe expands, more "suburban" regions play an important role in galaxy life. The IGM under the influence of a galaxy's gravitational pull slowly accumulates onto the galaxy at a rate of about one solar mass (equal to the mass of the sun) per year, which is about the rate of star formation in the disk of the Milky Way.

"IGM is the gas that feeds star formation in galaxies," Shull said. "If we didn't still have gas falling in, being pulled in by gravity, star formation would slowly grind to a halt as the gas [in the galaxy] gets used up."

To probe the IGM, astronomers also have started looking at fast radio bursts that come from distant galaxies. Using both this technique and by examining quasar light, astronomers continue to study the characteristics of the IGM to determine its varying temperatures and densities.

"By measuring the temperature of the gas, you can get a clue as to its origins," Shull said. "It allows us to know how it got heated and how it got there."

Although gas is pervasive between galaxies, it isn't the only thing out there; astronomers have also found stars. Sometimes called intergalactic or rogue stars, these stars are thought to have been flung from their birth galaxies by black holes or collisions with other galaxies.

In fact, stars sailing the void might be fairly common. A 2012 study published in The Astrophysical Journal reported more than 650 of these stars at the edge of the Milky Way, and by some estimates, there could be trillions out there.

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