Intergalactic space

**Intergalactic space** is the physical space between galaxies. Studies of the large scale distribution of galaxies show that the Universe has a foam-like structure, with clusters and groups of galaxies lying along filaments that occupy about a tenth of the total space. The remainder forms huge voids that are mostly empty of galaxies. Typically, a void spans a distance of (10–40) *h*−1 Mpc, where *h* is the Hubble constant in units of 100 km s−1 Mpc−1.Template:Sfn

## Medium

Surrounding and stretching between galaxies, there is a rarefied plasma that is organized in a galactic filamentary structure. This material is called the intergalactic medium (IGM). The density of the IGM is 5–200 times the average density of the Universe. It consists mostly of ionized hydrogen; i.e. a plasma consisting of equal numbers of electrons and protons. As gas falls into the intergalactic medium from the voids, it heats up to temperatures of 105 K to 107 K, which is high enough so that collisions between atoms have enough energy to cause the bound electrons to escape from the hydrogen nuclei; this is why the IGM is ionized. At these temperatures, it is called the warm–hot intergalactic medium (WHIM). (Although the plasma is very hot by terrestrial standards, 105 K is often called "warm" in astrophysics.) Computer simulations and observations indicate that up to half of 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 108 K and above in the so-called intracluster medium.

## References

1. ↑ Jafelice, Luiz C.; Opher, Reuven (July 1992), "The origin of intergalactic magnetic fields due to extragalactic jets", *Monthly Notices of the Royal Astronomical Society* (Royal Astronomical Society) **257** (1): 135–151, doi:10.1093/mnras/257.1.135, Bibcode: 1992MNRAS.257..135J.
2. ↑ Wadsley, James W. *et al*. (August 20, 2002), "The Universe in Hot Gas", *Astronomy Picture of the Day* (NASA), archived from the original on June 9, 2009, https://web.archive.org/web/20090609150938/http://antwrp.gsfc.nasa.gov/apod/ap020820.html, retrieved 2009-06-19.
3. ↑ 3.0 3.1 Fang, T. *et al*. (2010), "Confirmation of X-Ray Absorption by Warm-Hot Intergalactic Medium in the Sculptor Wall", *The Astrophysical Journal* **714** (2): 1715, doi:10.1088/0004-637X/714/2/1715, Bibcode: 2010ApJ...714.1715F.
4. ↑ Gupta, Anjali; Galeazzi, M.; Ursino, E. (May 2010), "Detection and Characterization of the Warm-Hot Intergalactic Medium", *Bulletin of the American Astronomical Society* **41**: 908, Bibcode: 2010AAS...21631808G.
5. ↑ Bykov, A. M.; Paerels, F. B. S.; Petrosian, V. (February 2008), "Equilibration Processes in the Warm-Hot Intergalactic Medium", *Space Science Reviews* **134** (1–4): 141–153, doi:10.1007/s11214-008-9309-4, Bibcode: 2008SSRv..134..141B.
6. ↑ Wakker, B. P.; Savage, B. D. (2009), "The Relationship Between Intergalactic H I/O VI and Nearby (z<0.017) Galaxies", *The Astrophysical Journal Supplement Series* **182**: 378, doi:10.1088/0067-0049/182/1/378, Bibcode: 2009ApJS..182..378W.
7. ↑ Mathiesen, B. F.; Evrard, A. E. (2001), "Four Measures of the Intracluster Medium Temperature and Their Relation to a Cluster's Dynamical State", *The Astrophysical Journal* **546**: 100, doi:10.1086/318249, Bibcode: 2001ApJ...546..100M.