Warm–hot_intergalactic_medium
Warm–hot intergalactic medium
Plasma that cosmologists believe exists between galaxies
The warm–hot intergalactic medium (WHIM) is the sparse, warm-to-hot (105 to 107 K) plasma that cosmologists believe to exist in the spaces between galaxies and to contain 40–50%[1][2] of the baryonic 'normal matter' in the universe at the current epoch.[3] The WHIM can be described as a web of hot, diffuse gas stretching between galaxies, and consists of plasma, as well as atoms and molecules, in contrast to dark matter. The WHIM is a proposed solution to the missing baryon problem, where the observed amount of baryonic matter does not match theoretical predictions from cosmology.[4]
Much of what is known about the warm–hot intergalactic medium comes from computer simulations of the cosmos.[5] The WHIM is expected to form a filamentary structure of tenuous, highly ionized baryons with a density of 1−10 particles per cubic meter.[6] Within the WHIM, gas shocks are created as a result of active galactic nuclei, along with the gravitationally-driven processes of merging and accretion. Part of the gravitational energy supplied by these effects is converted into thermal emissions of the matter by collisionless shock heating.[1]
Because of the high temperature of the medium, the expectation is that it is most easily observed from the absorption or emission of ultraviolet and low energy X-ray radiation. To locate the WHIM, researchers examined X-ray observations of a rapidly growing supermassive black hole known as an active galactic nucleus, or AGN. Oxygen atoms in the WHIM were seen to absorb X-rays passing through the medium.[7] In May 2010, a giant reservoir of WHIM was detected by the Chandra X-ray Observatory lying along the wall-shaped structure of galaxies (Sculptor Wall) some 400 million light-years from Earth.[7][8] In 2018, observations of highly-ionized extragalactic oxygen atoms appeared to confirm simulations of the WHIM mass distribution.[4] Observations for dispersion from fast radio bursts in 2020, further appeared to confirm the missing baryonic mass to be located at the WHIM.[9]