Thewarm–hot intergalactic medium (WHIM) is the sparse, warm-to-hot (105 to 107K)plasma thatcosmologists believe to exist in the spaces betweengalaxies and to contain 40–50%[1][2] of thebaryonic 'normal matter' in the universe at the currentepoch.[3] The WHIM can be described as a web of hot, diffuse gas stretching between galaxies, and consists of plasma, as well asatoms andmolecules, in contrast todark matter. The WHIM is a proposed solution to themissing baryon problem, where the observed amount ofbaryonic 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, gasshocks are created as a result ofactive galactic nuclei, along with the gravitationally-driven processes of merging and accretion. Part of thegravitational energy supplied by these effects is converted into thermal emissions of the matter bycollisionlessshock heating.[1] The gas shocks caused by AGNs drive gas out of a galaxy andquench it over time.[7]
Because of the high temperature of the medium, the expectation is that it is most easily observed from the absorption or emission ofultraviolet and low energyX-ray radiation. To locate the WHIM, researchers examined X-ray observations of a rapidly growingsupermassive 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.[8] In May 2010, a giant reservoir of WHIM was detected by theChandra X-ray Observatory lying along the wall-shaped structure of galaxies (Sculptor Wall) some 400 millionlight-years from Earth.[8][9] 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.[10]
Conceptually similar to WHIM, thecircumgalactic medium (CGM) is a halo of gas between theISM andvirial radii surrounding galaxies that is diffuse and nearly invisible.[11] It serves as the boundary between galaxies and the larger intergalactic medium. Current thinking is that the CGM is an important source of star-forming material and that it regulates a galaxy’s gas supply through gas inflows and outflows between galaxies and the intergalactic medium.[12]
Absorption measurements of large samples of galaxies are expected to show that gas inflows move along the major axis of a galaxy in acorotational fashion, while gas outflows move along the minor axis of a galaxy in a biconical fashion. Most major hydrodynamical simulations of this show that these gas inflows and outflows stretch to around ~100 kpc or above from the center of the source galaxy. This gas recycling process affects the metallicities of the ISM of source galaxies through the mass-metallicity relation, which states that metallicity of a galaxy is positively correlated to its mass.[7] The gas recycling process is also known as the baryon cycle and is facilitated by the movement of the cool gas phase of the CGM at T ~ 104 K, which is thought to be clouds of cool gas surrounded by a hotter CGM phase at T ~ 106 K.[13] Inelliptical galaxies, the hotter CGM phase is commonly considered to consist of the ejecta fromType IA supernovae andasymptotic giant branch (AGB) stars, while indisc galaxies, the hot gas is instead thought to consist ofType II supernova ejecta carried out into the CGM by gas outflows.[14] At temperatures higher than 106 K, nearbydust that is present in the CGM radiates away the thermal energy from collisions with the gas. The CGM contains dust that is carried alongside gas to the CGM through galactic outflows due to thedrag force.Radiation pressure may also be partly responsible for the presence of dust in the CGM. Together these processes can demonstrate how dust leaves galaxies and enters the IGM.[15]
If visible, the CGM of theAndromeda Galaxy (1.3-2 million ly) would stretch 3 times the size of the width of theBig Dipper—easily the biggest feature on the nighttime sky, and even bump into our own CGM, though that isn't fully known because we reside in it. There are two layered parts to Andromeda's CGM: an inner shell of gas is nested inside an outer shell. The inner shell (0.5 million ly) is more dynamic and is thought to be more dynamic and turbulent because of outflows from supernovae, and the outer shell is hotter and smoother.[16]
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