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% of the

baryon In particle physics, a baryon is a type of composite particle, composite subatomic particle that contains an odd number of valence quarks, conventionally three. proton, Protons and neutron, neutrons are examples of baryons; because baryons are ...

ic 'normal matter' in the universe at the current

epoch In chronology and periodization, an epoch or reference epoch is an instant in time chosen as the origin of a particular calendar era. The "epoch" serves as a reference point from which time is measured. The moment of epoch is usually decided b ...

. The WHIM can be described as a web of hot, diffuse gas stretching between galaxies, and consists of plasma, as well as

atom Atoms are the basic particles of the chemical elements. An atom consists of a atomic nucleus, nucleus of protons and generally neutrons, surrounded by an electromagnetically bound swarm of electrons. The chemical elements are distinguished fr ...

s and

molecule A molecule is a group of two or more atoms that are held together by Force, attractive forces known as chemical bonds; depending on context, the term may or may not include ions that satisfy this criterion. In quantum physics, organic chemi ...

s, in contrast to

dark matter In astronomy, dark matter is an invisible and hypothetical form of matter that does not interact with light or other electromagnetic radiation. Dark matter is implied by gravity, gravitational effects that cannot be explained by general relat ...

. The WHIM is a proposed solution to the missing baryon problem, where the observed amount of

baryonic matter In particle physics, a baryon is a type of composite subatomic particle that contains an odd number of valence quarks, conventionally three. Protons and neutrons are examples of baryons; because baryons are composed of quarks, they belong to ...

does not match theoretical predictions from cosmology. Much of what is known about the warmhot intergalactic medium comes from computer simulations of the cosmos. The WHIM is expected to form a filamentary structure of tenuous, highly ionized baryons with a density of 1−10 particles per cubic meter. 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 Gravitational energy or gravitational potential energy is the potential energy an object with mass has due to the gravitational potential of its position in a gravitational field. Mathematically, it is the minimum mechanical work that has to be do ...

supplied by these effects is converted into thermal emissions of the matter by collisionless shock heating. Because of the high temperature of the medium, the expectation is that it is most easily observed from the absorption or emission of

ultraviolet Ultraviolet radiation, also known as simply UV, is electromagnetic radiation of wavelengths of 10–400 nanometers, shorter than that of visible light, but longer than X-rays. UV radiation is present in sunlight and constitutes about 10% of ...

and low energy

X-ray An X-ray (also known in many languages as Röntgen radiation) is a form of high-energy electromagnetic radiation with a wavelength shorter than those of ultraviolet rays and longer than those of gamma rays. Roughly, X-rays have a wavelength ran ...

radiation. To locate the WHIM, researchers examined X-ray observations of a rapidly growing

supermassive black hole A supermassive black hole (SMBH or sometimes SBH) is the largest type of black hole, with its mass being on the order of hundreds of thousands, or millions to billions, of times the mass of the Sun (). Black holes are a class of astronomical ...

known as an active galactic nucleus, or AGN. Oxygen atoms in the WHIM were seen to absorb X-rays passing through the medium. 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. In 2018, observations of highly-ionized extragalactic oxygen atoms appeared to confirm simulations of the WHIM mass distribution. Observations for dispersion from fast radio bursts in 2020, further appeared to confirm the missing baryonic mass to be located at the WHIM.

Circumgalactic medium

Conceptually similar to WHIM, circumgalactic medium (CGM) is a halo of gas between the ISM and virial radii surrounding galaxies that is diffuse, and nearly invisible. Current thinking is that the CGM is an important source of star-forming material, and that it regulates a galaxy’s gas supply. If visible, the CGM of the Andromeda Galaxy (1.3-2 million ly) would stretch 3 times the size of the width of the Big 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 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 supernova, and the outer shell is hotter and smoother.

References

{{DEFAULTSORT:Warm-hot intergalactic medium Intergalactic media

Circumgalactic medium

See also

References