Warm–hot intergalactic medium

Space is not empty.

It has a big web of hot gas. This gas sits between galaxies. It is very thin and spread out. Scientists use computers to see it.

This gas helps solve a mystery. We thought some matter was missing. Now we think it is in this web. Some gas flows in and out of galaxies. This helps galaxies grow. It is a very big part of our world.

Space is not empty. It has a big web of hot gas.

Scientists call this the warm-hot intergalactic medium. We call it the WHIM for short. This gas sits in the spaces between galaxies. It is very thin and spread out. It is made of plasma. Plasma is a type of gas that has a charge.

The WHIM helps solve a mystery. Scientists once thought some matter was missing. This matter is called baryons. They call it the missing baryon problem. Now, they think the WHIM holds 40 to 50 percent of these baryons.

How does the gas get so hot? It gets heat from many things. Large black holes can create gas shocks. These shocks happen when gas moves fast. Merging galaxies also help make heat.

It is hard to see this gas. It does not give off much light. Scientists look for X-rays to find it. In 2010, a tool called the Chandra X-ray Observatory found a huge part of it. It was near the Sculptor Wall. This wall is a group of galaxies. It is 400 million light-years from Earth.

Space between galaxies is not empty. It contains a huge web of hot gas. Scientists call this the warm-hot intergalactic medium. They often call it the WHIM for short. This gas is a plasma. Plasma is a type of gas that has a charge.

The WHIM is very important to science. It helps solve the missing baryon problem. This is a mystery about missing normal matter. Scientists think the WHIM holds 40 to 50 percent of all baryons. Baryons are the building blocks of normal matter.

How does this gas get so hot? It gets heat through many different ways. Large black holes can create gas shocks. These shocks happen when gas moves very fast. Merging galaxies also create these shocks. The energy from these movements turns into heat. This is called collisionless shock heating. This process makes the gas reach very high temperatures. The gas can reach 105 to 107 Kelvin.

Finding the WHIM is a hard job. The gas is very thin and spread out. It has only 1 to 10 particles in each cubic meter. Scientists must look for ultraviolet light or X-ray radiation. In May 2010, a big discovery was made. The Chandra X-ray Observatory found a giant reservoir of WHIM. It was near a group of galaxies called the Sculptor Wall. This wall is 400 million light-years away from Earth.

Other gas lives closer to galaxies too. This is called the circumgalactic medium or CGM. The CGM acts like a boundary. It sits between galaxies and the larger WHIM. It helps regulate a galaxy's gas supply. This is part of a process called the baryon cycle. Gas moves in and out of galaxies. This movement helps create new stars.

We can use the Andromeda Galaxy to imagine this. If we could see its CGM, it would be huge. It would stretch three times the width of the Big Dipper. It might even touch our own CGM. The Andromeda CGM has two different layers. An inner shell is very turbulent. This inner part is about 0.5 million light-years wide. An outer shell is hotter and smoother.

The universe contains much more than just stars and galaxies. Between these massive structures lies a sparse, vast web of hot gas. Scientists call this the warm-hot intergalactic medium, or the WHIM for short. The WHIM is a plasma made of atoms and molecules. It is different from dark matter because it consists of baryonic matter. Baryons are the building blocks of the normal matter we see every day.

Cosmologists believe the WHIM is very important to our understanding of the universe. It may hold 40% to 50% of all baryonic matter in the current epoch. This makes it a primary solution to the missing baryon problem. This problem occurs because the observed amount of normal matter does not match theoretical predictions.

Understanding how the WHIM stays so hot requires looking at cosmic energy. The gas reaches temperatures between 10^5 and 10^7 Kelvin. This heat comes from processes like merging and accretion. These are gravitationally-driven processes that move matter around. Additionally, active galactic nuclei, or AGNs, create gas shocks. An AGN is a rapidly growing supermassive black hole at a galaxy's center. These shocks convert gravitational energy into thermal emissions through collisionless shock heating. This process can even drive gas out of a galaxy and quench it over time.

The WHIM is incredibly thin and difficult to detect. It has a density of only 1 to 10 particles per cubic meter. Because the gas is so hot, researchers look for ultraviolet and low-energy X-ray radiation. Scientists often study how this gas absorbs or emits radiation to find it. In May 2010, the Chandra X-ray Observatory made a major discovery. It detected a giant reservoir of WHIM along the Sculptor Wall. This structure of galaxies is located about 400 million light-years from Earth.

Later, in 2018, observations of highly-ionized oxygen atoms helped confirm simulations of WHIM mass. In 2020, observations of dispersion from fast radio bursts also appeared to confirm the location of this missing mass.

Closer to individual galaxies lies a different but related structure. This is the circumgalactic medium, or the CGM. The CGM is a diffuse halo of gas that surrounds a galaxy. It acts as a boundary between the galaxy and the larger intergalactic medium. The CGM is essential for regulating a galaxy's gas supply. It facilitates a process known as the baryon cycle. This cycle involves the movement of gas into and out of galaxies. This gas recycling affects the metallicity of a galaxy. Metallicity refers to the amount of elements heavier than hydrogen and helium. This is linked to the mass-metallicity relation, where a galaxy's metallicity correlates to its mass.

The movement of gas in the CGM follows specific patterns. Simulations show that gas inflows move along a galaxy's major axis. These inflows often move in a corotational fashion. In contrast, gas outflows move along the minor axis in a biconical fashion. These flows can stretch 100 kiloparsecs or more from the galaxy center. The CGM also contains a cool gas phase at about 10^4 Kelvin. This cool phase is thought to consist of clouds surrounded by a hotter phase at 10^6 Kelvin. The composition of this hot gas depends on the type of galaxy. In elliptical galaxies, the gas comes from Type IA supernovae and AGB stars. In disc galaxies, the hot gas comes from Type II supernova ejecta.

Dust also plays a role in the environment of the CGM. Dust is carried from galaxies into the CGM through galactic outflows. This happens because of the drag force acting on the particles. Radiation pressure might also help move dust into the intergalactic medium. At temperatures higher than 10^6 Kelvin, nearby dust radiates away thermal energy. This energy comes from collisions with the surrounding gas. These processes show how material leaves a galaxy to join the larger cosmos.

We can use the Andromeda Galaxy to visualize the scale of the CGM. If it were visible, the CGM would be massive. It would stretch three times the width of the Big Dipper. It might even reach our own CGM, though we cannot be sure since we live in one. The Andromeda CGM has two distinct layers. The inner shell is about 0.5 million light-years wide. This layer is more dynamic and turbulent due to supernova outflows. The outer shell is much hotter and smoother than the inner one.