Is a galaxy considered to be matter

The new image of the interstellar and intergalactic medium

by Philipp Richter

Illustration 1: Computer simulation of the distribution of the intergalactic medium.
Credit: Courtesy Renyue Cen

Diffuse gas, which is located between the stars and between the galaxies, represents an essential component of matter in the universe. Huge amounts of gas were aggregated in the early stages of the universe under the gravitational influence of dark matter and thus formed the first generation of galaxies and stars. Galaxies still contain large amounts of diffuse gas and are constantly converting it into new stars. The gaseous matter component in the universe therefore plays a key role in the formation and development of galaxies.

Only about 5 percent of the material / energy inventory of the universe consists of "conventional" (= baryonic) matter, i.e. the atoms and molecules from which the stars - but also ourselves - are composed. Under the influence of gravity, an initially relatively homogeneous dark matter distribution developed into an increasingly inhomogeneous, filament-like structured universe. The baryonic matter followed the gravity field of the dark matter and structured itself into ever greater density contrasts. The universe with its planets, stars, galaxies and cosmological filaments grew out of this structural development, just as we can observe it today.

When astronomers look into the depths of the universe with their large optical telescopes, they see a multitude of galaxies or the light of the many trillion stars that are in these galaxies. The fact is, however, that most of the baryonic matter in the universe is outside of galaxies in the form of diffuse gas in intergalactic space. This plasma is so thin that it hardly emits any radiation and is therefore very difficult to observe. When it comes to diffuse gas in the universe, astronomers differentiate between the intergalactic medium (IGM), i.e. gas, which is itself outside of galaxies and the interstellar medium (ISM), gas which is located between the stars within of galaxies located. The transition between ISM and IGM is fluid, as galaxies have no sharp borders. In order to understand the spatial distribution and the physical properties of the interstellar and intergalactic medium, we want to get an overview of the turbulent history of the diffuse, gaseous matter component in the universe in this article.

The cosmic primordial gas and the age of recombination

At the beginning of the universe, long before the first stars appear, baryonic matter exists in the form of a hot, ionized plasma. Ionized means that the electrons are not bound in atoms, but move freely with the atomic trunk in the plasma. The chemical elements in this cosmic “primordial soup” are primarily hydrogen (because it is ionized, i.e. free protons), helium, and traces of deuterium and lithium. Heavier elements (e.g. carbon and oxygen) are only produced for the first time later by nuclear fusion in the first generation of stars. The mass fraction of helium is about 25 percent. In addition to the atomic nuclei and the free electrons, the energy density of the plasma and its expansion are mainly dominated by photons (i.e. massless light quanta) and neutrinos. About 3 minutes after the Big Bang, the temperature of the plasma is still almost 1 billion degrees Kelvin, much too hot to be able to form stars and galaxies. But the plasma cools down quickly until the temperature has dropped more than 5 orders of magnitude to a few thousand degrees. The age of recombination begins, in which the free electrons and the atomic nuclei come together for the first time to form neutral atoms. The term “re-combination” is somewhat unfortunate in that it implies a repeated coming together of electrons and atomic nuclei in atoms that have never existed as such before. The term recombination has meanwhile become common.

Formation of the first stars from primordial gas

The recombination is complete at a redshift of about z ~ 1100 (about 300,000 years after the Big Bang). The originally extremely hot plasma has become an atomic gas, which mainly consists of neutral hydrogen and which, due to the physical properties of the hydrogen atom, absorbs all energetic radiation. The lack of transparency of this medium for light of short wavelengths makes it very difficult for astronomers to obtain direct information from this early epoch of the universe through observations. The neutral gas is not distributed homogeneously, but rather it follows the gravitational attraction of dark matter, which more and more agglomerates in filament-like structures with increasing age of the universe. The gas can collect in the largest and most massive of these dark matter structures and finally cool down to temperatures at which the formation of the first stars becomes possible. Since the gas does not contain any heavy elements at this point in time (it is therefore called “primordial”; Latin: “of first order”), the cooling of the gas necessary for star formation can only take place via the energy emitted by hydrogen. This is done first via the excited states of the atomic Hydrogen up to around 10,000 degrees Kelvin, after which the gas cools via the line radiation of the molecular Hydrogen further down to temperatures below 1000 degrees, at which the first stars can form. These stars are also called Population III stars in astrophysics. Little is known about the properties of the first regions of star formation. However, from physical considerations one can conclude that the masses of these structures must be at least 10,000 solar masses for the gas to be able to cool efficiently and produce stars. It is also certain that the properties of the first stars are very different from the properties of today's star generations. Because due to the lack of heavy elements in the primordial star gas, it is very transparent to radiation. That is why the first stars in the universe were extremely massive and luminous, but their lifespan was also very short.

The age of reionization

Figure 2: Reionization process according to the state of knowledge. The first stars are formed in relatively low-mass dark matter halos. Subsequent generations of stars form more efficiently in more massive halos. These stars ionize the surrounding gas. Due to the overlap of the ionized regions, the intergalactic medium is finally completely ionized.
Credit: Philipp Richter.

The formation of the first stars marks a truly epoch-making point in the history of the universe. Astronomers now assume that this point in time must have been at a redshift between z = 20 and 30, but the exact point in time is currently unclear. But what influence do the first stars have on the surrounding gas? The influence is certainly twofold. On the one hand, the first stars are extremely luminous and also very hot, so that a lot of high-energy radiation is released. The emission of this radiation leads to the surrounding molecular hydrogen gas being dissociated and finally ionized (this means that the molecular structure is broken up and the electrons are released from the atomic compound). The ionizing radiation eats its way through the surrounding gas and forms bubbles of ionized hydrogen gas around the stars (see Figure 2). First of all, this prevents further stars from forming. Shortly after the first stars have formed, they explode in very high-energy supernova explosions. The heavy elements (oxygen, carbon, nitrogen, etc.) generated in the stars for the first time by nuclear fusion are released and given off to the surrounding gas; they enrich it chemically. Presumably, the energy released by these first supernova explosions is so great that the gas surrounding the stars is completely blown away from the dark matter structures (also called "dark matter halos") and merges with the primordial gas outside the halos mixed. Due to the progressive clumping and amalgamation of the dark matter halos, larger and more massive halos are formed in this epoch, in which new stars can eventually emerge. The formation of stars in these first galaxy-like structures (“protogalaxies”) is now more efficient, since with these larger halo masses, the gas can also be cooled without molecular hydrogen. This second generation of stars releases so much high-energy, ionizing radiation that the ionized bubbles in the intergalactic space around the protogalaxies become larger and larger and eventually overlap (Figure 2). This process marks the age of Reionization. Now that stars and early types of galaxies have formed for the first time from the primordial gas, we can for the first time use the terms “interstellar” and “intergalactic” accordingly. The intergalactic medium is now almost completely ionized again (as in the epoch before the recombination) and is therefore again permeable to energetic radiation, since the absorbing neutral hydrogen has been almost completely ionized away. To this day, the intergalactic medium remains in this highly ionized state. This is ensured by the radiation of massive stars in the galaxies that are growing in number and mass and the particularly energetic radiation of the increasingly developing active galaxy nuclei.Only because of that we can even observe the light from early galaxies at high redshifts. If the intergalactic medium were neutral, the neutral hydrogen contained in it would swallow almost any light from these objects.

When exactly the era of reionization is over is a matter of controversy among astronomers. The fact that we can see the light from objects up to z ~ 6.5 proves that the intergalactic medium must already be almost completely ionized at this redshift, i.e. the reionization must have essentially already taken place at this redshift.

How a spiral galaxy is formed from cosmic gas

After the reionization is complete, the structure formation continues, denser areas in the universe become denser, areas with low matter density continue to thin out, because gravity drives the agglomeration of matter. More and more galaxies are forming from the huge cosmic gas reservoir. With the increasing formation and evolution of stars, the interstellar gas is increasingly enriched with heavy elements. In addition, interstellar dust, tiny particles (in the micrometer range) of silicates and graphites, which mix with the molecules, atoms and ions in the interstellar gas, form in the shells of giant red stars. However, it should be noted that even at z = 3, the baryon proportion of the intergalactic gas is still over 95 percent. Up to this point in time, only a small fraction of the matter in the form of stars was “processed” in galaxies. But how exactly does the formation of a galaxy from the gaseous material actually work? We now have a reasonably good idea of ​​how galaxies arise from the gas, even if many details are still unclear. In the following we want to visualize the formation of a spiral galaxy - like the Milky Way - as an example.

As already mentioned, the gaseous material in the universe follows the mass distribution of dark matter. Larger structures made of dark matter form potential pots in which the gas continuously collects. By falling into the dark matter halos, the gas converts potential energy from gravity into kinetic energy, and finally into thermal energy, so that the gas heats up to several million degrees. An equilibrium situation is created which prevents a further rapid gravitational collapse of the gas. In this "hydrostatic equilibrium" the gas density is higher in the center of the dark matter halo than in the outer areas. The collision processes of the gas particles in the inner areas of the halo, which increase with the density, lead to the emission of energy, especially in the form of high-energy X-rays. The radiation slowly cools down the gas and sinks further and further into the center of the dark matter halo. It is crucial that the gas retains its angular momentum (angular momentum is a physical conservation quantity). Because of this, the gas cloud rotates faster and faster around its own axis (like a figure skater in a pirouette rotates faster and faster when she draws her arms towards her body). A gas disk then forms from the cloud, the diameter of which is kept at a constant value by the conservation of angular momentum. After this gas disk has cooled down further, molecular clouds form in the densest areas, in which billions of stars are gradually formed.

Gas in vast galaxy halos

The process of gas accretion in dark matter halos describes in a simplified way the formation of a spiral galaxy. From this emergence scenario, the structure and the spatial structure of the interstellar gas in spiral galaxies result naturally, as we observe it today e.g. in the Milky Way. The first defining characteristics of this type of galaxy on large scales are of course the spiral arms, kinematic structures of increased matter density within the disk, in which both stars and gas are concentrated. In the inner, central area of ​​the disk is the bulge, a spherical, light thickening of the disk with a high star density. The disk is surrounded by the so-called halo, i.e. the outer area of ​​the galaxy that extends far into intergalactic space. In the halo there are isolated stars, globular clusters and very hot gas (based on the sun, one also speaks of "coronal" gas or "corona" for short). In the case of galaxies, the term “halo” should first be understood as a spatial indication. The hot coronal gas in the halo of a galaxy creates the spatial link between the relatively dense gaseous disk and the surrounding, filament-like intergalactic medium. The notion of a galaxy halo was established long before the concept of dark matter halos was introduced. Both terms are linked in terms of content, because it is the gravitational attraction of the dark matter halo that holds the stars and the interstellar gas in galaxies together. Therefore, the spatial extent of a galaxy (and thus the size of the galaxy halo) depends largely on the mass of the associated dark matter halo. Most of the galaxies in the universe are not isolated, but part of even larger structures, namely groups of galaxies and clusters of galaxies. In this hierarchical distribution of matter in the universe, the dark matter halos of galaxies cannot therefore be viewed in isolation. Let us take a closer look at this hierarchical principle using the example of the Milky Way.

The Milky Way is part of the "Local Group", a collection of at least 50 galaxies of different masses. The Milky Way and the Andromeda Galaxy are both large spiral galaxies and dominate the matter distribution of the Local Group.Within the gravitational sphere of influence of the Milky Way (i.e. within the dark matter halo of the Milky Way) there are several dwarf galaxies with their own, smaller dark matter halos (e.g. the Magellanic Clouds), which are gradually captured and incorporated by the Milky Way. It can also be assumed that the dark matter halos of the Milky Way and Andromeda overlap. Based on these considerations, it becomes clear that it is extremely difficult, even almost impossible, to precisely define the extent of an individual galaxy halo. This leads to great difficulties for the mass estimation of the interstellar gas belonging to galaxies. Because while the stellar mass of a galaxy like the Milky Way is essentially determined by the stars in the disk and can therefore be estimated relatively easily, a significant proportion of the interstellar gas in spiral galaxies is possibly in the form of hot, very thin coronal gas out the disc. However, since the exact extent of this coronal gas is unknown and the mass depends on the cube of the radius of the gas halo, the mass estimate of the coronal gas is extremely uncertain. For example, if we assume that the coronal gas halo of the Milky Way is 500,000 light years in diameter, the total mass of the gas in this huge volume would be around 1 billion solar masses despite the extremely low density of only a few dozen particles per cubic meter. This corresponds roughly to the mass of the interstellar gas in the disk, where the gas is much denser, but the volume is significantly smaller. So hot and cold gas could roughly be in balance in the Milky Way.

Interstellar gas in the disks of spiral galaxies

In the relatively thin disk of a spiral galaxy, the interstellar gas has the highest (particle) density.The majority of the gas in the disk consists of neutral hydrogen, which can be observed particularly well using the 21cm line with radio telescopes. The expansion of the gaseous, neutral gas disk is usually significantly larger than the stellar disk that is directly visible to us. Obviously, the physical conditions in the outer areas of the disk are such that only very few stars can emerge from the gas there. A good example of this is the galaxy NGC 2403, as shown in Figure 3. The diameter of the neutral gas disk (observed in 21cm emission) is more than twice as large as the stellar disk of the galaxy that is visible in the visual. It is even to be expected that the gas disk will be even more extensive. Due to the decreasing gas density and the limited sensitivity of the radio telescope, the neutral gas can only be observed up to a certain radius. It is clear that due to such uncertainties, the total mass of neutral gas disks in spiral galaxies can only be determined with limited accuracy, especially because the radius is included in the calculation of the total mass as a square. The exact diameter of the gas disk is also not known for our own Milky Way. It is currently assumed to have a diameter of around 200,000 light years, with the thickness of the neutral disk only around 800 light years. The ratio of thickness and diameter for the neutral gas disk of the Milky Way is therefore less than half that of a CD!

Figure 3: Neutral gas disk (HI 21cm line, left) and stellar disk (visual, right) of the spiral galaxy NGC 2403. The gas disk has a diameter that is more than twice as large as the stellar disk. This shows that star formation in this galaxy is essentially restricted to the inner part of the disk.
Credit: Image courtesy of NRAO / AUI and Tom Oosterloo, Astron, The Netherlands.

The gas in the disk of a spiral galaxy is not homogeneously distributed, but is structured on both large and small scales (box "Phases of interstellar gas in spiral galaxies"; see also article by P. Richter, SuW Spezial 1/2006, p. 48). As a result, the interstellar medium within the disk has significant density and temperature differences of several orders of magnitude. The neutral Gas (as the regions in the interstellar gas in which the hydrogen is present in neutral form) is relatively evenly distributed within the disk, but of course the distribution follows the global structures (e.g. the spiral arms). Warmer, neutral gas surrounds the somewhat more compact, cooler, neutral gas clouds. The densest and coldest areas of the interstellar medium are the large molecular clouds, which are mainly located in the inner area of ​​the disk. The gas clumps to high densities in the molecular clouds, so that it collapses under its own gravity and forms new stars. First, the atoms combine to form molecules. Due to the high density of gas and dust particles already at the edge of the cloud, the innermost part of the molecular cloud is shielded from the dissociating and ionizing radiation, so that the gas can continue to compress and cool down "undisturbed". How exactly the star formation then takes place is not yet sufficiently well understood. In particular, the role of magnetic fields and turbulence in the gas is not clear (see also article by S. Wolf, Th. Henning and R. Launhardt, SuW Spezial 1/2006, p. 63). Molecular clouds can be studied particularly well in the radio range using the emission lines of carbon monoxide (CO) (see box).

The newly formed stars, especially the most massive and brightest stars, dissociate and ionize the surrounding interstellar gas through the radiation they emit. This creates expanding bubbles of ionized gas around the star formation areas, which eat their way into the neutral gas disk like holes in Swiss cheese. Such so-called HII regions (HII stands for ionized hydrogen) can be observed very well at optical wavelengths, where the gas releases intense radiation that is generated by the recombination of electrons with atomic cores (so-called "recombination radiation", e.g. from hydrogen ( H alpha); see box). Massive stars and their stellar winds drive additional energy into these cavities, causing the ionized bubbles to expand further. When the most massive of the stars go up in a supernova explosion, an extremely large amount of energy is released in a very short time - this energy is captured by surrounding interstellar gas. This allows the gas to be heated to very high temperatures (up to several million degrees) and the HII regions can expand so far that they break vertically out of the neutral disk. The hot gas then flows into the galaxy halo. There it will eventually cool down and - trapped in the gravitational field of the galaxy - fall back to the disk as a neutral gas. This process is also known as the “galactic fountain”. Astronomers believe that the galactic fountain is at least partially responsible for a population of neutral gas clouds above and below the disk, known as "high-speed clouds" because of their high radial velocity (Figure 4; see also article by BP Wakker and P. Richter, Spektrum der Wissenschaft, April 2004, p. 46). The incidence of these gas clouds on the neutral disk leads to density fluctuations in the interstellar gas, which may result in new star formation. The transformation of interstellar gas into stars and back is also referred to in astrophysics as the "cosmic matter cycle".

Figure 4: "High-speed clouds" in the halo of the Milky Way, recorded in the 21cm line of neutral hydrogen. These neutral gas clouds are located above and below the galactic disk and move towards or away from the disk at high speeds. The clouds consist on the one hand of material that is accreted from the intergalactic medium and satellite galaxies, on the other hand of gas that is thrown from the disk as part of a galactic fountain into the halo.
Credit: Tobias Westmeier & Peter Kalberla (LAB-Survey). Images by Philipp Richter.

Interstellar gas in other types of galaxies

The example of disk galaxies shows the complex relationships between stellar and interstellar matter. But how do I relate to other types of galaxies and their gaseous matter components? The extremely massive, elliptical galaxies consist mainly of dark matter and old stars, which apparently move randomly around the center of the galaxy in a diffuse star cloud. In optical recordings, elliptical galaxies therefore often appear as structureless, "boring" structures. Early observations of elliptical galaxies showed that hardly any star formation takes place in them. Interstellar gas could not be detected at first. For a long time it was therefore thought that elliptical galaxies did not harbor any interstellar gas at all. Only with more sensitive measuring instruments could it be shown that elliptical galaxies contain cold and above all hot interstellar gas, but with a significantly smaller mass fraction than in spiral galaxies. With the help of satellite-based X-ray telescopes, one can observe that massive elliptical galaxies are enveloped by a corona of very hot gas with temperatures of several million degrees. The origin and physical conditions of this hot interstellar gas differ fundamentally from the properties of the interstellar medium in spiral galaxies. This is mainly due to the very different composition and spatial structure of spiral galaxies. The properties and composition of the interstellar gas in elliptical galaxies are determined by the ejection of stellar material from the mostly very old stars at the end of their evolutionary path and by supernova explosions. Due to the fact that the orbits of the stars in these galaxies do not follow any orderly movement, the radially expanding gas envelopes thrown off by the stars collide with one another and thus heat the newly released interstellar material to high temperatures, which causes it to emit X-rays. Sometimes so-called “cooling currents” develop in elliptical galaxies. In the process, colder gas flows to the center of the galaxy, is compressed by the gas pressure rising inwards and continues to cool. Eventually, in this way, new stars can eventually emerge in elliptical galaxies, albeit to a very limited extent.

Smaller galaxies, the so-called dwarf galaxies, also contain interstellar gas. Dwarf galaxies come in different shapes, they can be spiral or elliptical, or they can have irregular shapes. The gas content of these galaxies varies considerably, depending on the state of development and the environment. Dwarf galaxies are mainly located in groups of galaxies and appear there as satellite galaxies of larger spirals. The Milky Way also has a number of satellite galaxies around it, such as the Magellanic Clouds that can be observed in the southern sky. Due to the proximity to a much larger (typically more than a hundred times heavier) "mother galaxy", the development of the dwarf satellites is significantly influenced, in particular the development of the interstellar gas in them. The gravitational “pull” of the much more massive mother galaxy can, for example, cause density fluctuations in the gas of the dwarf galaxy, which can lead to a sudden increase in the star formation rate. Such a "starburst" can lead to the stellar winds and supernova explosions caused by the new stars simultaneously releasing an extremely large amount of energy, so that the interstellar medium is completely blown out of the dwarf galaxy and escapes into intergalactic space ("super wind"). If a dwarf galaxy flies very close to its parent galaxy, the interstellar gas of the dwarf galaxy can also be stripped off directly. On the one hand, this happens through the gravitational interaction between the two galaxies. On the other hand, in this case, the dwarf galaxy flies through the gaseous halo of the mother galaxy, so that the dynamic pressure that results makes it easier for the dwarf galaxy's gas to be stripped off. Ultimately, the gravitational interaction leads to the fact that the dwarf galaxies are torn apart and merge with their mother galaxies. Stellar and interstellar material, which originally came from the satellite galaxies, is thus incorporated into the mother galaxies. These gain in mass over time and grow steadily. An influx of gas from dwarf galaxies is also observed in the Milky Way. The "Magellanic Stream", a gigantic, neutral gas cloud in the halo of the Milky Way, which is also classified as a high-speed cloud, most likely consists of gas that was torn out of the Magellanic Clouds by the Milky Way and is now being incorporated into it.

The ionized intergalactic medium - a huge reservoir of matter

Figure 5: The Lyman Alpha Forest in the optical spectrum of a quasar. The many individual absorption lines are generated by neutral hydrogen in the intergalactic medium, which collects in the cosmic filaments. Due to the expansion of the universe, each of these filaments has its own characteristic redshift along the line of sight, which determines the position of the associated absorption line in the spectrum.
Credit: Philipp Richter, data from the Millennium Simulation.

While the transformation of interstellar into stellar matter takes place within the galaxies and thus drives their development, the density contrasts in the cosmic filaments develop more and more on much larger scales due to the influence of gravity over time. Due to the energetic radiation of massive stars and the active galaxy nuclei, the intergalactic medium remains highly ionized. Less than one in 1 million hydrogen atoms is in the neutral state (i.e. it is not ionized). Nevertheless, these relatively few neutral hydrogen atoms in the intergalactic gas leave characteristic absorption lines in the spectra of distant quasars and other active galaxy nuclei, which can be used to study the properties of the intergalactic gas in front of them (Figure 5). These absorption lines are mainly caused by the Lyman Alpha line of neutral hydrogen, which is in the ultraviolet wavelength range at a rest wavelength of about 121.6 nanometers. Due to the expansion of the universe, the intergalactic Lyman Alpha absorption lines are redshifted by the factor (1 + z), where z is the respective redshift of an intergalactic gas filament. If one looks through the complex network of intergalactic gas filaments, one obtains a characteristic pattern of absorption lines which, due to the redshift effect, extends far into the optical wavelength range. This so-called "Lyman-Alpha-Forest" appears to us like a cosmic barcode with which one can examine the distribution of dark and baryonic matter in the cosmological filaments.

But due to the increasing concentration of baryonic and dark matter in the cosmic filaments, the intergalactic gas gains additional energy in addition to the energetic radiation from active galaxy nuclei, which leads to heating. The reason for this is the gravitation itself: through the incidence of the gaseous matter in the dark matter halos, the gas gains potential energy, which is first converted into kinetic and then into thermal energy. This leads to an increase in the temperature of the gas. The conversion of potential gravitational energy into kinetic energy can be illustrated using a ball that is located on a surface and rolls into a depression or trough. When it hits the trough, the ball accelerates and thus increases its kinetic energy. If one neglects the friction, the gain in kinetic energy corresponds exactly to the loss in potential energy ("positional energy" of the sphere). The heating of the intergalactic gas and the increasing compression in the ever-increasing potential wells of the dark matter halos mean that ionization through collisions between the gas particles is becoming increasingly important compared to ionization through radiation. In the densest areas of the intergalactic filaments in today's universe, impact ionization dominates via ionization by radiation, which is why the proportion of neutral hydrogen in the gas is further reduced there. Due to the now barely measurable proportion of neutral hydrogen, this hot, intergalactic gas component "disappears" from the spectrum of the Lyman-Alpha forest and remains almost completely hidden from the observer. From the comparison of the number of Lyman alpha absorption lines at high redshift and today it can be estimated that around 50 percent of the intergalactic gas in the local universe must be in this hot phase, ionized by collisions.

Particularly hot intergalactic plasma with temperatures of up to 100 million degrees can be found in the largest gravitationally bound objects in the universe: the galaxy cluster. These gigantic but relatively rare objects represent the densest nodes in the cosmic web of matter and can contain several thousand galaxies. The enormous mass of these objects, typically 1 quadrillion (!) Solar masses, binds a large amount of hot gas. This heap gas is relatively dense and can emit radiation that can be measured with satellites in the X-ray range. Similar to the elliptical galaxies - only now on much larger scales - cooling currents can develop in galaxy clusters, which allow colder gaseous material to migrate into the center of the cluster.

Despite the many billions of galaxies that have formed from the cosmic gas reservoir since the age of reionization, only a fraction of this material has been converted into stellar matter. At z = 0, i.e. in today's universe, there is still about 70 percent (!) Of the baryonic matter in the form of diffuse intergalactic gas. This gas forms the “fuel reserve” for many future generations of stars in the galaxies. The influx of intergalactic material onto the galaxies is still of great importance for their development today, especially for disk galaxies like the Milky Way. However, it is unclear whether the accreted gas hits the galaxies as hot, predominantly ionized gas, or whether the gas cools and "rains" on the disk as neutral gas clouds. For the Milky Way, the latter scenario would mean that the incoming intergalactic gas takes the form of high-speed clouds, which also represent accreted gas from satellite galaxies and gas from the galactic fountain (see above). As a result, it is extremely difficult to distinguish whether the gas in a high-velocity cloud comes from the intergalactic medium or from a satellite galaxy or from the galactic fountain and is only possible if the chemical composition of the gas can be determined. To do this, the absorption lines of heavy elements in the high-speed clouds must be examined in suitable spectra.Studies of this type that have recently been carried out with the FUSE Satellites and the space telescope Hubble have shown that all three scenarios (intergalactic gas, gas from satellite galaxies and galactic fountains) actually play a role in the formation of the high-speed clouds (see Figure 4).

The galactic-intergalactic cycle of matter

Figure 6: The matter cycle between interstellar and intergalactic gas using the example of a disk galaxy (like the Milky Way). Gas from the intergalactic medium flows into the nodes of the dark matter distribution and forms galaxies there. The influx of intergalactic gas continues to this day. In addition, in the course of hierarchical structural development, galaxies accret stars and gas from neighboring dwarf galaxies. Star formation and the resulting supernova explosions drive gas from the disk into the galaxy's halo. All of these processes create a population of gas clouds around galaxies known as "high-velocity clouds" (see Figure 4).
Credit: Philipp Richter

Our journey through the history of the gaseous matter component of the universe illustrates that the evolution of galaxies over time and the properties of the stellar and interstellar matter they contain can only be viewed in connection with their intergalactic environment. To some extent, this contradicts our visual impression of galaxies that we get from the many spectacular images taken with large optical telescopes. There galaxies appear as majestic spirals or mighty ellipses, which remain static in space and whose starlight suggests a clear demarcation from the outside. But this is a mirage, because in such images the outer regions of the galaxies, especially the interstellar gas in the extended disks and the hot coronae, are not visible at all. In addition, such images (like all images) are snapshots that cannot capture the development of these objects over time. Only observations with the most modern radio telescopes and X-ray satellites as well as cosmological simulations show a different face of galaxies today: Galaxies as dynamic, constantly growing nodes in the cosmic network, in which gas is converted into stars and which interact strongly with their surroundings. It is therefore clear that intensive observations of the gaseous intergalactic environment of galaxies are of great importance in order to study the accretion processes of gas from the intergalactic medium and from satellite galaxies and to characterize their role in the evolution of galaxies (see Figure 6). New observation instruments that will be put into operation in the next few years will also make an important contribution to such studies. This will be the case for the repair mission of the space telescope planned for May 2009 Hubble a new, highly sensitive spectrograph installed ("Cosmic Origin Spectrograph", short: COS), which is supposed to collect new data about the interstellar and above all intergalactic gas in the nearby universe. With this and other new instruments, astronomers hope to gain new insights into the fascinating, complex properties of the gaseous interstellar and intergalactic matter components in the universe.


Cold molecular gas

Cold molecular gas is the birthplace of new stars. Molecular gas exists in lumpy structures and is embedded in the neutral gas disk. This gas phase has temperatures of around 10-30 degrees Kelvin and densities of around 100 to 1 million particles per cubic centimeter. The picture shows molecular gas clumps at the edge of the Rosette Nebula. You can see the emission of the carbon monoxide (CO) molecule, which shows the densest areas in the gas. New stars can arise there.

Found at:; Credit: unclear


Dust is an essential part of the interstellar medium. This dust consists of solid particles, which are mostly less than a micrometer and are composed of silicates and graphites, among other things. Interstellar dust absorbs the visual light very efficiently. A shadowing of the background light caused by interstellar dust shows the image of a dense gas / dust cloud in the star formation region IC 2944.

Credit: NASA and The Hubble Heritage Team (STScI / AURA)

Cold and warm neutral gas

Cold and warm neutral gas are distributed relatively evenly in the disks of spiral galaxies. Typical temperatures and gas densities are 30-10000 Kelvin and 1-100 particles per cubic centimeter. The picture shows neutral hydrogen in the gas disk of the Circinus spiral galaxy (ESO 97-G13). The left picture shows the distribution of the neutral gas along the spiral arms, the right picture shows the radial velocity of the gas, with the help of which the rotational movement of the disk can be studied.

Credit: ATCA HI image by B. Koribalski (ATNF, CSIRO), K. Jones, M. Elmouttie (University of Queensland) and R. Haynes (ATNF, CSIRO)

Warm and hot ionized gas

Warm and hot ionized gas exist in the disk in bubbles ionized by massive stars, as well as in the extended galactic halo (“galactic corona”). The temperature of this gas is typically 10,000 to 100 million degrees Kelvin, the gas densities are very low with 0.0001 to 1 particle per cubic centimeter. The picture shows sections of the hot gas halo of the galaxy NGC 4631. Blue is the X-ray emission of the hot gas in the halo, red is the H alpha emission of the ionized gas near massive stars in the disk. Presumably through supernova explosions of such stars, the halogen gas is heated to temperatures of several million degrees.

Credit: Daniel Wang et al. Chandra, NASA. Astronomy Picture of the Day 25 July 2001