# Sources/Intergalactic medium

The intergalactic medium (IGM) is a rarefied plasma.
"The Chandra observations found evidence for the massive and hot intergalactic medium filaments by noting a slight dimming in distant quasar X-rays likely caused by hot gas absorption."

## Astronomy

In astronomy, the interstellar medium (or ISM) is the matter that exists in the space between the star systems in a galaxy. This matter includes gas in ionic, atomic, and molecular form, dust, and cosmic rays. It fills interstellar space and blends smoothly into the surrounding intergalactic medium, or intergalactic space.

## Radiation

"The situation with regard to the intergalactic medium (IGM) is analogous [to that of the interstellar medium (ISM)]. First we needed to be convinced that it existed, and the evidence, when it came in (QSO) absorption lines and X-ray-emitting gas), was quite analogous to that which compelled belief in the ISM (interstellar absorption lines and emission nebulae). Then, the known UV sources (primarily QSOs) were allowed for with an ad hoc introduction of other possible sources such as shock heating (e.g., Ikeuchi & Ostriker 1983) to estimate the thermal balance in the IGM."

## Medium

Def. the nature of the surrounding environment is called a medium.
An "epoch will exist when the thermodynamic properties of most of the IGM may be determined by these X-rays [of the X-ray background (XRB)]. [...] the properties of this volume [involve] ionization and thermal evolution in a uniform primordial medium."
For "ionization solutions: [there are] photoionization, collisional ionization, case B radiative recombination, dielectronic recombination for He I, and the coupling between H and He caused by the radiation fields from the He I 24.6 eV recombination continuum and from the bound-bound transitions of He I (photon energies at 19.8 eV, 21.2 eV, and the two-photon continuum with an energy sum of 20.6 eV)."
For "the effects of the secondary ionizations and excitations of H I and He I [these are] due to the energetic photoelectrons liberated by the X-rays (Shull & van Steenberg (1985); henceforth SVS85). These introduce a further coupling between the ionization equilibria of H and He. Secondary ionization dominates over direct photoionization for H I in the specialized circumstance where X-rays are the sole source of photoionization. A typical X-ray photon is far more likely to be absorbed by He I rather than H I. The ejected photoelectron, however, will ionize many more H I atoms than He I, as H I is more abundant. As a result, secondary ionizations from He I photoelectrons and the radiation associated with He I recombination and excitation are the primary sources of H I ionization."
The "thermal evolution of the IGM [include] the following processes: photoelectric heating from the secondary electrons of H and He, [generates] less heating relative to a prescription where 100% of the photoelectron’s excess energy goes into heating the IGM), and heating from the H I photoelectrons liberated by the bound-bound transitions or the 24.6 eV recombination continuum of He I (here the loss of excess energy to heat is taken to be 100% as further ionizations of H I or He I are not possible). Cooling terms include radiative and dielectronic recombination, thermal bremsstrahlung, Compton scattering off the CMB, collisional ionization and excitation, and the adiabatic expansion of the IGM. The values of the recombination and cooling coefficients for temperatures ≲ 10$^4$ K were taken from Hummer (1994) and Hummer & Storey (1998), and the heating contribution from the He I two-photon process was calculated using the photon frequency distribution given in Drake et al. (1969)."

## Intergalactics

Def. occurring between galaxies is called intergalactic.
Def. originating
* outside the Milky Way galaxy or
* outside of a galaxy
is called extragalactic.
Intergalactic space is the physical space between galaxies. The huge spaces between galaxy clusters are called the voids.
Surrounding and stretching between galaxies, there is a rarefied plasma that is organized in a cosmic filamentary structure. This material is called the intergalactic medium (IGM). The density of the IGM is 5-200 times the average density of the Universe. It consists mostly of ionized hydrogen; i.e. a plasma consisting of equal numbers of electrons and protons. As gas falls into the intergalactic medium from the voids, it heats up to temperatures of 10$^5$ K to 10$^7$ K, which is high enough so that collisions between atoms have enough energy to cause the bound electrons to escape from the hydrogen nuclei; this is why the IGM is ionized. At these temperatures, it is called the warm–hot intergalactic medium (WHIM). (Although the gas is very hot by terrestrial standards, 10$^5$ K is often called "warm" in astrophysics.) Computer simulations and observations indicate that up to half of the atomic matter in the Universe might exist in this warm-hot, rarefied state. When gas falls from the filamentary structures of the WHIM into the galaxy clusters at the intersections of the cosmic filaments, it can heat up even more, reaching temperatures of 10$^8$ K and above in the so-called intracluster medium.

## Planetary sciences

"The UV is an essential spectral interval for all fields in astrophysical research; imaging and spectral coverage at UV wavelengths provides access to diagnostic indicators for diffuse plasmas in space, from planetary atmospheres to elusive gas in the intergalactic medium (IGM)."

## Colors

"Using HST observations, Gallagher et al. (2001) found 115 candidate star clusters, most of them distributed among the tidal debris of SQ. From color-color diagrams they estimated their ages ranging from 2–3 Myr up to several Gyr. The distribution of ages sheds light on the star formation history in SQ: The youngest star clusters (with ages of less than 10 Myr) are found in SQ A and south of NGC 7318a/b, while somewhat older star clusters, with ages between 10 and 500 Myr are in the young tidal tail and around NGC 7319. This is consistent with the picture that the eastern tidal tail was produced in a previous interaction whereas the collision and star formation around SQ A is ongoing."

## Intergalactic medium theory

"Whether the thermal IGM is collisional or collisionless at scales smaller than the Coulomb scale [charge interaction distance] depends on the effect of reduced mean free path that is mediated by the plasma instabilities. Consequently the way compressible turbulence is damped and particle[s] are reaccelerated in the IGM depends on the interplay between several reference scales (wavenumbers): the collisionless scale, k$_{coll}$, the Coulomb scale, k$_C$, the turbulence cut-off scale due to collisionless damping with thermal particles, $k_{c}^{th}$, and that due to collisionless damping with relativistic particles, $k_{c}^{CR}$ [...]."
With respect to the image on the right: "A schematic diagram giving the evolution with scale of turbulence (solid lines = fast modes and dashed lines = Alfvén and slow modes) assuming collisionless interaction with the thermal IGM (e.g. [null Brunetti & Lazarian 2007]; left panel) and collisional interaction mediated by plasma instabilities (this paper; right panel). We also mark the generation (thin arrows) of fast modes from the cascading of Alfvén and slow modes (both panels) and that of solenoidal modes from plasma instabilities (right panel). The positions of the relevant scales, Coulomb scale (k$_C$), MHD scale (l$^{−1}$$_A$∼k$_A$), thermal and relativistic cut-off scales (kth$_c$ and k$^{CR}$$_c$, respectively) are reported, together with that of the effective collisionless scale k$_{coll}$ (in the right panel; k$_{coll}$=k$_C$ in the left panel). The exponential cut-off in the spectrum of fast modes marks the effect of collisionless damping with the thermal IGM (left panel) and with relativistic particles (right panel)."
Here's a theoretical definition:
Def. the electromagnetism and matter between galaxies is called the intergalactic medium.
As there are many locations for intergalactic media, there are many intergalactic media.

## Entities

The production of a double layer requires regions with a significant excess of positive or negative charge, that is, where quasi-neutrality is violated. In general, quasi-neutrality can only be violated on scales of the order of the Debye length. The thickness of a double layer is of the order of ten Debye lengths, which is a few centimeters in the ionosphere, a few tens of meters in the interplanetary medium, and tens of kilometers in the intergalactic medium.

## Sources

"Do we know enough about the intergalactic medium to trust measurements of background sources seen through foreground structure?"

## Objects

"We know that within 1.5 billion years after the Big Bang, some galaxies had formed; this is evidenced by galaxies observed to z ∼6. Even at this epoch most of the intergalactic medium was ionized as evidenced by the lack of continuum absorption redward of Lyman α (the Gunn-Peterson effect, Gunn & Peterson 1965). It must be concluded that some objects must have existed earlier that produced sufficient UV flux to ionise nearly all the baryonic matter in the Universe."
Mayall's Object (also classified under the Atlas of Peculiar Galaxies as Arp 148) is the result of two colliding galaxies located 500 million light years away within the constellation of Ursa Major. ... When first discovered, Mayall's Object was described as a peculiar nebula, shaped like a question mark. Originally theorized to represent a galaxy reacting with the intergalactic medium, it is now thought to represent the collision of two galaxies, resulting in a new object consisting of a ring-shaped galaxy with a tail emerging from it. It is thought that the original collision between the two original galaxies created a shockwave that initially drew matter into the center which then formed the ring.

## Strong forces

The "scaling of the acceleration efficiency with IGM temperature derived assuming a collisionless IGM may also extend to the case of a weakly-collisionless IGM implying that the conclusion that stochastic acceleration is stronger in the hottest clusters holds for a wide range of (micro-)physical conditions."
The "turbulent magnetic compressions on the scale of the mean free path and less are the most effective for inducing the instability$^*$. As the scattering happens on magnetic perturbations induced by the instability, the mean free path of particles decreases as a result of the operation of the instability. This results in the process being self-regulating, i.e. the stronger the turbulence at the scale of injection, the smaller is the mean free path of plasma particles and the larger is the span of scales over which the fluid behaves as essentially collisional."
"$^*$MHD turbulence theory has a long history (see Biskamp 2003) and its details are still a subject of hot debates. However, recent numerical calculations are roughly consistent with the model of strong Alfvenic turbulence in Goldreich & Sridhar (1995) (see Beresnyak & Lazarian 2009) and cofirm scaling of compressible modes reported in Cho & Lazarian (2003) (see Kowal & Lazarian 2010)."

## Electromagnetics

"Spectral and timing properties of astronomical sources of very high-energy γ-rays could be strongly affected by the development of electromagnetic cascades on the way from the source to the Earth. These cascades could be initiated by interactions of the γ-rays with the ambient radiation fields inside the γ-ray source, in the source host galaxy and in the Milky Way galaxy, as well as with cosmological photon fields in the intergalactic space."

## Weak forces

The "decrease of the size of extended source with the increase of energy allows to measure weak magnetic fields with magnitudes in the range from ≤ 10$^{−16}$ G to 10$^{−12}$G if they exist in the voids of the Large Scale Structure."

## Continua

A "detection of, or a significant upper limit to, the Lyman continuum can constrain the fraction of photons escaping absorption within the galaxy and ionizing the surrounding intergalactic medium."

## Emissions

"Direct evidence for in situ particle acceleration mechanisms in the intergalactic medium (IGM) is provided by the diffuse Mpc-scale synchrotron emissions observed from galaxy clusters."

## Absorptions

The "intergalactic medium (IGM) may be ionized by photons emitted from a cosmological distribution of massive neutrinos."
"The absence of absorption troughs in quasar spectra due to atomic hydrogen and helium, and the possible presence of a trough due to singly ionized helium, would then imply that the neutrino mass lies between 50 and 110 eV."
A "calculated lifetime depends critically on whether a mechanism called GIM suppression is operating (de Rujula & Glashow 1980). [...] However, if GIM suppression does not operate (e.g. if there are four neutrino flavours) [... and] the C$^{IV}$ observed by IUE high up in our galactic halo owes its ionization to photons from decaying neutrinos which dominant the halo [...] 96 eV ≤ m$_ν$ ≤ 110 eV τ ~ 10$^{27}$ s. These latter ideas might be tested by searching above the atmosphere for a faint narrow emission line (Δλ ~ 10$^{-3}$λ) at high galactic latitudes with a photon energy lying between 47.9 and ~ 55 eV."

## Bands

An "early XRB does not create significant amounts of new molecular hydrogen in the IGM, despite the increased population of free electrons. This is primarily due to the strong photo-destruction of H$^−$ and H$_2$ by the near IR/optical (henceforth IR/O) and the far-UV (UV photons in the Lyman-Werner bands; henceforth FUV) backgrounds, in the respective energy ranges 0.755–11.2 eV and 11.2–13.6 eV, associated with any XRB generated by QSOs."
"H$_2$ forms by associative detachment of H I and H$^−$, and is destroyed by collisions with electrons and with H I, by charge exchange with protons, and by photodissociation in the Lyman and Werner bands by photons of energies 11.2–13.6 eV."

## Backgrounds

The "most important collisionless damping of fast modes is due to the Transit-Time-Damping (TTD) resonance with relativistic particles (eg. Schlickeiser & Miller 1998; Yan & Lazarian 2004; Brunetti & Lazarian 2007, 2010) [dependent on] B$_0$ [which] is the background (unperturbed) magnetic field".
There are several astrophysical components contributing to the sky background: these could be sets of point sources like faint asteroids, Galactic stars and far away galaxies, as well as diffuse sources like dust in the Solar System, in the Milky Way, and in the intergalactic space.

## Meteors

"The temperature distribution of the intergalactic medium (IGM) depends on whether or not there exist strong galactic winds."
It "may be possible for a heated IGM with sufficient pressure to inhibit outflows from star-forming protogalaxies. Mass loss from early objects through galactic winds or evaporation is often invoked to explain the ubiquitous presence of metals in the Lyα forest clouds at z ∼ 3; a pre-heated IGM could, however, hinder such outflows of metal-enriched gas from host galaxies at sufficiently high redshifts (prior to reionization)."
At right is a Spitzer Infrared Telescope image that shows in red the outflow from galaxy M82 of polycyclic aromatic hydrocarbons (PAH)s embedded in a gaseous wind propelled out of the galaxy by hot stars in the stellar disk (blue).
In November 2010, two gigantic gamma-ray bubbles were detected at the heart of our galaxy. These bubbles appear as a mirror image of each other. These bubbles of high-energy radiation are suspected as erupting from a massive black hole or evidence of a burst of star formations from millions of years ago. These bubbles have been measured and span 25,000 light-years across. They were discovered after scientists filtered out the "fog of background gamma-rays suffusing the sky". This discovery confirmed previous clues that a large unknown "structure" was in the center of the Milky Way.
The bubbles stretch up to Grus and to Virgo on the night-sky of the southern hemisphere.

## Cosmic rays

The "maximum energy budget available for cosmic rays in the IGM can be efficiently constrained from the recent upper limits to the gamma ray emission from nearby galaxy clusters (eg. Aharonian et al. 2009; Ackermann et al 2010) and to the Mpc-scale radio emission in clusters without radio halos (eg. Brunetti et al 2007), and this provide[s] important information for theoretical models."

## Neutrals

The "transmission of the Ly α forest [is] produced by neutral hydrogen scattering in the intergalactic medium [... It has been measured] between redshifts 2 and 6.3 using high signal-to-noise, high-resolution (R≥ 5000) observations of 50 quasars spread over the redshift range."
The "time of first light in universe, the ‘epoch of reionization’, [is] when the UV emission from the first stars and (accreting) supermassive black holes reionizes the neutral intergalactic medium."

## Neutrons

The "high-redshift IGM [may be] a tracer of cosmic structure formation by gravitational instability. In such a scenario, diffusely distributed baryonic material (the protons and neutrons that constitute ordinary matter) responds to the gravitational influence of the underlying dark matter."

## Protons

"Mergers between galaxy clusters are the most energetic events in the present Universe. During these collisions a fraction of the gravitational binding-energy of massive Dark Matter halos can be channelled into shocks and turbulence that may accelerate relativistic protons and electrons (e.g. Ryu et al 2003; Cassano & Brunetti 2005; Brunetti & Lazarian 2007; Hoeft & Brëuggen 2007; Pfrommer et al 2008; Skillman et al 2008; Vazza et al 2009), while collisions between the accelerated protons and the thermal protons generate secondary particles (e.g. Blasi & Colafrancesco 1999; Pfrommer & Ensslin 2004)."
The "mean free path of thermal protons due to Coulomb collisions in the hot IGM is very large, ten to hundred kpc (e.g. Sarazin 1986)."
The "proton-wave boiler, was observed in the case of reacceleration by a hypothetical spectrum of isotropic Alfvenic waves (Brunetti et al 2004) where indeed the damping of the modes was dominated by gyro-resonance with relativistic protons."
In the fast regime the damping due to relativistic particles is initially small making the acceleration efficiency large. Under these conditions relativistic protons rapidly gain energy with the consequence that the damping of the turbulent modes by these protons increases with time and makes the reacceleration process less efficient."
In "the slow regime relativistic protons do not increase significantly their energy implying a quasi-constant damping of the modes".
The "evolution with time of the spectrum of relativistic electrons and protons [is] subject to reacceleration by fast modes assuming a collisional IGM. [... Considering] only primary protons and the secondary electrons produced by inelastic collisions between these protons and the IGM [...] the damping of the modes is largely dominated by that with relativistic protons."
"Relativistic protons do not experience relevant energy losses in the IGM and TTD resonance in the IGM may reaccelerate supra-thermal protons up to high energies."
Cosmic "ray protons contribute to a few percent of the thermal cluster energy, consistent with the recent limits derived from FERMI observations of nearby clusters (Aharonian et al. 2009; Ackermann et al 2010)."
Larger "injection rates of turbulence do not make the reacceleration process substantially more efficient, due to the damping by the relativistic protons that self-regulates the acceleration efficiency in a few acceleration times."
The "effect of proton back-reaction on the acceleration efficiency becomes less important in the (more realistic) case of intermittent (or patchy) turbulence [...] that implies that less energy in channelled into cosmic ray protons; in this case larger acceleration efficiencies are maintained for longer periods of time".

## Beta particles

"If the [extragalactic magnetic field] EGMF strength is below 10$^{−12}$ G (plausible assumption, in the view of the cosmological models of the origin of magnetic fields), one expects that the deflections of the secondary electrons and positrons by EGMF should result in the appearance of an EGMF-dependent extended emission around initially point sources and/or in an EGMF-dependent time delay of the emission from the secondary pairs."

## Electrons

"X-rays, which have large mean free paths relative to EUV photons, and their photoelectrons can have significant effects on the thermal and ionization balance."
Hydrogen "ionization is dominated by the X-ray photoionization of neutral helium and the resulting secondary electrons."
An "early XRB does not create significant amounts of new molecular hydrogen in the IGM, despite the increased population of free electrons."
"A partially ionized IGM can affect the CMB through Thomson scattering of the CMB photons off the free electrons in the IGM."

## Positrons

When dark matter "DM decays/annihilations result in the production of an electron–positron pair, it is useful to distinguish between electrons and positrons."

## Neutrinos

The "evolution of the energy deposition in the intergalactic medium (IGM) by dark matter (DM) [consists of] decays/annihilations for both sterile neutrinos and light dark matter (LDM) particles. At z > 200 sterile neutrinos transfer a fraction f$_{abs}$ ∼ 0.5 of their rest mass energy into the IGM; at lower redshifts this fraction becomes ≲0.3 depending on the particle mass."

## Gamma rays

"[T]hree-dimensional electromagnetic cascade[s are] initiated by interactions of the multi-TeV γ-rays with the cosmological infrared/optical photon background in the intergalactic medium. Secondary electrons in the cascade are deflected by the intergalactic magnetic fields before they scatter on [cosmic microwave background] CMB photons. This leads to extended 0.1-10 degree scale emission at multi-GeV and TeV energies around extragalactic sources of very-high-energy γ-rays."
"The ubiquity of particle cascades has two-fold consequences. On one, pessimistic, side, they complicate the interpretation of the observational data in the very-high-energy (VHE, 0.1-10 TeV) γ-ray band. On the other, optimistic, side, with enough spectral and angular resolution, one can quantify the influence of the cascade on the observed source signal and not only reconstruct the spectrum of the primary γ-ray source, but also use the information about properties of the cascade to study the physical characteristics of the medium in which the cascade has developed."

## X-rays

X-ray observations of galaxy clusters and groups have discovered a large amount of hot, metal-rich gas.
Clusters of galaxies are the largest bound systems known, with their baryonic mass dominated by X-ray emitting plasma (coronal clouds), which is ten times the mass of the sum of the constituent galaxies.
For highly ionized, hot gas at T ~ 10$^6$ K, absorption lines from heavy elements are probably the only method of detection, since hydrogen is highly ionized and its absorption lines are very weak. These absorption lines are denoted by the phrase X-ray forest. The number of absorption lines measures the product of the baryon density times the metallicity. The ratio of strengths of the O VII and O VIII lines probes the distribution of gas temperature and density.
The X-ray forest is produced by a hot intergalactic medium in the form of filamentary and sheetlike structures connected to galaxy clusters and groups.
The "puddles of X-ray emission seen around the centres of galaxies were not the only diffuse X-ray sources. Around the galaxy clusters in Virgo and Coma, puddles measuring nearly 3° across could be seen, and other, more distant, galaxy clusters were accompanied by similar, smaller halos. The familiar optical images of these clusters revealed that they typically contained hundreds to thousands of galaxies, spread over tens of millions of light years, but the X-ray emission showed that there was also some smooth distribution of material in between the galaxies – an intergalactic medium. The significance of its X-ray emission was that for the first time, the amount of gas lying in the vast spaces between galaxies could be estimated, and surprisingly, it seemed that there was ten times as much invisible gas between galaxies as there was within them. This gas was understood to have reached temperatures of tens of millions of degrees in the process of falling in towards the immense gravitational attraction of the galaxy cluster, and hence to have become a source of X-rays."

## Ultraviolets

The "pre-reionization universe may be described by a model in which the first luminous sources and their individual EUV Strömgren spheres are embedded in a pre-heated, partially ionized IGM, rather than in a cold, completely neutral IGM."
"The FUV includes the photons relevant for H$_2$ photodissociation in the Lyman-Werner bands (11.2–13.6 eV)."
The "minimum IR/O and FUV backgrounds associated with any putative XRB from high-z quasars more than compensate for any positive feedback from the XRB in the form of an increased electron fraction in the IGM (see, however, Ricotti et al. (2001) on the positive feedback for H$_2$ formation in the vicinity of individual ionization fronts generated by hard stellar spectra)."

## Opticals

An "early XRB does not create significant amounts of new molecular hydrogen in the IGM, despite the increased population of free electrons. This is primarily due to the strong photo-destruction of H$^−$ and H$_2$ by the near IR/optical [...] and the far-UV [...] backgrounds, in the respective energy ranges 0.755–11.2 eV and 11.2–13.6 eV, associated with any XRB generated by QSOs."
The "XRB-enhanced electron fraction in the IGM prior to reionization increases the total optical depth to electron scattering."

## Visuals

"Far Ultraviolet Explorer (FUSE) observations of the He II absorption [are planned] toward the bright (visual magnitude V = 16.1) z = 2.885 quasar HE2347–4342 at high spectral resolution. Previous observations of HE2347–4342 at longer ultraviolet (UV) wavelengths showed it to be one of the brightest candidates for such observations (22, 23). As with the other quasars observed with HST, the He ii absorption in this object is mostly opaque, but there are wavelength intervals of high transmission that suggest we can see the beginnings of the He$^+$ re-ionization in the IGM in the redshift range accessible toward this quasar (22)."

## Violets

"Lyman alpha (Lyα forest lines in the spectra of QSOs are generally believed to be produced by intergalactic clouds (Sargent et al. 1980), though some evidence exists for their being associated with galaxies (Lanzetta et al. 1995; Le Brun, Bergeron & Boisse 1996; Bowen et al. 1996)."
"The near-UV observations of B2 1225 + 317 were obtained on the nights of 23 and 26 March 1987, with the Kitt Peak Maya1l4-m telescope plus echelle spectrograph. The instrumental configuration included the 58.5 lines mm$^{-1}$ 63° echelle grating with the blaze peak centred on the format. The 226-1 cross-disperser grating was used in second order through a copper sulphate filter to allow coverage of orders 96 through 65, or 3130-4500 Å. The poor seeing on both nights of 3-4 arcsec FWHM and the intermittent clouds on the second night imposed a substantial throughput penalty to maintain the resolution through the 1-arcsec slit. A decker masked the slit to a height of 10 arcsec, necessary for separating the orders at their closest spacing at the violet end of the echellogram. The spectrograph was rotated between the 3000-s exposures to orient the slit approximately along the parallactic angle to avoid selective losses in the far UV. Each object exposure was preceded and followed by Th-Ar arc lamp images for interpolation of the wavelength solution. The total exposure time on the quasar was 392 min on 23 March and 342 min on 26 March."

## Blues

"Because the FUSE bandpass stops short of wavelengths where the unobscured continuum of HE2347–4342 is visible (λ > 1190 Å), we obtained low-resolution HST spectra covering 1150–3200 Å on 21 August and 16 October 2000 to establish the continuum level. These observations each consisted of a 1060 s exposure using the Space Telescope Imaging Spectrograph (STIS) with grating G140L and a 600 s exposure using grating G230L. HE2347–4342 was slightly fainter (by 7%) in the October observation, but otherwise the spectra were identical. We scaled the October observation up to the August flux levels, and fitted a simple power law, F$_λ$ = 3.31 × 10$^{−15}$(λ/1000 Å)$^{−2.40}$ erg cm$^{−2}$ s$^{−1}$ Å$^{−1}$, with an extinction correction to spectral regions free of galactic absorption lines. We used a mean galactic extinction curve with ratio of selective to total extinction RV = 3.1 (27) and a color excess E(B − V ) = 0.014 (28), where B is the blue-band magnitude."

## Reds

The "He II Lyα absorption as a discrete forest of absorption lines in the redshift range 2.3 to 2.7."
"The distribution of absorption features according to redshift (z) and the column densities of gaseous material in different ions reveal the structure of the IGM and its density and ionization state."
Using "the Hubble Space Telescope (HST) (4) along the line of sight to the quasar Q0302-003 (z = 3.29). The observation showed the IGM to be essentially opaque at redshifts of z ∼ 3, and it seemed possible that the material in the H I Lyα forest was sufficient to account for the He II opacity (5)."
"The quantitative character of the evolution in opacity [is] with redshift".
"The mean opacity over the redshift interval 2.3 to 2.7 is τ$_{HeII}$ = 0.91±0.01 (29), similar to the sightline towards HS1700+64 (6)."

## Infrareds

Most "of the photodissociation occurs at near IR and optical wavelengths, given the nature of the H$^−$ cross" section.

## Submillimeters

"The intergalactic medium is a strong source of diffuse X-ray radiation by free-free emission (Jones & Forman 1984). It is observed at submillimeter wavelengths too, via the Sunyaev-Zeldovich (hereafter SZ) effect: a spectral distorsion of the Cosmic Microwave Background (hereafter CMB) due to the interaction of the electrons of the hot ionized gas with the photons of the CMB (Zel’dovich & Sunyaev 1969, Sunyaev & Zeldovich 1972). If the electronic temperature is determined from X-ray data, together with a model of the gas distribution, SZ data allow to derive the Compton optical depth (τ ) of the intergalactic gas. This parameter directly provides the gas mass, if it is integrated over solid angles. The association of X-ray and SZ data allows to estimate the Hubble constant, H$_0$, independently of the usual standard candles methods (see Holzapfel et al. 1997 for instance). The peculiar velocity of several clusters can also be derived from the Doppler effect, so that it should be possible to detect the large scale gravitational field which is produced by the dark matter."

## Radios

"Radio observations of galaxy clusters probe these complex processes through the study of cluster-scale synchrotron emission generated by relativistic electrons that gyrate in the magnetic fields of the IGM. Giant radio halos are the most spectacular, and best studied, examples of cluster-scale synchrotron sources."
"They are steep-spectrum, low brightness diffuse emissions that extend similarly to the hot X-ray emitting gas (eg. Ferrari et al 2008 for a review) and that are found in merging clusters (eg. Cassano et al 2010 and ref therein). The morphological and spectral properties of a number of radio halos suggest that the emitting electrons are accelerated by spatially distributed and "gentle" (i.e. poorly efficient, with acceleration time ~ 10$^8$yrs) mechanisms (e.g. Brunetti et al 2008)."

## Plasma objects

Coronal clouds of the hot intergalactic medium are likely in the Local Group and intergalactic medium, i.e., extragroup. The copious production of hot intragroup and intergalactic gas is a natural consequence of white dwarf-dominated halos.
"MHD turbulence, generated during cluster-cluster mergers, may be a source of particle reacceleration in the IGM."
Acceleration may be "by compressible turbulence in galaxy clusters, where the interaction between turbulence and the IGM is mediated by plasma instabilities and maintained collisional at scales much smaller than the Coulomb mean free path. In this regime most of the energy of fast modes is channelled into the reacceleration of relativistic particles and the acceleration process approaches a universal behaviour being self-regulated by the back-reaction of the accelerated particles on turbulence itself."
Relativistic "protons contribute to several percent (or less) of the cluster energy, consistent with the FERMI observations of nearby clusters, [...] compressible turbulence at the level of a few percent of the thermal energy can reaccelerate relativistic electrons at GeV energies, that are necessary to explain the observed diffuse radio emission in the form of giant radio halos."

## Gaseous objects

"The intergalactic medium (IGM) is the gaseous reservoir that provides the raw material for the galaxies that dominate our view of the visible universe."
"The distribution of absorption features according to redshift (z) and the column densities of gaseous material in different ions reveal the structure of the IGM and its density and ionization state. From the ionization state of the gaseous species, we can also infer the processes responsible for ionizing the gas, e.g., radiation from quasars in the early universe, or from early bursts of star formation."

## Astrochemistry

The relatively high metallicity, Z ~ 0.3 Z? for clusters and Z ~ 0.1 Z? for groups, indicates the gas has undergone a significant amount of stellar processing during an earlier epoch of star formation which would also produce stellar remnants, mostly white dwarfs. This stellar processed material is released into the interstellar medium to form its hot component via supernova explosions.

## Hydrogen

"On the basis of depletion of deuterium in the intergalactic medium (IGM), it has been noted by Chuvenkov and Vainer (1989) that exchange between the Milky Way Galaxy and the primordial IGM occurs at a rate of 3% of the galactic mass per 10$^9$ yr."

## Oxygens

"While it does not compose a significant fraction of the total oxygen in the Galactic ISM, and is never the dominant form of oxygen (Sutherland and Dopita 1993), the lithium-like ion O VI is an important diagnostic of the hot intercloud medium, as well as gas in the Galactic halo and the intergalactic medium."

## Silicons

"X-ray observations [consist] of excess silicon ejected by first-generation stars into the hot intergalactic medium of massive galaxy clusters."

## Interstellar medium

"In astronomy, the interstellar medium (or ISM) is the gas and cosmic dust that pervade interstellar space: the matter that exists between the star systems within a galaxy. It fills interstellar space and blends smoothly into the surrounding intergalactic medium. The interstellar medium consists of an extremely dilute (by terrestrial standards) mixture of ions, atoms, molecules, larger dust grains, cosmic rays, and (galactic) magnetic fields. The energy that occupies the same volume, in the form of electromagnetic radiation, is the interstellar radiation field."

## Milky Way

The existence of hot coronal gas within the Milky Way halo is one of the major discoveries of the ROSAT mission.
For no galactic corona, the gas pressure above some height (H) above the galactic disk is small compared to that in the disk. The galactic halo region is that region at a distance greater than H. When the only source of energy and mass for the halo is the galactic disk, gas streams into the halo until the pressure gradient is such that the pressure in the halo approaches the disk pressure, on a timescale of about 5 x 10$^7$ yr.
The X-ray luminosity (L$_x$) of the galactic coronal cloud is 1.5 x 10$^{40}$ erg s$^{-1}$.

## Local hot bubble

The 'local hot bubble' is a "hot X-ray emitting plasma within the local environment of the Sun." "This coronal gas fills the irregularly shaped local void of matter (McCammon & Sanders 1990) - frequently called the Local Hot Bubble (LHB)."

## Intracluster medium

The intracluster medium (ICM) is the superheated plasmas present at the center of a galaxy cluster. This is gas heated to temperatures of between roughly 10 and 100 megakelvins and consisting mainly of ionized hydrogen and helium, containing most of the baryonic material in the cluster. The ICM strongly emits X-ray radiation.
Studying the composition of the ICM at varying redshift (which results in looking at different points back in time) can therefore give a record of element production in the universe if they are typical.
Although the hard X-ray background for the range 2-10 keV has been nearly completely resolved into individual sources, primarily active galactic nuclei, there is a minor contribution from the hot, intergalactic medium in rich galaxy clusters.

## Galaxies

In 1974, radio sources were divided ... into two classes ... Fanaroff and Riley Class I (FRI), and Class II (FRII).
The distinction was originally made based on the morphology of the large-scale radio emission (the type was determined by the distance between the brightest points in the radio emission): FRI sources were brightest towards the centre, while FRII sources were brightest at the edges.
There is a reasonably sharp divide in luminosity between the two classes: FRIs were low-luminosity, FRIIs were high luminosity.
The morphology turns out to reflect the method of energy transport in the radio source. FRI objects typically have bright jets in the centre, while FRIIs have faint jets but bright hotspots at the ends of the lobes. FRIIs appear to be able to transport energy efficiently to the ends of the lobes, while FRI beams are inefficient in the sense that they radiate a significant amount of their energy away as they travel.
The FRI/FRII division depends on host-galaxy environment in the sense that the FRI/FRII transition appears at higher luminosities in more massive galaxies. FRI jets are known to be decelerating in the regions in which their radio emission is brightest,
The hotspots that are usually seen in FRII sources are interpreted as being the visible manifestations of shocks formed when the fast, and therefore supersonic, jet (the speed of sound cannot exceed c/√3) abruptly terminates at the end of the source, and their spectral energy distributions are consistent with this picture.

## Warm–hot intergalactic medium

The warm–hot intergalactic medium (WHIM) refers to a sparse, warm-to-hot (10$^5$ to 10$^7$ K) plasma that [may] exist in the spaces between galaxies and ... contain 40–50% of the baryons (that is, 'normal matter' which exists as plasma or as atoms and molecules, in contrast to dark matter) in the universe at the current epoch. Because of the high temperature of the medium, it is more readily observed from the ultraviolet and low energy X-ray emission. This was detected in the 0.4–0.6 keV energy band as of 2010 using the orbiting XMM-Newton observatory. This emission forms 12% ± 5% of the diffuse X-ray background radiation.
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.
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.
"These computer simulations [at right] show a swarm of dark matter clumps around our Milky Way galaxy. Some of the dark-matter concentrations are massive enough to spark star formation. Thousands of clumps of dark matter coexist with our Milky Way galaxy, shown in the center of the top panel. The green blobs in the middle panel are those dark-matter chunks massive enough to obtain gas from the intergalactic medium and trigger ongoing star formation, eventually creating dwarf galaxies. In the bottom panel, the red blobs are ultra-faint dwarf galaxies that stopped forming stars long ago."

## Hot intergalactic medium

The hot intergalactic medium is the hot intragroup gas within galaxy clusters and groups such as the Local Group of galaxies. This hot intragroup gas may have a total luminosity (L$_{bol}$) ~ 1.5 x 10$^{40}$ erg s$^{-1}$ and a temperature of 0.26 keV.
Where it is observed, the gas is a substantial fraction of the baryonic mass (the dominant baryonic component of clusters) comparable to that of the galactic component in groups. It appears that most of the baryons in the universe reside in the coronal clouds that compose the hot intergalactic medium.
The presence of a hot intergalactic medium in rich galaxy clusters is resulting in the stripping away of the interstellar medium. While ram pressure of the hot interstellar medium can be sufficient to push this gas bodily from a galaxy, it is not the only means by which the gas can be stripped. The rate of stripping of a galaxy moving through a hot intracluster medium is substantial and often exceeds that due to ram pressure alone. Thermal evaporation due to the long Coulomb mean free paths in the intracluster medium can also be the dominant cause of mass loss. And, Kelvin-Helmholtz instability may lead to substantial stripping rates under some circumstances.

## Interacting galaxies

Galaxies are generally classified according to ellipticity, spiralness, or irregularity. But, interacting galaxies may be a class by themselves, as is the science that determines two or more nearby galaxies are interacting, usually through the intergalactic medium.
"Stephan's Quintet (SQ) is a system consisting of at least four interacting galaxies which is well known for its complex dynamical and star formation history. It possesses a rich intergalactic medium (IGM), where hydrogen clouds, both atomic and molecular, associated with two starbursts (refered to as SQ A and B) have been found."
The composite image at right shows "Stephan’s Quintet [with] a diffuse arc of atomic hydrogen emission, indicated in green, roughly coincident with a shock front observed in the X-ray domain. Spitzer observations reveal powerful H$_2$ emission originating from the center of this 10$^3$ km s$^{-1}$ shock."
"One of [SQs] most striking properties is that the major part of the gas is in the intragroup medium, most likely the result of interactions in the past and present. [...] a few times 10$^8$ yr ago the group experienced a collision with NGC 7320c, a galaxy ∼4 arcmin to the east of NGC 7319 but with a very similar recession velocity (6583 kms$^{−1}$ [...] to the other galaxies in SQ. This collision removed most of the gas of NGC 7319 towards the west and east, and produced the eastern tidal tail which connects to NGC 7319. Presently, the group is experiencing another collision with the “intruder” galaxy NGC 7318b which strongly affects the interstellar medium (ISM) removed during the first collision."
"UGC 8335 [in the image at left] is a strongly interacting pair of spiral galaxies resembling two ice skaters. The interaction has united the galaxies via a bridge of material and has yanked two strongly curved tails of gas and stars from the outer parts of their bodies. Both galaxies show dust lanes in their centers. UGC 8335 is located in the constellation of Ursa Major, the Great Bear, about 400 million light-years from Earth. It is the 238th galaxy in Arp's Atlas of Peculiar Galaxies."
"This Hubble image [at second lower right] displays a beautiful pair of interacting spiral galaxies with swirling arms. The smaller of the two, dubbed LEDA 62867 and positioned to the left of the frame, seems to be safe for now, but will probably be swallowed by the larger spiral galaxy, NGC 6786 (to the right) eventually. There is already some disturbance visible in both components. The pair is number 538 in Karachentsev's Catalog of Pairs of Galaxies. A supernova was seen to explode in the large spiral in 2004. NGC 6786 is located in the constellation of Draco, the Dragon, about 350 million light-years away."
"UGC 4881 [at second left], known as the "The Grasshopper," is a stunning system consisting of two colliding galaxies. It has a bright curly tail containing a remarkable number of star clusters. The galaxies are thought to be halfway through a merger the cores of the parent galaxies are still clearly separated, but their disks are overlapping. A supernova exploded in this system in 1999 and astronomers believe that a vigorous burst of star formation may have just started. This notable object is located in the constellation of Lynx, some 500 million light-years away from Earth. UGC 4881 is the 55th galaxy in Arp's Atlas of Peculiar Galaxies."
"NGC 5257/8 (Arp 240) [at third right] is an astonishing galaxy pair, composed of spiral galaxies of similar mass and size, NGC 5257 and NGC 5258. The galaxies are visibly interacting with each other via a bridge of dim stars connecting the two galaxies, almost like two dancers holding hands while performing a pirouette. Both galaxies harbor supermassive black holes in their centers and are actively forming new stars in their disks. Arp 240 is located in the constellation Virgo, approximately 300 million light-years away, and is the 240th galaxy in Arp's Atlas of Peculiar Galaxies. With the exception of a few foreground stars from our own Milky Way all the objects in this image are galaxies."
"NGC 6050/IC 1179 (Arp 272) [at third left] is a remarkable collision between two spiral galaxies, NGC 6050 and IC 1179, and is part of the Hercules Galaxy Cluster, located in the constellation of Hercules. The galaxy cluster is part of the Great Wall of clusters and superclusters, the largest known structure in the universe. The two spiral galaxies are linked by their swirling arms. Arp 272 is located some 450 million light-years away from Earth and is the number 272 in Arp's Atlas of Peculiar Galaxies."
"NGC 454 [at fourth right] is galaxy pair comprising a large red elliptical galaxy and an irregular gas-rich blue galaxy. The system is in the early stages of an interaction that has severely distorted both components. The three bright blue knots of very young stars to the right of the two main components are probably part of the irregular blue galaxy. Although the dust lanes that stretch all the way to the center of the elliptical galaxy suggest that gas has penetrated that far, no signs of star formation or nuclear activity are visible. The pair is approximately 164 million light-years away [in the constellation Phoenix]."
Fourth at left "is a stunning pair of interacting galaxies, the barred spiral Seyfert 1 galaxy NGC 7469 (Arp 298, Mrk 1514), a luminous infrared source with a powerful starburst deeply embedded into its circumnuclear region, and its smaller companion IC 5283. This system is located about 200 million light-years away from Earth in the constellation of Pegasus, the Winged Horse."
Fifth at right, "NGC 6621/2 (VV 247, Arp 81) is a strongly interacting pair of galaxies, seen about 100 million years after their closest approach. It consists of NGC 6621 (to the left) and NGC 6622 (to the right). NGC 6621 is the larger of the two, and is a very disturbed spiral galaxy. The encounter has pulled a long tail out of NGC 6621 that has now wrapped behind its body. The collision has also triggered extensive star formation between the two galaxies. Scientists believe that Arp 81 has a richer collection of young massive star clusters than the notable Antennae galaxies (which are much closer than Arp 81). The pair is located in the constellation of Draco, approximately 300 million light-years away from Earth. Arp 81 is the 81st galaxy in Arp's Atlas of Peculiar Galaxies."
Fifth at left, "The galaxy system NGC 1614 has a bright optical center and two clear inner spiral arms that are fairly symmetrical. It also has a spectacular outer structure that consists principally of a large one-sided curved extension of one of these arms to the lower right, and a long, almost straight tail that emerges from the nucleus and crosses the extended arm to the upper right. The galaxy appears to be the result of a tidal interaction and the resulting merger of two predecessor systems. The system has a nuclear region of quasar-like luminosity, but shows no direct evidence for an active nucleus. It is heavily and unevenly reddened across its nucleus, while infrared imaging also shows a ridge of dust. The linear tail to the upper right and extended arms to the lower right are likely the remains of an interacting companion and the tidal plume(s) caused by the collision. NGC 1614 is located about 200 million light-years away from Earth in the constellation of Eridanus, the River."

## Galaxy filaments

"This is a map of the universe [at right] within 500 million light years. It shows most of the major galaxy superclusters that surround the Virgo supercluster. These superclusters are not isolated in space but together with many other smaller concentrations of galaxies they form parts of extensive walls of galaxies surrounding large voids. Three of the biggest walls near us are marked on the map as well as several of the largest voids. There are several hundred thousand large galaxies within 500 million light years, so even on this scale our galaxy is a very insignificant object."
"The Telescope for Habitable Exoplanets and Interstellar/Intergalactic Astronomy (theia), which would observe in both the optical and ultraviolet, would have as one of its core science goals an improved understanding of the cosmic web."
For example, the Local Group "is just one of a hundred or so groups and clusters belonging to the Virgo Supercluster. [...] the Virgo Supercluster is part of a gigantic structure [...] called the Pisces-Cetus Supercluster Complex."
The "cosmic web" is a "web that links the cosmos".

## Cosmic webs

On the right is a map "showing gas filaments of the cosmic web (blue) running from the top to the bottom of the image, detected using the MUSE instrument at the Very Large Telescope. The white dots embedded within these filaments are very active star forming galaxies which are being fed by the filaments, and which are detected using the Atacama Large Millimeter/submillimeter array."
"I originally did not expect to see such cosmic web filaments ... they were thought to be much fainter and very hard to see."
"Using the Multi Unit Spectroscopic Explorer instrument on the European Southern Observatory's Very Large Telescope in Chile, [...] focused on the SSA22 Protocluster, which lies about 12 billion light-years away from Earth in the constellation Aquarius [...] light emitted by hydrogen gas excited by ultraviolet rays from galaxies within the protocluster [was detected and mapped]."
"The main obstacle to see the filaments is their faintness. To circumvent this, one can turn to a region where the filaments are much brighter than the usual case. In this work, we focus on a core of the protocluster where a number of star-bursting galaxies illuminate the filaments."
"These strands are the brightest threads of the cosmic web found yet, but they're still quite dim. The emission levels from the very outskirts of the filaments are as low as 5% that of the ambient background light from the rest of the sky."
"These gaseous structures were predicted for years theoretically, but astronomers have struggled to map them directly. Our work shows that mapping cosmic web filaments is now possible, which means that we have obtained a novel tool to understand the formation of galaxies and supermassive black holes."
"They are thought to fuel the intense activity seen in galaxies — star formation and the growth of supermassive black holes. Thus, our research adds a key piece to understand how galaxies and supermassive black holes acquire their fuels."

## Evolution of the intergalactic medium

The theory of the Big Bang includes an early period for the universe of massive star formation.
"The metallicity at z = 5 exceeds 3.5 × 10$^{-4}$, which in turn implies that this fraction of the universal massive star formation took place beyond this redshift. This is sufficient to have ionized the intergalactic medium."
"An alternative and attractive explanation is that the radio sources rotate relative to the intergalactic medium, the axis of rotation being preferentially aligned with a universal vorticity; for, if the Universe has net angular momentum, this will be conserved and provide a natural axis of rotation aligned over cosmic scales."

## Voids

Voids located in high-density environments are smaller than voids situated in low-density spaces of the universe.
"[V]oids [are] now considered as regular astronomical entities in their own rights, [and] are clustered."
"In cosmogonic schemes involving cold dark matter, there is no equally obvious process that might inhibit galaxy formation in the incipient voids".

## Astrophysics

In astronomical spectroscopy, the Gunn–Peterson trough is a feature of the spectra of quasars due to the presence of neutral hydrogen in the intergalactic medium (IGM). The trough is characterized by suppression of electromagnetic emission from the quasar at wavelengths less than that of the Lyman-alpha line at the redshift of the emitted light.
In 2001, a quasar with a redshift z = 6.28 using data from the Sloan Digital Sky Survey, that a Gunn–Peterson trough was finally observed.

## Technology

The Large-Aperture Experiment to Detect the Dark Ages (LEDA) is designed to detect the spectrum of the 21 cm Hydrogen line from the intergalactic medium (IGM) at redshifts of 15-30, when the Universe was just ~1% of its present age. LEDA principally comprises a "large-N" array correlator (512 inputs over ~ 60 MHz), calibration & imaging system, and instrumentation for measurement of calibrated total-power. These systems will use the station 1 of the Long Wavelength Array as an aperture. LEDA is one of several efforts seeking to study cosmological reionization and the preceding Dark Ages. Others include the Precision Array for Probing the Epoch of Reionization (PAPER), Low Frequency Array (LOFAR), Murchison Widefield Array (MWA), and Giant Metrewave Radio Telescope (GMRT). LEDA will feature array-based calibration to improve the accuracy of foreground subtraction from the total-power signal.

## Sounding rockets

"Two calorimeter arrays have already measured cosmic x-rays as part of a joint undertaking of the University of Wisconsin-Madison and NASA's Goddard Space Flight Center known as the X-ray Quantum Calorimeter (XQC) project. Carried aloft on a sounding rocket [image at left], the instrument observed the diffuse soft x-ray emission from a large solid angle at high galactic latitude for 240 seconds in 1996 and again, in March of [1999], with improved hardware. The spectrum [...] is from a very preliminary analysis of data from the 1999 flight. Although the data await further, improved analysis, lines are clearly seen at the energies of highly ionized oxygen."
"The 36-pixel array, consisting of ion-implanted, micromachined silicon thermistors and mercury telluride thermalizing x-ray absorbers, was operated at 60 mK and had an energy resolution as good as 5 eV in the 0.05--1 keV band. The ongoing XQC project, led by one of us (McCammon) at Wisconsin and by Andrew Szymkowiak and Scott Porter at Goddard, serves as a testbed for new calorimeter technology optimized for soft x-rays."
"The X-Ray Spectrometer (XRS) calorimeter instrument, scheduled for launch as part of the Japanese-US x-ray astronomy satellite ASTRO-E in early 2000, has an array of 32 microcalorimeters at the focal plane. The arrays for XRS, like those for XQC, were developed and fabricated at Goddard and consist of silicon thermistors and mercury telluride absorbers. Typical resolution is 9--10 eV at 3 keV and is 11--12 eV at 6 keV --- an order of magnitude better than a silicon x-ray ionization detector. This broad band (0.4--12 keV) instrument was developed jointly by a Goddard-Wisconsin, team led Richard Kelley and a Japanese team from several institutions led by the Institute of Space and Astronautical Science. During its two years of operation, XRS will apply the power of x-ray spectroscopy to unravel the mysteries of the hot and energetic universe."

## Hypotheses

* The intergalactic medium has at least one chemical characteristic that separates it from the interstellar medium and the interplanetary medium.
"O VI detection rate is slightly lower (14%) if we use the DS08 b LW measurements to define the non-BLA control group."
"The cluster and control group galaxies were then distributed in two color-magnitude diagrams (CMDs), and any extinction and reddening of the cluster group was searched by shifting the cluster CMD both in magnitude and color with respect to the control CMD until the two CMDs matched."
"The camera is a proof of concept for the 2m-class Antarctic Cosmic Web Imager telescope, a dedicated experiment to directly detect and map the redshifted lyman alpha [...] emission of the Intergalactic Medium (IGM)."
"Our project will operate as the proof of concept for a number of key [Square Kilometer Array of the Australian Square Kilometre Array Pathfinder (ASKAP)] SKA-related technologies."