Interstellar medium

The ISM is usually far from thermodynamic equilibrium. Collisions establish a Maxwell–Boltzmann distribution of velocities, and the 'temperature' normally used to describe interstellar gas is the 'kinetic temperature', which describes the temperature at which the particles would have the observed Maxwell–Boltzmann velocity distribution in thermodynamic equilibrium. However, the interstellar radiation field is typically much weaker than a medium in thermodynamic equilibrium; it is most often roughly that of an A star (surface temperature of ~10,000 K) highly diluted. Therefore, bound levels within an atom or molecule in the ISM are rarely populated according to the Boltzmann formula . Depending on the temperature, density, and ionization state of a portion of the ISM, different heating and cooling mechanisms determine the temperature of the gas. Heating mechanisms ; Heating by low-energy cosmic rays: The first mechanism proposed for heating the ISM was heating by low-energy cosmic rays. Cosmic rays are an efficient heating source able to penetrate in the depths of molecular clouds. Cosmic rays transfer energy to gas through both ionization and excitation and to free electrons through Coulomb interactions. Low-energy cosmic rays (a few MeV) are more important because they are far more numerous than high-energy cosmic rays. ; Photoelectric heating by grains: The ultraviolet radiation emitted by hot stars can remove electrons from dust grains. The photon is absorbed by the dust grain, and some of its energy is used to overcome the potential energy barrier and remove the electron from the grain. This potential barrier is due to the binding energy of the electron (the work function) and the charge of the grain. The remainder of the photon's energy gives the ejected electron kinetic energy which heats the gas through collisions with other particles. A typical size distribution of dust grains is n(r) ∝ r, where r is the radius of the dust particle. Assuming this, the projected grain surface area distribution is πrn(r) ∝ r. This indicates that the smallest dust grains dominate this method of heating. ; Photoionization: When an electron is freed from an atom (typically from absorption of a UV photon) it carries kinetic energy away of the order E − E. This heating mechanism dominates in H II regions, but is negligible in the diffuse ISM due to the relative lack of neutral carbon atoms. ; X-ray heating: X-rays remove electrons from atoms and ions, and those photoelectrons can provoke secondary ionizations. As the intensity is often low, this heating is only efficient in warm, less dense atomic medium (as the column density is small). For example, in molecular clouds only hard x-rays can penetrate and x-ray heating can be ignored. This is assuming the region is not near an x-ray source such as a supernova remnant. ; Chemical heating: Molecular hydrogen (H2) can be formed on the surface of dust grains when two H atoms (which can travel over the grain) meet. This process yields 4.48 eV of energy distributed over the rotational and vibrational modes, kinetic energy of the H2 molecule, as well as heating the dust grain. This kinetic energy, as well as the energy transferred from de-excitation of the hydrogen molecule through collisions, heats the gas. ; Grain-gas heating: Collisions at high densities between gas atoms and molecules with dust grains can transfer thermal energy. This is not important in HII regions because UV radiation is more important. It is also less important in diffuse ionized medium due to the low density. In the neutral diffuse medium grains are always colder, but do not effectively cool the gas due to the low densities. Grain heating by thermal exchange is very important in supernova remnants where densities and temperatures are very high. Gas heating via grain-gas collisions is dominant deep in giant molecular clouds (especially at high densities). Far infrared radiation penetrates deeply due to the low optical depth. Dust grains are heated via this radiation and can transfer thermal energy during collisions with the gas. A measure of efficiency in the heating is given by the accommodation coefficient: \alpha = \frac{T_2 - T}{T_d - T} where T is the gas temperature, Td the dust temperature, and T2 the post-collision temperature of the gas atom or molecule. This coefficient was measured by as α = 0.35. ; Other heating mechanisms: A variety of macroscopic heating mechanisms are present including: :* Gravitational collapse of a cloud :* Supernova explosions :* Stellar winds :* Expansion of H II regions :* Magnetohydrodynamic waves created by supernova remnants Cooling mechanisms ; Fine structure cooling: The process of fine structure cooling is dominant in most regions of the Interstellar Medium, except regions of hot gas and regions deep in molecular clouds. It occurs most efficiently with abundant atoms having fine structure levels close to the fundamental level such as: C II and O I in the neutral medium and O II, O III, N II, N III, Ne II and Ne III in H II regions. Collisions will excite these atoms to higher levels, and they will eventually de-excite through photon emission, which will carry the energy out of the region. ; Cooling by permitted lines: At lower temperatures, more levels than fine structure levels can be populated via collisions. For example, collisional excitation of the n = 2 level of hydrogen will release a Ly-α photon upon de-excitation. In molecular clouds, excitation of rotational lines of CO is important. Once a molecule is excited, it eventually returns to a lower energy state, emitting a photon which can leave the region, cooling the cloud. ==Observations of the ISM==

Despite its extremely low density, photons generated in the ISM are prominent in nearly all bands of the electromagnetic spectrum. In fact the optical band, on which astronomers relied until well into the 20th century, is the one in which the ISM is least obvious. • Ionized gas radiates at a broad range of energies via bremsstrahlung. For gas in the warm phase (104 K) this is mostly detected in microwaves, while bremsstrahlung from the million-kelvin coronal gas is prominent in soft X-rays. In addition, many spectral lines are produced, including the ones significant for cooling mentioned in the previous section. One of these, a forbidden line of doubly-ionized oxygen, gives many nebulae their apparent green colour in visual observations, and was once thought to be a new element, nebulium. Spectral lines from highly excited states of hydrogen are detectable at infra-red and longer wavelengths, down to radio recombination lines which, unlike optical lines, are not absorbed by dust and so can trace ionized regions throughout the disk of the Galaxy. Coronal gas emits a different set of lines, since atoms are stripped of a larger fraction of their electrons at its high temperature. • The warm neutral medium produces most of the 21-cm line emission from hydrogen detected by radio telescopes, although atomic hydrogen in the cold neutral medium also contributes, both in emission and by absorption of photons from background warm gas ('H I self-absorption', HISA). While not important for cooling, the 21-cm line is easily observable at high spectral and angular resolution, giving us our most detailed view of the WNM. • Molecular clouds are detected via spectral lines produced by changes in the rotational quantum state of small molecules, especially carbon monoxide, CO. The most widely used line is at 115 GHz, corresponding to the change from 1 to 0 quanta of angular momentum. Hundreds of other molecules have been detected, each with many lines, which allows physical and chemical processes in molecular clouds to be traced in some detail. These lines are most common at millimetre and sub-mm wavelengths. By far the most common molecule in molecular clouds, H2, is usually not directly observable, as it stays in its ground state except when excited by rare events such as interstellar shock waves. There is some 'dark gas', regions where hydrogen is in molecular form and therefore does not emit the 21-cm line, but CO molecules are broken up so the CO lines are also not present. These regions are inferred from the presence of dust grains with no matching line emission from gas. • Interstellar dust grains re-emit the energy they absorb from starlight as quasi-blackbody emission in the far infrared, corresponding to typical dust grain temperatures of 20–100 K. Very small grains, essentially fragments of graphene bonded to hydrogen atoms around their edges (polycyclic aromatic hydrocarbons, PAHs), emit numerous spectral lines in the mid-infrared, at wavelengths around 10 micron. Nanometre-sized grains can be spun up to rotate at GHz frequencies by a collision with a single ultraviolet photon, and dipole radiation from such spinning grains is believed to be the source of anomalous microwave emission. • Cosmic rays generate gamma-ray photons when they collide with atomic nuclei in ISM clouds. The electrons amongst cosmic ray particles collide with a small fraction of photons in the interstellar radiation field and the cosmic microwave background and bump up the photon energies to X-rays and gamma-rays, via inverse Compton scattering. Due to the galactic magnetic field, charged particles follow spiral paths, and for cosmic-ray electrons this spiralling motion generates synchrotron radiation which is very bright at low radio frequencies. == Radiowave propagation ==

/km as a function of frequency over the EHF band. Peaks in absorption at specific frequencies are a problem, due to atmosphere constituents such as water vapor (H2O) and carbon dioxide (CO2). Radio waves are affected by the plasma properties of the ISM. The lowest frequency radio waves, below ≈ 0.1 MHz, cannot propagate through the ISM since they are below its plasma frequency. At higher frequencies, the plasma has a significant refractive index, decreasing with increasing frequency, and also dependent on the density of free electrons. Random variations in the electron density cause interstellar scintillation, which broadens the apparent size of distant radio sources seen through the ISM, with the broadening decreasing with frequency squared. The variation of refractive index with frequency causes the arrival times of pulses from pulsars and Fast radio bursts to be delayed at lower frequencies (dispersion). The amount of delay is proportional to the column density of free electrons (Dispersion measure, DM), which is useful for both mapping the distribution of ionized gas in the Galaxy and estimating distances to pulsars (more distant ones have larger DM). A second propagation effect is Faraday rotation, which affects linearly polarized radio waves, such as those produced by synchrotron radiation, one of the most common sources of radio emission in astrophysics. Faraday rotation depends on both the electron density and the magnetic field strength, and so is used as a probe of the interstellar magnetic field. The ISM is generally very transparent to radio waves, allowing unimpeded observations right through the disk of the Galaxy. There are a few exceptions to this rule. The most intense spectral lines in the radio spectrum can become opaque, so that only the surface of the line-emitting cloud is visible. This mainly affects the carbon monoxide lines at millimetre wavelengths that are used to trace molecular clouds, but the 21-cm line from neutral hydrogen can become opaque in the cold neutral medium. Such absorption only affects photons at the line frequencies: the clouds are otherwise transparent. The other significant absorption process occurs in dense ionized regions. These emit photons, including radio waves, via thermal bremsstrahlung. At short wavelengths, typically microwaves, these are quite transparent, but their brightness approaches the black body limit as \propto \lambda^{2.1}, and at wavelengths long enough that this limit is reached, they become opaque. Thus metre-wavelength observations show H II regions as cool spots blocking the bright background emission from Galactic synchrotron radiation, while at decametres the entire galactic plane is absorbed, and the longest radio waves observed, 1 km, can only propagate 10-50 parsecs through the Local Bubble. The frequency at which a particular nebula becomes optically thick depends on its emission measure EM = \int n_e^2\, dl, the column density of squared electron number density. Exceptionally dense nebulae can become optically thick at centimetre wavelengths: these are just-formed and so both rare and small ('Ultra-compact H II regions') The general transparency of the ISM to radio waves, especially microwaves, may seem surprising since radio waves at frequencies > 10 GHz are significantly attenuated by Earth's atmosphere (as seen in the figure). But the column density through the atmosphere is vastly larger than the column through the entire Galaxy, due to the extremely low density of the ISM. ==History of knowledge of interstellar space==

HH 110 ejects gas through interstellar space. The word 'interstellar' (between the stars) was coined by Francis Bacon in the context of the ancient theory of a literal sphere of fixed stars. Later in the 17th century, when the idea that stars were scattered through infinite space became popular, it was debated whether that space was a true vacuum or filled with a hypothetical fluid, sometimes called aether, as in René Descartes' vortex theory of planetary motions. While vortex theory did not survive the success of Newtonian physics, an invisible luminiferous aether was re-introduced in the early 19th century as the medium to carry light waves; e.g., in 1862 a journalist wrote: "this efflux occasions a thrill, or vibratory motion, in the ether which fills the interstellar spaces." In 1864, William Huggins used spectroscopy to determine that a nebula is made of gas. Huggins had a private observatory with an 8-inch telescope, with a lens by Alvan Clark; but it was equipped for spectroscopy, which enabled breakthrough observations. From around 1889, Edward Barnard pioneered deep photography of the sky, finding many 'holes in the Milky Way'. At first he compared them to sunspots, but by 1899 was prepared to write: "One can scarcely conceive a vacancy with holes in it, unless there is nebulous matter covering these apparently vacant places in which holes might occur". These holes are now known as dark nebulae, dusty molecular clouds silhouetted against the background star field of the galaxy; the most prominent are listed in his Barnard Catalogue. The first direct detection of cold diffuse matter in interstellar space came in 1904, when Johannes Hartmann observed the binary star Mintaka (Delta Orionis) with the Potsdam Great Refractor. Hartmann reported that absorption from the "K" line of calcium appeared "extraordinarily weak, but almost perfectly sharp" and also reported the "quite surprising result that the calcium line at 393.4 nanometres does not share in the periodic displacements of the lines caused by the orbital motion of the spectroscopic binary star". The stationary nature of the line led Hartmann to conclude that the gas responsible for the absorption was not present in the atmosphere of the star, but was instead located within an isolated cloud of matter residing somewhere along the line of sight to this star. This discovery launched the study of the interstellar medium. Interstellar gas was further confirmed by Slipher in 1909, and then by 1912 interstellar dust was confirmed by Slipher. Interstellar sodium was detected by Mary Lea Heger in 1919 through the observation of stationary absorption from the atom's "D" lines at 589.0 and 589.6 nanometres towards Delta Orionis and Beta Scorpii. In the series of investigations, Viktor Ambartsumian introduced the now commonly accepted notion that interstellar matter occurs in the form of clouds. Subsequent observations of the "H" and "K" lines of calcium by revealed double and asymmetric profiles in the spectra of Epsilon and Zeta Orionis. These were the first steps in the study of the very complex interstellar sightline towards Orion. Asymmetric absorption line profiles are the result of the superposition of multiple absorption lines, each corresponding to the same atomic transition (for example the "K" line of calcium), but occurring in interstellar clouds with different radial velocities. Because each cloud has a different velocity (either towards or away from the observer/Earth), the absorption lines occurring within each cloud are either blue-shifted or red-shifted (respectively) from the lines' rest wavelength through the Doppler Effect. These observations confirming that matter is not distributed homogeneously were the first evidence of multiple discrete clouds within the ISM. . The growing evidence for interstellar material led to comment: "While the interstellar absorbing medium may be simply the ether, yet the character of its selective absorption, as indicated by Kapteyn, is characteristic of a gas, and free gaseous molecules are certainly there, since they are probably constantly being expelled by the Sun and stars." The same year, Victor Hess's discovery of cosmic rays, highly energetic charged particles that rain onto the Earth from space, led others to speculate whether they also pervaded interstellar space. The following year, the Norwegian explorer and physicist Kristian Birkeland wrote: "It seems to be a natural consequence of our points of view to assume that the whole of space is filled with electrons and flying electric ions of all kinds. We have assumed that each stellar system in evolutions throws off electric corpuscles into space. It does not seem unreasonable therefore to think that the greater part of the material masses in the universe is found, not in the solar systems or nebulae, but in 'empty' space" . noted that "it could scarcely have been believed that the enormous gaps between the stars are completely void. Terrestrial aurorae are not improbably excited by charged particles emitted by the Sun. If the millions of other stars are also ejecting ions, as is undoubtedly true, no absolute vacuum can exist within the galaxy." In September 2012, NASA scientists reported that polycyclic aromatic hydrocarbons (PAHs), subjected to interstellar medium (ISM) conditions, are transformed, through hydrogenation, oxygenation and hydroxylation, to more complex organics, "a step along the path toward amino acids and nucleotides, the raw materials of proteins and DNA, respectively". Further, as a result of these transformations, the PAHs lose their spectroscopic signature, which could be one of the reasons "for the lack of PAH detection in interstellar ice grains, particularly the outer regions of cold, dense clouds or the upper molecular layers of protoplanetary disks." for tracking polycyclic aromatic hydrocarbons (PAHs) in the universe. According to scientists, more than 20% of the carbon in the universe may be associated with PAHs, possible starting materials for the formation of life. PAHs seem to have been formed shortly after the Big Bang, are widespread throughout the universe, and are associated with new stars and exoplanets. In April 2019, scientists, working with the Hubble Space Telescope, reported the confirmed detection of the large and complex ionized molecules of buckminsterfullerene (C60) (also known as "buckyballs") in the interstellar medium spaces between the stars. In September 2020, evidence was presented of solid-state water in the interstellar medium, and particularly, of water ice mixed with silicate grains in cosmic dust grains. ==See also==