BACKGROUND RADIATION, INFRARED MARTIN HARWIT Even on the darkest nights, a diffuse glow permeates the sky. Part of this glow condenses into stellar images in the focal plane of large telescopes, but a component remains which is genuinely diffuse. Some diffuse sources are nebular patches on the sky, gas clouds in the vicinity of stars, or entire galaxies at extreme distances. Still other sources of radiation produce a totally isotropic background having one and the same surface brightness in whatever direction we look. An isotropic background flux is the most difficult to measure accurately. INFRARED BACKGROUND MEASUREMENTS Any warm body emits radiation-glows. At room temperature and below, this radiated energy is emitted at infrared wavelengths that span the range of 1 - 1000 **. Ambient temperature telescopes, therefore, emit infrared radiation, and that emission would normally not be distinguishable from a diffuse cosmic glow were it not for the possibility of cooling telescopes and detectors to extremely low temperatures, around 2 K. That way, practically all infrared radiation incident on the astronomer's detectors must be arriving at the telescope from space, rather than from the telescope itself. The most ambitious instrument constructed to detect the diffuse infrared background is a space telescope cooled to just such low temperatures. It is the Cosmic Background Explorer, (COBE), built under the auspices of the U.S. National Aeronautics and Space Administration, NASA. In Earth orbit, above any glow from the atmosphere, it has been able to peer out into the universe to detect faint glows across most of the infrared wavelength domain. Before the advent of COBE in late 1989, such measurements could only be undertaken in short-lived rocket observations using similarly cooled telescopes. The extreme cooling of telescopes is not strictly necessary in observations conducted at short infrared wavelengths, around 1 **, and some such observations can be undertaken from mountaintop observatories, although the Earth's atmosphere then produces a limiting foreground glow. SOURCES OF DIFFUSE INFRARED RADIATION An astronomer's telescope receives light not only from the star or galaxy being observed, but from a wide variety of other sources also present in the telescope's field of view. Those sources are at vastly differing distances from the telescope (see Fig. 1) and contribute radiation through a wide selection of different mechanisms. AIRGLOW Airglow emanating from the terrestrial atmosphere enters the telescope from the lowest few hundred kilometers above Earth's surface. For the astronomer this radiation is a nuisance that must be weeded out to reveal the genuine astronomical sources beyond the atmosphere. At the shortest wavelengths, ranging out to about 4 **, collisionally excited OH radicals at high altitudes emit radiation at scores of well-defined wavelengths, each wavelength corresponding to a particular transition of a collisionally excited OH radical from one vibrationally and rotationally excited state to a lower one. At wavelengths between 1 and 1000 **, the atmosphere also emits over broad wavelength ranges, through emission by water vapor, carbon dioxide, methane, N2O, ozone, and other atmospheric constituents. In much of this wavelength regime, the atmosphere is also opaque, so that astronomical observations must be carried out from high altitudes or from space. ZODIACAL LIGHT The Sun is enclosed in a cloud of finely dispersed dust that orbits the Sun in a disk-shaped volume symmetrically distributed about the ecliptic plane. This zodiacal cloud contains roughly micrometer-sized grains that scatter light at visible and near-infrared wavelengths. At wavelengths longward of 3 **, the thermal emission from these grains begins to dominate scattered sunlight. For observations on diffuse astronomical sources, this foreground emission can represent a serious contaminant that needs to be subtracted before the intrinsic surface brightness of more distant astronomical nebulosity can be assessed. The uncertainty of whether infrared radiation from the telescope, from the atmosphere, or from zodiacal dust has been adequately subtracted from flux levels recorded by infrared detectors makes the detection of an isotropic, cosmic background flux particularly difficult. For patchy, diffuse glow emanating, say, from the galactic plane, a reference signal can always be obtained at higher galactic latitudes; but isotropic background radiation can only be determined through an absolute flux level measurement, independent of any other reference region in the sky. GALACTIC STARLIGHT As we leave the solar system and penetrate further out into the Milky Way, we find four other sources of diffuse radiation, stars, dust, gaseous-atomic, ionic, and molecular-constituents, and electrons both at thermal energies and at the high energies of cosmic rays. The glow emanating from these broadly distributed sources is called the diffuse Galactic emission. 1. Unresolved stars produce a diffuse glow. Most of this radiation lies in the visible part of the spectrum and in the near infrared where stars emit much of their energy. The component of this light that reaches us from remote regions preferentially lies in the infrared, because much of the ultraviolet and visible radiation is absorbed in interstellar dust clouds along the way. 2. The interstellar dust heated by starlight appears to consist of two quite distinct components. The first consists of small grains in the size range around 10***** (0.1 **). This dust has an equilibrium temperature that is much cooler than the illuminating star and, by Planck's radiation law, correspondingly reemits the absorbed energy at much longer wavelengths-around 10 ** when the absorbing dust cloud lies relatively close to the illuminating stars, and 100 ** when the dust is at larger distances. The emitted radiation has a continuum spectrum, somewhat akin to a blackbody spectrum except peaked toward shorter wavelengths. This spectral shift comes about because small grains radiate with systematically decreasing efficiency at wavelengths substantially longer than the grain diameter. A second component formerly thought to be dust recently has been identified with macromolecules, possibly polyaromatic hydrocarbon (PAH) molecules. Upon absorbing a single quantum of starlight, parts of these molecules are excited to energies equivalent to temperatures of several hundred degrees kelvin. The molecular heat capacities are so small that the tiny amounts of energy carried by a single absorbed quantum can raise temperatures to these heights, causing the PAHs to emit at substantially shorter wavelengths than larger grains. Being small, PAHs also act more like molecules than like macroscopic objects, and they emit radiation at rather well-defined energies, in contrast to the continuum radiation emitted by larger grains. Characteristic PAH wavelengths lie at 3.3, 6.2, 7.7, 8.6, and 11.3 **. Because this radiation is precipitated by the absorption of single photons of radiation, its spectrum tends to be independent of a PAH grain's distance from the illuminating star or stars. As a result, PAH emission can be observed in emission from clouds of dust at high galactic latitudes, from regions that might be as far as 100 pc from the galactic plane, producing a diffuse though patchy glow straddling the entire plane. 3. Gas clouds between the stars heated and ionized by starlight emit radiation when their atomic, ionic, or molecular constituents become collisionally excited and cascade down to the ground state emitting quanta of radiation at well-defined wavelengths characterizing the active atom or molecule. In hot, ionized gases, electrons undergoing close encounters with ions are electrostatically deflected in their trajectories and emit radiation through free-free transitions; that is, the electron is bound to the ion neither in its initial approach nor after deflection. Free-free emission is observed in diffuse ionized regions throughout the galactic plane and is most readily identified at radio wavelengths, though the emission takes place at shorter infrared wavelengths as well. 4. Electrons accelerated to relativistic energies constitute part of the cosmic ray particle component traversing interstellar space. They scatter-collide with-radiation in what is termed Compton scattering, a process in which the scattered energy can be substantially changed from the incident energy. When the electron loses significant amounts of energy to the quantum of incident radiation, in Compton scattering, we speak of inverse Compton scattering and the associated energy increase of the quanta of radiation expresses itself in a shortening of the wavelength. In particular, it can lead to the conversion of radio waves into infrared radiation. That kind of a diffuse infrared glow appears primarily to be emitted in active galaxies and quasars exhibiting powerful radio as well as x-ray emission in a compact nuclear region, but it may be present in galactic emission as well. Highly energetic electrons gyrating in the interstellar magnetic field can also produce synchrotron radiation at infrared wavelengths, that is, radiation associated with the accelerations the electrons experience in their motions across the magnetic field. EXTRAGALACTIC EMISSION Beyond the Milky Way, stretching to the cosmic horizon, lie perhaps a hundred billion galaxies and millions of quasars. These sources appear to be the primary contributors to the extragalactic background light. Most of these sources are believed to emit infrared radiation by virtue of the various processes enumerated previously, but there may be exceptions among objects whose sources of energy are not well understood. The most extreme among these are the quasars, which appear to be galaxies in whose nuclei violent processes hundreds of times more powerful than the emission from normal galaxies take place. A number of other extragalactic contributors to the diffuse infrared radiation have been postulated but never observed. 1. Individual stars and individual globular clusters may be tidally removed from galaxies during intergalactic collisions and left to drift in extragalactic space. If so, their stellar emission could be a significant contributor to the near-infrared diffuse background. 2. The earliest galaxies to have been formed may all have had their origin in one brief outburst when the universe was still young. Many stars formed at that time could have been massive and highly luminous. Such massive stars are thought to be the primary contributors to the formation of heavy elements, such as carbon, oxygen, nitrogen, and possibly iron, and their injection into the interstellar medium through supernova explosions. Such elements are found in appreciable abundances in all stars observed today, suggesting that all known stars formed in the wake of an earlier generation of massive-and therefore luminous- precursors. Granted these assumptions, we would expect a diffuse background flux due to early galaxies whose radiation, though originally at visible and ultraviolet wavelengths, would now be observed redshifted into the near infrared. 3. Dust ejected into intergalactic space, shortly after the first galaxies formed, would have radiated at wavelengths around 100 **, and the redshifted flux received today would most likely be found at submillimeter wavelengths. MICROWAVE BACKGROUND RADIATION In addition to all of the previously mentioned sources of background radiation, there is the ubiquitous microwave background radiation, a remnant of the earliest hot phases of the universe (see Background Radiation, Microwave). A portion of this radiation stretches from the radio region into the infrared domain. As seen in Fig. 2, this cosmic background radiation contains far more radiant energy than sources at all other wavelengths combined. Additional Reading Bowyer, S. and Leinert, C., eds. (1989). Galactic and Extragalactic Background Radiation. IAU Symposium 139. Kluwer, Dordrecht. Clegg, P.(1978). Cosmic background: Measurements of the spectrum. In Infrared astronomy, G.Setti and G.G.Fazio, eds., NATO Advanced Studies Institutes Series (Series C, Mathematical and Physical Sciences). D.Reidel, Dordrecht, p. 181. Harrison, E.(1987). Darkness at Night. Harvard University Press, Cambridge, MA. Harwit, M.(1978). Infrared astronomical background radiation. In Infrared Astronomy, G.Setti and G.G.Fazio, eds., NATO Advanced Studies Institutes Series (Series C, Mathematical and Physical Sciences). D.Reidel, Dordrecht, p. 173. See also Background see also Background Radiation, Microwave; Diffuse Galactic Light; Galaxies, Infrared Galaxies, Infrared Emission; Infrared Astronomy, Space Missions;Zodiacal Light and Gegenschein.