B
Glossary

21-cm line: The hyperfine transition of neutral hydrogen (H I) with a corresponding rest-frame wavelength of 21 cm, or a frequency of 1.420 GHz. If the line is optically thin it is a direct measure of the neutral hydrogen content along a given line of sight.

56Ni: Radioactive isotope of nickel, produced in large quantities in Type Ia supernovae.

α Cep: A rapidly rotating A-type star in the constellation of Cepheus. It has the traditional name Alderamin.

fNL: A dimensionless parameter describing the amplitude of non-gaussianity of the fluctuations in the CMB.

Absorption line spectroscopy: A spectroscopic technique to measure the absorption of radiation as a function of frequency or wavelength in order to probe the nature of foreground material. Material along the line of sight absorbs light from the background source at wavelengths corresponding to atomic transitions that depend on its chemical composition, temperature and velocity. The pattern of absorption lines in the spectrum therefore yields information on the composition, temperature, and motion of the intervening matter.



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B Glossary 21-cm line: The hyperfine transition of neutral hydrogen (H I) with a correspond- ing rest-frame wavelength of 21 cm, or a frequency of 1.420 GHz. If the line is optically thin it is a direct measure of the neutral hydrogen content along a given line of sight. 56Ni:Radioactive isotope of nickel, produced in large quantities in Type Ia supernovae. α Cep: A rapidly rotating A-type star in the constellation of Cepheus. It has the traditional name Alderamin. fNL: A dimensionless parameter describing the amplitude of non-gaussianity of the fluctuations in the CMB. Absorption line spectroscopy: A spectroscopic technique to measure the absorp- tion of radiation as a function of frequency or wavelength in order to probe the nature of foreground material. Material along the line of sight absorbs light from the background source at wavelengths corresponding to atomic transitions that depend on its chemical composition, temperature and velocity. The pattern of absorption lines in the spectrum therefore yields information on the composition, temperature, and motion of the intervening matter. 513

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Panel rePorts—new worlds, new HorIzons 514 Accretion disk: A flattened cloud of material that accretes onto a central gravitat- ing mass (star or black hole). Accretion-induced collapse (AIC): The collapse of a white dwarf, initially formed in a binary system, into a neutron star. Acoustical peaks: Peaks in the temperature fluctuations of the cosmic microwave background as functions of the multipole moment l. Active galactic nucleus/nuclei (AGN): Active accretion of mass onto the supermas- sive black hole at the center of a galaxy. Such black holes appear to be a feature of most galaxies, but in most cases, the rate of accretion is minimal. When the accre- tion rate is high, the AGN releases energy well in excess of the host galaxy starlight, all of it originating near the center of the galaxy, hence the name. Adaptive optics (AO): A technology used to improve the resolution of ground- based telescopes by compensating for atmospheric distortions in real time. Adiabatic fluctuations: Adiabatic fluctuations are fluctuations in the density of dark matter, ordinary matter, and radiation with the ratio of the density of matter components to the density of the radiation components held spatially constant. AGN (quasar) winds: Outflows or jets generated by quasars or AGN. See also Jets. Akari: Japanese satellite for infrared astronomy. Allen Telescope Array (ATA): ATA-42 is an array of 42 6-meter-diameter radio telescope dishes at Hat Creek Observatory, California. Arecibo: A 305-meter radio telescope in Puerto Rico, operated by the National Astronomy and Ionosphere Center (NAIC). Asphericity: Deviation from a spherical shape. Asteroseismology: The study of the internal structure of stars through observation of surface vibrations. Astro2010: The National Research Council’s decadal survey of astronomy and astrophysics published in 2010 as New Worlds, New Horizons in Astronomy and Astrophysics.

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aPPendIx b 515 Astro-H: Japanese X-ray satellite, scheduled for launch in 2013; see http://astro-h. isas.jaxa.jp/ for details. Asymptotic giant branch (AGB): A phase of stellar evolution undergone by a low to intermediate mass star (0.6 to 10 solar masses) late in its life and appearing in a second red giant phase. They have a dormant, helium-filled core surrounded by a helium-fusing shell and then a hydrogen-fusing shell. Atacama Cosmology Telescope (ACT): A 6-meter-diameter, millimeter-wave- length telescope on Cerro Toco in the Atacama Desert of northern Chile, used to study the cosmic microwave background radiation (CMB). Atacama Large Millimeter/submillimeter Array (ALMA): An international inter- ferometer located at Llano de Chajnantor in the Atacama Desert of northern Chile. Auger: The Pierre Auger Cosmic-Ray Observatory. Array of light detectors and water tanks in Argentina used to detect Čerenkov radiation produced by relativistic charged particles as they pass through Earth’s atmosphere to study extremely high energy cosmic rays. Axion: A postulated elementary particle. One of the mysteries of the standard model of physics is the lack of CP (charge conjugation and parity) symmetry viola- tion in the strong interaction (that holds nuclei together). One of the solutions of the “CP problem” is to postulate a new symmetry, the Peccei-Quinn symmetry. If this symmetry exists in nature and is at the appropriate scale, then copious num- bers of axions would be produced in the early universe. The axion is suggested as a possible explanation of dark matter. B-mode polarization: Component of the CMB polarization map that can be ex- pressed as the curl of a vector field and has zero divergence. This contrasts with the E-mode polarization, which can be expressed as the gradient of a scalar field (and hence is curl free). B-mode polarization can arise because of gravity waves from the inflation at the beginning of the universe. Baryon: Strictly speaking, “baryon” is the collective term given to particles com- posed of three quarks (with the neutron and proton being the two most common examples). In the present context, the term is used to encompass all normal mat- ter that interacts via electromagnetic forces. The vast majority of dark matter is believed to be non-baryonic.

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Panel rePorts—new worlds, new HorIzons 516 Baryon acoustic oscillation (BAO): Sound waves in the early universe generate fluctuations both in the cosmic microwave background spectrum and the matter (baryon) fluctuation spectrum. Be stars: Luminous blue stars, often rapidly rotating, with prominent hydrogen emission lines. Big bang nucleosynthesis (BBN): The production of nuclei during the first 10 minutes of the universe. Binary millisecond pulsars: Orbiting pairs of rapidly rotating neutron stars that emit radio signals. Black hole: A body so massive and dense that its gravity prevents even light from escaping. Black hole feedback: The release of energy generated by an AGN into the sur- rounding interstellar or intergalactic medium in clusters of galaxies, and the effect of that energy on processes like star formation or galaxy formation. Blazars: AGN with relativistic jets aligned close to the line of sight, such that the jet emission is Doppler beamed, an effect in Einstein’s theory of special relativity whereby radiation from a moving source appears to be concentrated in the for- ward direction. Blazar emission is therefore strongly jet-dominated, emitting at radio through gamma-ray wavelengths, and blazars appear to be highly variable and polarized. Blue stragglers: Stars in open or globular clusters that are hotter and bluer than other cluster stars whose initial mass should have evolved it away from the main sequence and thereby exhibiting atypical stellar evolution. Bose condensate: A state of matter of a dilute gas of weakly interacting bosons con- fined in an external potential and cooled to temperatures very near to absolute zero. Brown dwarfs: Cool, low-mass objects roughly at the boundary of mass between stars and planets. Cadence: Regular time interval of observation. Cas A: Young supernova remnant (exploded near 1680 AD) in the constellation Cassiopeia.

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aPPendIx b 517 Cataclysmic variable (CV): Binary star systems that have a white dwarf and a companion star. The companion star loses material onto the white dwarf and forms an accretion disk. Strong UV and X-ray emission is often seen from the accretion disk. The accretion disk may be prone to instability leading to outbursts of rapidly burning hydrogen. Centrifugal decretion disk: Disk produced around a rapidly rotating (usually type Be) star (see also Decretion disk). Čerenkov radiation: Electromagnetic radiation emitted by a charged particle mov- ing faster than the phase velocity of light in a dielectric medium such as water or air. Chandra: The Chandra X-ray Observatory, launched in 1999, one of NASA’s four Great Observatories. Chandrasekhar limit/mass: The mass (about 1.44 solar masses) supported by quantum-mechanical electron degeneracy pressure above which a star will ulti- mately collapse into a neutron star or black hole. Charge parity symmetry: The combination of charge conjugation symmetry and parity symmetry, states that the laws of physics should be the same if a particle were interchanged with its antiparticle (charge conjugation symmetry) and left and right were swapped (parity symmetry). Chemical enrichment/Metal enrichment: Processes related to the creation and distribution of all elements heavier than helium, which are synthesized during the evolution of stars. Chirp: A signal with rapidly increasing frequency. Chromosphere: The thin, outer layer of a star at T ~ 10,000 K that lies above the photosphere. Circumgalactic: Around a galaxy. Circumgalactic medium (CGM): The matter pervading the space and the gaseous medium located 100-200 kpc around a galaxy that are strongly influenced by their gravity and by chemical and mechanical feedback. Clusters of galaxies: A large group of galaxies bound together gravitationally.

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Panel rePorts—new worlds, new HorIzons 518 Clusters of stars: A group of stars formed at about the same time. CMBPol: A next-generation successor to the Planck spacecraft designed to measure CMB B-mode polarization. Coherent emission from brown dwarfs: Emission process in the magnetized re- gion around a low-mass dwarf in which bunches of electrons produce very intense radio radiation. Column density: The total amount of material along a path (e.g., the path line of sight from a quasar or galaxy to us), expressed as the number of atoms of hydrogen or mass per square centimeter. Compact binary: A close two-star system that contains a compact object—a white dwarf star, neutron star, or black hole. Compact stellar remnant: The endpoint of stellar evolution—a white dwarf star, neutron star, or black hole. Compton: The Compton Gamma-Ray Observatory, launched in 1991 and deor- bited in 2000. Confusion limit: Brightness level in a given image below which it becomes impos- sible to determine the properties of individual sources with any confidence, because of fluctuations in a large underlying population of still fainter unresolved sources. At a particular wavelength, confusion can be ameliorated by re-observing the same target field at higher angular resolution. Convective star: A star in which most of its energy is transported by moving matter rather than by the diffusion of radiation or conduction. Cool cores: The centers of clusters of galaxies in which the temperature drops below the temperature in the surrounding gas. Clusters with cool cores have high central surface brightnesses and high central gas densities and short apparent cooling times. Core-collapse supernova: A supernova explosion produced when the core of a massive star runs out of nuclear fuel and collapses to neutron-star densities. Corona: The outermost part of the stellar atmosphere, a tenuous but extremely hot (million K) plasma.

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aPPendIx b 519 Correlator: A device employed to correlate two signals, used for imaging and spectroscopy. It is necessary to combine signals from separate antennas in a radio interferometer. Cosmic microwave background (CMB): Blackbody radiation pervading the uni- verse, which is left over from the hot big bang (the initial state of the universe) and cooled by the expansion to a current temperature of 2.73 K above absolute zero. Fluctuations in the CMB intensity correspond to density fluctuations that grow over time through gravity. Cosmic Origins Spectrograph (COS): UV spectrograph on HST, installed during the 2009 servicing mission. Cosmic strings: Filaments formed by symmetry breaking in various particle phys- ics models that may stretch across the visible universe. Cosmic variance: The statistical uncertainty of a limited sample size in observa- tions of the universe at very large distances, based on the idea that it is possible to observe only part of the universe at a particular time, thus making it difficult to draw statistical statements about cosmology on the scale of the entire universe. Cosmic web: The structure of the universe created from intergalactic gas and galax- ies, embedded in filaments, stretching between voids. Dark ages: Term for the long period when the universe is opaque between the emission of the CMB at redshift z ~ 1,100 and the formation of the first radiating objects at redshift z ~ 30 to 50. Dark energy: Hypothetical form of energy that causes the present-day expansion of the universe to accelerate. Dark Energy Survey (DES): A ground-based survey to study the nature of dark energy by observing the distributions of galaxies and clusters of galaxies, weak gravitational lensing, and supernovae. Dark matter: The hypothetical form of matter that dominates the mass budget of galaxies, clusters, and the universe and is thought to interact only through gravita- tional forces. The presence of dark matter is inferred from its gravity; experiments to detect the dark matter particles directly are underway. Dark matter halo: Collection of dark matter particles that has evolved from an

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Panel rePorts—new worlds, new HorIzons 520 initially overdense region of the universe, separated from the global expansion of the universe, and collapsed into a virialized configuration. Dark matter halos range in mass from subgalactic to cluster scales, and their associated gravitational potential wells bind together the baryonic components of galaxies and clusters. Debris disk: A disk of matter around a young star that contains dust grains, gener- ated by collisions of protoplanets or asteroids. Deconfined quark matter: At sufficiently high temperatures or densities, hadronic matter should evolve into a new phase of matter containing almost free quarks and gluons, also known as a quark-gluon plasma or quark matter. Deconfined quark matter may have existed in the first few microseconds after the big bang and might exist inside the cores of dense neutron stars. Decretion: The centrifugal ejection of material. Decretion disk: An equatorial disk formed by the centrifugal ejection of material (see also Centrifugal decretion disk). Degenerate core: The dense central region of an evolved star supported by electron degeneracy pressure. Dome C: A high-altitude site on the ice sheet atop the Antarctic Plateau. Site of the Concordia Research Station operated by France and Italy, more than 1,000 km from the coast. Doppler tomography: Rotation velocities in stars or disks produce Doppler shifts, which can be used to reconstruct the geometry of the rotating object. Double pulsar: The system J0737-3039, an exceptionally relativistic binary pulsar discovered in 2003. Dynamical dark energy: If the energy density of the dark energy can evolve with time and the dark energy can cluster, then the dark energy is called “dynamical.” Quintessence models are an example of a model with dynamical dark energy. If the dark energy is vacuum energy, then it is not dynamical. Early-type stars: Stars hot enough to keep hydrogen ionized at their surface, typi- cally with spectral classifications O, B, and A.

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aPPendIx b 521 Einstein ring: The image formed when a background source located directly be- hind a massive galaxy undergoes gravitational lensing. Electromagnetic Observations from Space (EOS): One of the four Astro2010 Program Prioritization Panels. Ellipticals: Galaxies with a smooth brightness profile and an ellipsoidal shape that ranges from nearly spherical to highly flattened. Most ellipticals show signs that they are made primarily of old stars and have little current star formation. e-MERLIN: The enhanced version of the United Kingdom’s Multi-Element Radio Linked Interferometer Network (MERLIN), an interferometer array of radio tele- scopes. In e-MERLIN, the microwave links between dishes are replaced by optical fibers. EMIR: Near-infrared multiobject spectrograph for the Gran Telescopio de Canarias. E-mode polarization: Component of the CMB polarization map that can be ex- pressed as the gradient of a scalar field (and hence is curl free). This contrasts with the B-mode polarization, which can be expressed as the curl of a vector field and has zero divergence. Temperature and density fluctuations at the last scattering surface generate a pure E-mode polarization. Epoch of reionization: The period during which the baryonic content of the uni- verse is gradually ionized. The onset of reionization coincides with the emergence of the first ionizing (luminous) sources, at the end of the dark ages, and ends when the intergalactic medium is fully ionized. eRosita/Spectrum RG: Extended Röntgen Survey with an Imaging Telescope Array, a German X-ray instrument designed to fly on the Russian Spectrum RG satellite in 2012. Event horizon: The boundary in space-time, most often surrounding a black hole, from within which no signals can reach an outside observer. Exoplanet: An extrasolar planet, i.e., a planet orbiting a star other than the Sun. Exozodiacal light: Emission from dust in debris disks. Expanded Very Large Array (EVLA): Upgrade to the Very Large Array radio in-

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Panel rePorts—new worlds, new HorIzons 522 terferometer in New Mexico, which will yield greatly increased sensitivity, spectral capabilities, and other improvements. Extremely Large Telescope (ELT): A generic reference to the kind of 20-meter to 40-meter-class optical and infrared telescopes currently being designed in the United States and Europe. Faber-Jackson relation: An empirical correlation between the central velocity dispersions and luminosities of elliptical galaxies Facility for Rare Isotope Beams (FRIB): An ongoing Department of Energy project to establish an accelerator facility to study rare, unstable nuclei. Far-infrared: Wavelengths from 20 to 200 microns (also see Infrared). Feedback: Energy input from various physical processes—such as supernovae explosions, ionizing radiation, stellar and galactic winds, black hole activity, jets, and more—that is emitted back into the surrounding medium around stars, gal- axies, and the intracluster and intergalactic medium, affecting its energetics and distribution. Fermi GLAST: The Fermi Gamma-Ray Space Telescope, formerly GLAST (Gamma- ray Large Area Space Telescope), a large-area gamma-ray satellite launched by NASA in 2008. Field of view (FOV): The maximum angular extent observable through an optical or other device. Filaments: See Cosmic web. Flamingos II: A near-infrared, multiobject spectrometer and wide-field imager built for Gemini South by the University of Florida. Fossil field: Magnetic field left from a previous evolutionary phase. Fundamental plane: A relation between effective radius, surface brightness, and central velocity dispersion for elliptical galaxies. Gaia: A European Space Agency astrometry space mission to provide precise as- trometric data and spectroscopic information.

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aPPendIx b 523 Galactic Neighborhood (GAN): One of the five Astro2010 Science Frontiers Panels. Galaxies Across Cosmic Time (GCT): One of the five Astro2010 Science Frontiers Panels. Galaxy Evolution Explorer (GALEX): An orbiting space telescope that observes galaxies and gas in ultraviolet light. Galaxy kinematics: The motions of stars and gas in a galaxy. Gamma ray (γ ray): Light with wavelength shorter than X-ray light, roughly 0.1 MeV (100 keV) to more than 100 TeV, or frequency range log n ~ 20.5 to 28.5 Hz. Gamma-ray burst (GRB): Extremely bright flashes of gamma rays, probably pro- duced by compact objects in distant galaxies. Long-duration (>2 seconds) GRBs are probably associated with the explosions of very massive stars. Short-duration (<2 seconds) GRBs may be produced by merging neutron stars. Gas accretion: Infall of gas from the intergalactic medium onto star-forming re- gions of galaxies or extended gaseous halos. Such infall provides a source of raw material to form new stars. Gauss: A unit of magnetic field strength. Gaussian fluctuations: Fluctuations in the matter distribution or CMB tempera- ture are Gaussian if the probability distribution of deviations follows a standard bell curve, with p(D) proportional to exp(-D2/2 s2). Gemini: The Gemini Observatory, which consists of two 8-m telescopes, one in Hawaii and one in Chile. General relativity: Einstein’s theory of gravitation in terms of the curvature of space-time produced by mass and energy density. Gigayear (Gyr): One billion years. Globular cluster: A nearly spherical cluster of 105 to 106 stars. Grand-design galaxy: A spiral galaxy in which the spiral arms are especially prominent.

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Panel rePorts—new worlds, new HorIzons 530 Metallicity: The proportion of a star’s matter that is made up of chemical elements other than hydrogen and helium. Microlensing: See Gravitational microlensing. Mid-infrared: Light with wavelengths between 5 and 20 microns (see also Infrared). Milagro: An observatory in New Mexico that detects gamma rays and Čerenkov ra- diation from cosmic-rays: A water Čerenkov system that detects airshower particles produced by ultrahigh-energy cosmic rays passing through Earth’s atmosphere. Milky Way: The spiral galaxy in which we live. Millimeter: Wavelength region from 1 to ~5 millimeters. Mode: See E-mode polarization, B-mode polarization. Multiscale physics: The study of how microscopic properties of systems can lead to macroscopic behavior. The thermal regulation of the gas in a cluster of galaxies (millions of light-years across) by accretion onto a central black hole (similar in size to the solar system) is one example of multiscale physics. Murchison Widefield Array (MWA): Formerly the Mileura Widefield Array, this is a low-frequency radio (at frequencies from 80 to 300 MHz) interferometer array to be built in Western Australia. Near-infrared: Wavelengths from ~1 to 5 microns (also see Infrared). Neutrinos: Weakly interacting elementary particles that are emitted, for example, in beta decay. They come in three “flavors” with differing masses and oscillate between those flavors as they travel. Neutrinos emitted by nuclear reactions in the core of the Sun or in the formation of a neutron star in a core-collapse supernova can be detected on Earth. Neutron skin: Thin layer of pure neutron matter covering the surface of many nuclei. Neutron star: A collapsed star with a mass comparable to that of the Sun crammed into the dimensions of a city, giving it a density comparable to that of an atomic nucleus.

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aPPendIx b 531 Newton: See XMM-Newton. Non-thermal radio emission: Radiation emitted by particles for reasons other than the high temperature of the source. The spectrum of non-thermal radiation is different from that predicted by Planck’s law for a blackbody. Nuclear cross section: The effective size (area in cm2) of a nucleus for a reaction of a given type for a given collision energy. OB association: A group of high-mass stars, looser than a cluster. Omega (W): The ratio of the density of the universe to the critical density of the universe. If omega is greater than one, the universe will turn around and begin collapsing; if omega is one, the universe will eventually stop expanding but will not collapse; and if omega is less than one, the universe will continue expanding. Opacity: A measure of the ability of a gas to absorb or scatter radiation. Optical: Wavelength range of light to which the human eye is sensitive, namely, 3,500 to 8,000 Å. Optical and Infrared Astronomy from the Ground (OIR): One of the four As- tro2010 Program Prioritization Panels. Outflows: Gas flowing outward and escaping from gravitating bodies. Examples include stellar winds, galactic winds powered by supernovae in star-forming galax- ies, winds from the surfaces of accretion disks, and jets emerging from the vicinity of accreting black holes. PanStarrs-1: Panoramic Survey Telescope and Rapid Response System, a wide-field (1.8 meter) imaging facility being developed at the University of Hawaii’s Institute for Astronomy. Parity-violating electron scattering: An experimental technique to determine the neutron distribution in a nucleus by weak-force scattering of polarized electrons. Particle Astrophysics and Gravitation (PAG): One of the four Astro2010 Program Prioritization Panels. Periastron advance: Angular change in the periastron point in the orbital plane;

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Panel rePorts—new worlds, new HorIzons 532 the periastron is the point in the orbit of one component of a binary system where it is nearest the other component. Phased-array feed: A way to provide multibeaming with a radio telescope. Photometric redshifts: The recession velocity based on measuring galaxy pho- tometry in multiple optical and near-IR wavelength bands. These measurements of the shape of the galaxy energy emission can be obtained more quickly than spectroscopic redshifts. However, they are less accurate. Photometry: Measurement of the brightness of an object. Planck: An ESA satellite launched in 2009 to map tiny fluctuations in the cosmic microwave background radiation. Planetary Systems and Star Formation (PSF): One of the five Astro2010 Science Frontiers Panels. Point spread function (PSF): The point spread function describes the angular response of a telescope to a point-like source of light. Because of atmospheric tur- bulence and the finite resolving power of a telescope, light from point-like object is spread out in angle. Polarimetry: The measurement and interpretation of the polarization of transverse electromagnetic waves. Population III: The first stars of zero metallicity, which formed before the heavy elements were produced by stars. Post-Swift GRB: Gamma-ray burst studied after the completion of the Swift mission. Pre-main sequence: Collapsing protostellar cores that have not yet ignited hydro- gen fusion in their interiors. Program Prioritization Panels (PPPs): Four panels of scientists constituted by the National Research Council to write reports on priorities across specific disciplines for the Astro2010 Decadal Survey. Prolate: A spheroid with a shape like an ellipse rotated about its major axis, i.e., football-shaped.

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aPPendIx b 533 Protoplanetary disk: A primordial disk around a young star that has just begun to form planets. Pulsar: A rotating neutron star that emits periodic radio pulses of periods ranging from milliseconds to seconds. Pulsar/black-hole binary: A binary system in which one star is a pulsar and the other is a black hole. Quantum chromodynamics (QCD): A quantum field theory that describes the strong nuclear forces and interactions of interaction of quarks and gluons. Quantum gravity: A physical theory that describes the gravitational interactions of matter and energy, which are themselves described by quantum theory. Quasar (AGN) winds: Outflows or jets generated by quasars or AGN. See also Jets. Quasar sightline: Line of sight through interstellar and intergalactic material of interest toward a background quasar. Radiative cooling: The loss of energy by a gas as the result of the emission of radiation. Radio: Light that has wavelengths longer than the far-infrared, roughly beginning at 200 microns. Radio galaxy: Galaxy that emits strong radio emission from powerful jets and lobes. Radio jet: See Jets. Radio lobes: Diffuse extended radio-emitting plasma fed by jets and formed when the material in the jet decelerates and interacts with the intergalactic medium. Radio, Millimeter, and Submillimeter Astronomy from the Ground (RMS): One of the four Astro2010 Program Prioritization Panels. Radio transients: Time variable radio source. Red giant branch (RGB): A phase of stellar evolution in which the stars have ex- panded their outer envelope in response to the extra energy generated by gravita- tional contraction, hydrogen shell fusion, and ultimately helium fusion in the core.

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Panel rePorts—new worlds, new HorIzons 534 Redshift: The increase in wavelength of electromagnetic radiation (doppler shift) caused by the motion of an object as described by the theory of special relativity, or by luminous material in a gravitational field as described by the general theory of relativity. In cosmology, it refers to the fractional increase in the wavelength of a photon received from a distant object, because of the expansion of the universe be- tween emission and reception. Coupled with a cosmological model, redshifts can be used to determine the distance and look-back time of phenomena in the universe. Redshift space distortions: Effects of inhomogeneities in light propagation. On small scales, random motions will cause particles at the same distance to have slightly different redshifts. This elongates structures along the line of sight. On very large scales, the opposite happens and overdensities are enhanced. Redward loops: Paths of post-main-sequence stellar evolution that proceed toward the red spectral range in the Hertzsprung-Russell diagram. Reionization: See Epoch of reionization. Relativistic jet: See Jets. Relativistically broadened line: A spectral line that appears wider to an observer because of the movement of its source at speeds approaching the speed of light. Rotation curve: Representation of the orbital velocity of the stars or gas in a galaxy as a function of radius from the center of the galaxy. S0 galaxy: A disk galaxy similar to a spiral galaxy, but shows no spiral arms. Schmidt-Kennicutt relation: An empirical relation between the surface density of star formation rate and the surface density of gas in galaxies. Schwarzschild radius: A characteristic radius associated with every quantity of mass, generally referring to a collapsing celestial object, which is (3 km) times the black hole mass (measured in solar-mass units). It is the radius of the object at which the force of gravity would be so great that no known force could stop the mass from continuing to collapse into a point of infinite density. Science Frontiers Panel (SFPs): Five panels of scientists constituted by the National Research Council to write reports on science priorities for the Astro2010 Decadal Survey.

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aPPendIx b 535 SFSR inflation: Single-field, slow-roll inflation, a simple model for inflation whereby inflation is driven by the displacement of a scalar field from the minimum of its potential. In slow-roll inflationary scenarios, the homogeneous part of the inflation field rolls slowly down its potential toward a minimum. Shapiro delay: The delay induced in the travel time of a light ray passing close to a massive object by the presence of space-time curvature; named after Irwin Shapiro, who first suggested searching for the delay associated with the space-time curvature induced by the Sun’s mass using radar ranging to Venus. Shock: Supersonic interactions with ambient gas, as happens when the supersonic wave of gas from a supernova hits the ambient gas. Shock-heated: Heated by supersonic interactions with ambient gas, as happens, for instance, when infalling gas accelerated by gravity hits ambient gas within a galaxy’s halo. SMBH (supermassive black hole) merger tree: History of black hole mergers. Galaxies form as small pieces that gradually merge to create larger and larger gal- axies. Black holes in the centers of these galaxies are thought to merge in the same interactions, although the details of evolution from a tight black hole binary to a single merger remnant are not understood. Solar-neutrino experiments: Experiments designed to detect weakly interacting neutrinos that are emitted during nuclear reactions in the core of the Sun. South Pole Telescope (SPT): A microwave/millimeter-wave telescope with a 10-m diameter, located at the Amundsen-Scott South Pole Station, Antarctica. Its main scientific mission is to survey several thousand clusters of galaxies, in order to constrain the nature of the dark energy and other aspects of cosmic evolution. Spectroscopic abundance: The chemical composition of a star as inferred from analysis of absorption and emission features formed in the star’s outer layers. Spectroscopic redshift: The recession velocity based on measuring the shift of spectral lines from their usual position. Spindown time: The characteristic time for a star to slow its rotation due to loss of angular momentum through its stellar wind and/or magnetic field. It is typically much shorter in magnetically active stars.

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Panel rePorts—new worlds, new HorIzons 536 Spitzer: NASA’s Space Infrared Telescope Facility, launched in 2003. Standard model of particle physics: The theory of fundamental interactions and elementary particles that make up all the visible matter in the universe; it does not include gravitation, dark matter, or dark energy. Starburst galaxy: A galaxy, or a region of a galaxy, with an exceptionally high rate of star formation. Stars and Stellar Evolution (SSE): One of the five Astro2010 Science Frontiers Panels. Starspot: The equivalent of sunspots (cool surface features) on other stars. Stokes Q/U/V sensitivity: Measurement of the surface magnetism of stars by measuring the polarization of light in various spectral bands. Stokes parameters describe the polarization state of electromagnetic radiation. Strömgren sphere: A sphere of ionized hydrogen gas produced by intense UV radiation from a massive, young star balanced by ion-electron captures (recombinations). Subgrid: Below the resolution of the grid in a numerical calculation. Submillimeter: Wavelengths between ~0.2 millimeters (200 microns) and 1 mil- limeter (1,000 microns). Suborbital: NASA’s program of sounding rockets and balloons. Subsonic burning: A flame that moves slower than the sound speed, i.e., a defla- gration, in contrast to a detonation. Sunyaev-Zel’dovich effect (SZE): The scattering of cosmic microwave background photons by the electrons in hot, ionized gas, which appears as a deficit or enhance- ment (depending on observing frequency) in radio continuum maps. Clusters of galaxies can be detected through the change they produce in the CMB spectrum. The SZ signal is proportional to the integral of the pressure (density of electrons times temperature) through the cluster and is independent of distance. The mag- nitude of the effect depends on the path integral of the electron density, and so cluster detection roughly scales as the cluster mass.

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aPPendIx b 537 Superbubble: An extremely large bubble of very hot gas produced in the interstellar medium by multiple supernova explosions or stellar winds. Supergiant: An extremely luminous, massive star. Supermassive black hole (SMBH): A black hole with a mass in excess of a million solar masses. Supernova neutrino background: When a massive star collapses down to a neu- tron star, it produces a core collapse supernova explosion. Most of the energy in a supernova explosion is radiated in neutrinos. The neutrinos radiated from all of the core collapse supernovae make up the neutrino background. Supersymmetry: A particle-physics symmetry relating elementary particle differ- ing by one-half unit of spin. Superwind: A powerful outflow of hot gas from a starburst galaxy, driven by the combined effects of supernova explosions and winds from massive stars. Suzaku: Japanese X-ray satellite, launched in 2005, featuring high spectroscopic resolution and a very wide energy band. Swift: A NASA medium-class gamma-ray (and UV/X-ray) Explorer mission to provide rapid identification of GRBs and multiwavelength follow-up. Synoptic solar magnetometry: Long-term and daily measurements of the solar photospheric and coronal magnetic fields. Tau neutrino: One of the three flavors of neutrinos. The tau neutrino, along with the associated tauon, forms the third generation of leptons. TeV experiment: An experiment designed to measure radiation from astronomi- cal sources in the TeV (1012 eV) energy range of gamma rays. Most modern TeV experiments operate from the ground using the imaging atmospheric Čerenkov techniques (see HESS, MAGIC, and Veritas). Thermal disk component of the X-ray continuum spectrum: Emission from the hot surface of an accretion disk around a compact object. Time-domain survey: A survey of objects that focuses on obtaining their fluxes as a function of time, perhaps in several wavelength bands. The critical elements are

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Panel rePorts—new worlds, new HorIzons 538 high sampling rate, signal-to-noise ratio, and time span of coverage with as few gaps as possible. Transitional disk: A protoplanetary disk with a gap, or in other usage, a disk in which the dust has grown in size and settled. Tully-Fisher relation: An empirical correlation between the rotation velocities and luminosities of spiral galaxies. Type Ia supernova: Observationally, a supernova without hydrogen and with strong lines of ionized silicon; theoretically the thermonuclear explosion of an accreting white dwarf in a binary system with a light curve powered by the decay of radioactive 56Ni and 56Co. Type II supernova: Observationally, a supernova with spectroscopic evidence of hydrogen; theoretically, typically the result of core collapse to a neutron star or black hole in a star with an extended hydrogen envelope, though thermonuclear varieties are possible. Uhuru: The first satellite dedicated to X-ray astronomy. Ultradense matter: Exotic new states of matter, e.g., deconfined quark matter or Bose condensates. Ultrahigh-energy cosmic rays (UHECRs): Fast-moving particles with detected energy occasionally over 1020 eV. They come from unidentified cosmic accelerators outside our galaxy. We detect them when they collide with atoms in the upper at- mosphere and produce a shower of particles and faint blue light from fluorescence. The flux of UHECRs is a sharply declining function of energy. Ultraviolet (UV): Light with wavelength slightly shorter than visible light, from 3,000 Å down to 912 Å (far ultraviolet or FUV) and ~100 Å (extreme ultraviolet or EUV). Velocity dispersion: The spread of the random velocities of stars, gas, or galaxies. Velocity map: An image of the distribution of speeds and directions of stars or gas in a galaxy. Very Energetic Radiation Imaging Telescope Array System (VERITAS): Located

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aPPendIx b 539 in Arizona, this is an imaging atmospheric Čerenkov array of four telescopes used for gamma-ray astronomy in the 100-GeV to 10-TeV energy range. Very Large Telescope (VLT): An array of four 8.2-m-diameter optical-IR telescopes operated by European Southern Observatory (ESO) at the Paranal Observatory in the Atacama Desert of northern Chile. The telescopes can be used independently or combined into an interferometer. Very Long Baseline Array (VLBA): An array of 10 radio telescopes, operated by the U.S. National Radio Astronomy Observatory, which functions as the world’s largest dedicated, full-time astronomical instrument. By using the technique of very long baseline interferometry, the VLBA attains extremely high angular resolution. Virial radius: The radius of a sphere around a galaxy or cluster within which virial equilibrium holds (a balance between kinetic and potential energy). Visible and Infrared Survey Telescope for Astronomy (VISTA): A 4-m-class, wide- field telescope located at the Paranal Observatory in Chile and currently operated by the European Southern Observatory. Warm-hot intergalactic medium (WHIM): Gas in the intergalactic medium resid- ing at sufficiently high temperature that hydrogen is fully ionized (very little neutral gas remains), and the gas is too diffuse for detectable X-ray emission; a nominal temperature range is ~105 to 107 K. This term sometimes refers to “missing bary- ons,” since it is estimated theoretically that 30 to 50 percent of the cosmic baryons at the present epoch may reside in this phase, hidden from current observational capabilities. Weak lensing: Fluctuations in the large-scale distribution of matter produce varia- tions in the “shape” of space that alter the path that light takes from a distant galaxy to our telescopes. This gravitational lensing changes the shape of a galaxy and makes it appear more elliptical. By measuring the shapes of large numbers of galaxies, astronomers can detect this “weak lensing” effect and infer the large-scale distribution of matter. Weakly interacting massive particle (WIMP): A broad class of hypothetical par- ticles that have small interaction cross-sections with ordinary matter and radia- tion. Postulated WIMPs include the lightest supersymmetric particle. WIMPs are suggested as a possible explanation of dark matter. White dwarf: The compact remnant of an evolved star from approximately 0.1

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Panel rePorts—new worlds, new HorIzons 540 to 8 solar masses. The star is supported against gravitational collapse by electron degeneracy pressure. XMM-Newton: ESA’s X-ray Multi-Mirror mission, named after Sir Isaac Newton, is an orbiting X-ray observatory launched in 1999. It features a large collecting area for X-ray spectroscopy. X-ray: Light with wavelength considerably shorter than visible light, roughly ~100 Å to ~0.01 Å, often referred to by the equivalent energy, 0.1 keV to 1 MeV, or fre- quency range log n ~16.5 to 20.5 Hz. X-ray polarimetry: The measurement and interpretation of the polarization of transverse electromagnetic waves, at X-ray wavelengths. The main physical mecha- nisms producing polarization in the X-ray band are thought to be synchrotron radiation (primarily from blazars), Compton scattering from accretion disks, radia- tive transfer in strong magnetic fields, and geometrical effects due to light bending in strong gravitational fields. X-ray spectroscopy: The study of the distribution of X-ray intensity as a function of energy, used to analyze the characteristics of the emitted radiation and chemical composition of the gas. Zodiacal light: Scattered light from dust particles orbiting the Sun, the dominant source of mid-infrared sky brightness seen from Earth. Zody or zodi: See Zodiacal light.