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 residing 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 baryons,” 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 variations 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 particles that have small interaction cross-sections with ordinary matter and radiation. 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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