nanojanskys

Nanojanskys (nJy) are a unit of spectral flux density used in radio astronomy. One nanojansky equals one billionth of a jansky, and one jansky (Jy) equals 10^-26 watts per square meter per hertz. Therefore, 1 nJy = 10^-35 W m^-2 Hz^-1. The nJy scale is used when describing extremely faint radio sources or diffuse background signals, where flux densities are far below a single jansky.

The nanojansky arises from the long-standing use of the jansky as a practical unit in radio astronomy. Expressing very small signals with nJy allows for clearer comparison and planning of observations. In practice, achieving nJy-level sensitivity requires long integration times, careful calibration, and advanced data processing to mitigate instrumental and environmental noise. Contexts for nJy-level measurements include deep-field surveys, studies of the faint end of radio source populations, and measurements of the cosmic radio background. Modern and planned facilities, such as the Square Kilometre Array (SKA) and next-generation very long baseline interferometry networks, aim to reach or approach nanojansky-scale sensitivities in specific observing modes.

Notes: The Jansky is a non-SI unit commonly used in astronomy, so the nanojansky is an adopted, practical unit within the field. When converting to energy flux, flux density per unit frequency is involved, which may require integrating over a bandwidth.