uJy

The microjansky, symbol μJy or uJy, is a unit of spectral flux density used in radio astronomy to quantify faint radio sources. It is a subdivision of the jansky, the standard unit for flux density.

One microjansky equals one millionth of a jansky: 1 μJy = 10^-6 Jy. Given that 1 Jy is 10^-26 W m^-2 Hz^-1, 1 μJy corresponds to 10^-32 W m^-2 Hz^-1. The unit is commonly abbreviated as μJy, with uJy used in ASCII contexts where the mu character is not easily available.

In practice, μJy is used to report flux densities of various radio sources, including radio galaxies, quasars, star-forming galaxies, and other faint objects observed with sensitive radio telescopes such as the Very Large Array, LOFAR, ALMA, and future facilities like the Square Kilometre Array. Catalogs may quote flux densities in μJy or μJy per beam when referring to peak surface brightness.

The μJy unit represents spectral flux density, meaning power per unit area per unit frequency, and is not a total flux measurement. In radio astronomy analyses, measurements are often given as flux density per frequency channel or per beam, depending on the instrument and method used.

See also: Jansky, Flux density, Jy per beam.