MJy

mJy, short for millijansky, is a unit of spectral flux density used in radio astronomy to express the amount of radio energy received from an astronomical source per unit area per unit bandwidth at a given frequency. It is commonly applied to sources such as radio galaxies, quasars, pulsars, and star-forming regions, particularly in deep surveys where signals are relatively faint.

The unit is based on the Jansky (Jy), defined as 1 Jy = 10^-26 watts per square meter per hertz (W m^-2 Hz^-1). Accordingly, 1 mJy = 10^-3 Jy = 10^-29 W m^-2 Hz^-1, and 1 μJy = 10^-6 Jy = 10^-32 W m^-2 Hz^-1. The Jansky is not part of the International System of Units (SI), but it is an established, widely used non-SI unit in radio and submillimeter astronomy, named after Karl G. Jansky, an early radio astronomer.

In practice, mJy is used to report flux densities at specific observing frequencies. Measurements can depend on telescope sensitivity, beam size, and bandwidth, and values are often reported with an associated spectral index if observed at multiple frequencies. Deep radio surveys routinely reach the mJy or microjansky (μJy) level, enabling studies of faint galaxy populations and early universe objects.

See also: Jansky, flux density, spectral flux density, radio astronomy units.