Millijansky

Millijansky is a unit of spectral flux density used in radio astronomy. It is equal to one thousandth of a jansky, with 1 Jy defined as 10^-26 W m^-2 Hz^-1. Accordingly, 1 mJy = 10^-29 W m^-2 Hz^-1, and 1000 mJy equals 1 Jy. The term is abbreviated mJy and is commonly used to describe the flux density of astronomical radio sources at a given observing frequency.

Flux density, denoted S_nu, represents the power received per unit area per unit frequency. In radio catalogs, sources are often listed with S_nu in mJy at a specific frequency, such as 1.4 GHz or 150 MHz. The mJy scale covers a wide range: bright radio sources can be tens or hundreds of Jy, while many faint sources lie in the mJy or microjansky (μJy) regime. Microjansky corresponds to 10^-6 Jy.

The millijansky is a practical unit for comparing radio brightness across objects and surveys, reflecting the sensitivity of modern radio telescopes like the Very Large Array, LOFAR, and ALMA. Because flux density is frequency dependent, values in mJy are tied to the observing frequency, and spectral indices are often used to compare measurements at different frequencies.

The unit is named after Karl G. Jansky, whose discovery of cosmic radio emission led to the adoption of the jansky as a standard unit in astronomy.