microjansky

Microjansky (symbol μJy) is a unit of spectral flux density used in astronomy, particularly radio astronomy. It denotes one millionth of a Jansky, with 1 Jy defined as 10^-26 watts per square meter per hertz (W m^-2 Hz^-1). Therefore 1 μJy equals 10^-32 W m^-2 Hz^-1. The Jansky was named in honor of Karl Jansky, a pioneer in radio astronomy; the microjansky follows SI prefixes and is used to express faint radio emission.

In practice, μJy is used to report the flux densities of very faint radio sources, such as distant star-forming galaxies and active galactic nuclei, as detected by sensitive interferometers. Modern facilities including the Very Large Array (VLA), LOFAR, MeerKAT, and other deep-field surveys routinely reach μJy-level sensitivities at many frequencies. Deep surveys can reach tens of μJy, and even lower levels with long integration times or gravitational lensing.

Because μJy is a flux density per unit frequency, it applies to a specific observing frequency; converting to total flux over a band requires integrating over the bandwidth. Measurements are subject to calibration errors, thermal noise, and confusion limits when many sources lie within a telescope beam. The unit is often written as uJy in plain-text or data tables, though μJy is preferred in formal writing.