Nanojansky

A nanojansky is a unit of spectral flux density used in radio astronomy. It is equal to 10^-9 Jansky, and since 1 Jansky corresponds to 10^-26 watts per square meter per hertz, 1 nanojansky equals 10^-35 W m^-2 Hz^-1. The nanojansky is used to express extremely faint radio sources and signals in deep-field observations and cosmic background studies.

The term stems from the Jansky, the standard unit named after Karl Jansky, who helped establish the field of radio astronomy in the 1930s. As observational capabilities advanced, astronomers adopted smaller units such as the nanojansky to quantify flux densities near the limits of detectability for distant or weak radio emitters.

In practical terms, measurements at the nanojansky level require very large collecting areas, long integration times, and meticulous calibration. Current radio facilities typically probe microjansky (μJy) levels for many surveys, while next-generation instruments aim to approach nanojansky sensitivities in targeted bands or with intensive observing campaigns. Observations at nJy levels are often limited by confusion noise, instrumental systematics, or radio-frequency interference, and they may rely on wide bandwidths and sophisticated data analysis to separate faint sources from backgrounds.

Nanojansky-scale measurements are relevant for studying extremely faint populations, such as distant star-forming galaxies, weak active galactic nuclei, and subtle fluctuations in the cosmic radio background. As telescope technology and data processing advance, nJy sensitivity is a benchmark for the deepest radio surveys and for probing the faint end of the radio source population.

See also: Jansky, radio astronomy, Square Kilometre Array, Very Large Array.