fixedsurfacebrightness

Fixed surface brightness is a term used in astronomy to describe a condition or approach in which the surface brightness of an extended astronomical source is treated as constant within a given analysis or dataset. Surface brightness is the flux per unit angular area of an object, typically expressed in magnitudes per square arcsecond. In many observational surveys, a fixed surface brightness threshold is imposed: only sources with surface brightness above the limit are detected or catalogued. This selection effect biases samples toward intrinsically brighter or more compact objects at larger distances and can influence inferred properties such as size and luminosity.

In cosmology, the concept is closely related to the Tolman surface brightness test. In an expanding universe, the observed surface brightness of a standard candle or a standard-size object is expected to decrease with redshift as (1+z)^-4, due to photon energy loss, time dilation, and geometric effects. Measurements of surface brightness over a range of redshifts can thus be used to test cosmological models and to study the evolution of galaxies.

Practical measurements of fixed surface brightness must account for several factors, including the instrument point-spread function, sky background, K-corrections, cosmological dimming, and dust extinction. The term is also used, more loosely, to describe analyses that assume a constant intrinsic surface brightness for a class of objects when deriving scaling relations, which can introduce biases if the assumption does not hold.

Overall, fixed surface brightness concerns how brightness per unit area is used in selecting, comparing, and interpreting extended astronomical sources, with important implications for survey design and cosmological tests.