source count(number count) A compilation of the number, N, of sources per unit solid angle that are brighter than a flux density level S. In Euclidean space the radius of the sphere out to which sources of a certain luminosity are observed varies as S –½; the number of sources contained therein thus varies as S –3/2. A plot of log N versus log S is a straight line of slope –1.5 and is independent of source luminosity. Deviations from this value may indicate source evolution, departures from a uniform space distribution, cosmological effects, or the effect of an intervening obscuring medium. Differential source counts (number per unit flux interval) are to be preferred when dealing with data since the errors on each point are then independent of other points. Radio source counts provided the first evidence for the Big Bang theory as against steady-state theory. See also radio-source catalog.
Collins Dictionary of Astronomy © Market House Books Ltd, 2006