The source counts distribution of radio-sources from a
radio-astronomical survey is the
cumulative distribution of the number of sources (''N'') brighter than a given flux density (''S''). As it is usually plotted on a log-log scale its distribution is known as the log ''N'' – log ''S'' plot. It is one of several
cosmological test
Cosmology () is a branch of physics and metaphysics dealing with the nature of the universe. The term ''cosmology'' was first used in English in 1656 in Thomas Blount's ''Glossographia'', and in 1731 taken up in Latin by German philosopher ...
s that were conceived in the 1930s to check the viability of and compare new
cosmological models
Physical cosmology is a branch of cosmology concerned with the study of cosmological models. A cosmological model, or simply cosmology, provides a description of the largest-scale structures and dynamics of the universe and allows study of f ...
.
Early work to catalogue radio sources aimed to determine the source count distribution as a discriminating test of different cosmological models. For example, a uniform distribution of radio sources at low redshift, such as might be found in a 'steady-state Euclidean universe,' would produce a slope of −1.5 in the cumulative distribution of log(''N'') versus log(''S'').
Data from the early Cambridge
2C survey (published 1955) apparently implied a (log(''N''), log(''S'')) slope of nearly −3.0. This appeared to invalidate the
steady state
In systems theory, a system or a process is in a steady state if the variables (called state variables) which define the behavior of the system or the process are unchanging in time. In continuous time, this means that for those properties ' ...
theory of
Fred Hoyle,
Hermann Bondi and
Thomas Gold
Thomas Gold (May 22, 1920 – June 22, 2004) was an Austrian-born American astrophysicist, a professor of astronomy at Cornell University, a member of the U.S. National Academy of Sciences, and a Fellow of the Royal Society (London). Gold was ...
. Unfortunately many of these weaker sources were subsequently found to be due to 'confusion' (the blending of several weak sources in the side-lobes of the interferometer, producing a stronger response).
By contrast, analysis from the contemporaneous
Mills Cross
The Mills Cross Telescope was a two-dimensional radio telescope built by Bernard Mills in 1954 at the Fleurs field station of the Australian Commonwealth Scientific and Industrial Research Organisation in the area known now as Badgerys Creek, about ...
data (by Slee and Mills) were consistent with an index of −1.5.
Later and more accurate surveys from Cambridge,
3C, 3CR, and
4C, also showed source count slopes steeper than −1.5, though by a smaller margin than 2C. This convinced some cosmologists that the steady state theory was wrong, although residual problems with confusion provided some defense for Hoyle and his colleagues.
The immediate interest in testing the steady-state theory through source-counts was reduced by the discovery of the 3K
microwave background radiation in the mid 1960s, which essentially confirmed the
Big-Bang model.
Later radio survey data have shown a complex picture
— the 3C and 4C claims appear to hold up, while at fainter levels the source counts flatten substantially ''below'' a slope of −1.5. This is now understood to reflect the effects of both density and luminosity evolution of the principal radio sources over cosmic timescales.
See also
*
Tolman surface brightness test
References
{{DEFAULTSORT:Source Counts
Physical cosmology