Monday, February 11, 2013

1302.2010 (Ian Heywood et al.)

Sample variance dominates the observed scatter in the counts of faint radio sources    [PDF]

Ian Heywood, Matt J. Jarvis
[Abridged] The shape of the curves defined by the counts of radio sources was one of the earliest cosmological probes. Radio source counts continue to be an area of interest as they can be used to study the relative populations of galaxy types in the Universe (as well as investigate any cosmological evolution in their respective luminosity functions). They are also a vital consideration for determining how source confusion may limit the depth of a radio interferometer observation, and are essential for characterising the extragalactic foregrounds in CMB experiments. There is currently no consensus as to the relative populations of the faintest (sub-mJy) source types, where the counts turn up. Most of the source count data in this regime are gathered from multiple observations that each use a deep, single pointing with an interferometric radio telescope. These independent count measurements exhibit large amounts of scatter (factors of order a few) that significantly exceeds their respective stated uncertainties. In this article we use a simulation of the extragalactic radio continuum emission to asses the level at which sample variance may be the cause of the scatter. We find that the scatter induced by sample variance in the simulated counts decreases strongly towards lower flux density bins. It is large enough to comfortably dominate the fluctuations in the measurements of counts derived from deep observations that consist of a single pointing, and could even be the sole cause at >50 micro-Jy. We present a method for evaluating the flux density limit that a radio survey must reach in order to reduce the count uncertainty induced by sample variance to a specific value. A conclusive empirical constraint on the effect of sample variance at these low luminosities is unlikely to arise until the completion of future large-scale radio surveys with next-generation radio telescopes.
View original: http://arxiv.org/abs/1302.2010

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