Abstract
We investigate the contribution that a local over- or under-density can have on linear cosmic dipole estimations. We focus here on radio surveys, such as the NRAO VLA Sky Survey (NVSS), and forthcoming surveys such as those with the LOw Frequency ARray (LOFAR), the Australian Square Kilometre Array Pathfinder (ASKAP) and the Square Kilometre Array (SKA). The NVSS has already been used to estimate the cosmic radio dipole; it was shown recently that this radio dipole amplitude is larger than expected from a purely kinematic effect, assuming the velocity inferred from the dipole of the cosmic microwave background. We show here that a significant contribution to this excess could come from a local void or similar structure. In contrast to the kinetic contribution to the radio dipole, the structure dipole depends on the flux threshold of the survey and the wave band, which opens the chance to distinguish the two contributions.
Highlights
The dipole anisotropy in radio surveys, such as the NRAO VLA Sky Survey (NVSS) catalogue (Condon et al 1998), has been investigated (e.g. Blake & Wall 2002; Singal 2011; Gibelyou & Huterer 2012; Rubart & Schwarz 2013; and Kothari et al 2013)
It appears that the cosmic radio dipole has a similar direction to the one found in the cosmic microwave background (CMB), but with a significantly higher amplitude
In this work we investigate one possible effect which can increase the dipole amplitude observed in radio surveys, with respect to the CMB dipole
Summary
The dipole anisotropy in radio surveys, such as the NVSS catalogue (Condon et al 1998), has been investigated (e.g. Blake & Wall 2002; Singal 2011; Gibelyou & Huterer 2012; Rubart & Schwarz 2013; and Kothari et al 2013). The local structures considered in this work are not in conflict with the Copernican principle, as they are much smaller than the Hubble scale and a fine tuning of the position of the observer with respect to the centre of a void is not required. Where ri is the normalized direction of source i on the sky as seen by an observer in the centre of the observed universe The fact that this estimator is linear is a big advantage here, since we can sum up the contributions of the background, of voids and of over-densities in an additive way. We assume a constant density contrast δ in the void, and an offset rv of the void in direction z, 2π
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