A velocity dipole in the distribution of radio galaxies


The motion of our Galaxy through the Universe is reflected in a systematic shift in the temperature of the cosmic microwave background1—because of the Doppler effect, the temperature of the background is about 0.1 per cent higher in the direction of motion, with a correspondingly lower temperature in the opposite direction. This effect is known as dipole anisotropy. If our standard cosmological model is correct, a related dipole effect should also be present as an enhancement in the surface density of distant galaxies in the direction of motion2. The main obstacle to finding this signal is the uneven distribution of galaxies in the local supercluster, which drowns out the small cosmological signal. Here we report a detection of the expected cosmological dipole anisotropy in the distribution of galaxies. We use a survey of radio galaxies that are mainly located at cosmological distances, so the contamination from nearby clusters is small. When local radio galaxies are removed from the sample, the resulting dipole is in the same direction as the temperature anisotropy of the microwave background, and close to the expected amplitude. The result therefore confirms the standard cosmological interpretation of the microwave background.

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Figure 1: The NVSS suffers from systematic fluctuations in source surface density across the sky at low flux densities.
Figure 2: Our measured dipole direction agrees with that of the CMB dipole.
Figure 3: The clustering dipole originates from redshifts z < 0.03.
Figure 4: The PSCz and RCBG3 catalogues include almost all nearby NVSS galaxies.


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We thank L. Miller and S. Rawlings for comments on early drafts. We also thank J. Condon for comments and suggestions.

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Correspondence to Chris Blake.

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Blake, C., Wall, J. A velocity dipole in the distribution of radio galaxies. Nature 416, 150–152 (2002). https://doi.org/10.1038/416150a

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