Dipole anisotropy in sky brightness and source count distribution in radio NVSS data

Abstract

We study the dipole anisotropy in number counts and flux density weighted number counts or sky brightness in the NRAO VLA Sky Survey (NVSS) data. The dipole anisotropy is expected due to our local motion with respect to the CMBR rest frame. We analyse data with an improved fit to the number density, n(S), as a function of the flux density S, which allows deviation from a pure power law behaviour. We also impose more stringent cuts to remove the contribution due to clustering dipole. In agreement with earlier results, we find that the amplitude of anisotropy is significantly larger in comparison to the prediction based on CMBR measurements. The extracted speed is found to be roughly 3 times the speed corresponding to CMBR. The significance of deviation is smaller, roughly 2 σ, in comparison to earlier estimates. For the cut, S > 30 mJy, the speed is found to be 1110 ± 370 Km/s using the source count analysis. The direction of the dipole anisotropy is found to be approximately in agreement with CMBR. We find that the results are relatively insensitive to the lower as well as upper limit imposed on the flux density. Our results suggest that the Universe is intrinsically anisotropic with the axis of anisotropy axis pointing roughly towards the CMBR dipole direction. Finally we present a method which may allow an independent extraction of the local speed and an intrinsic dipole anisotropy, provided a larger data set becomes available in future.

Testing the standard model of cosmology with the SKA: the cosmic radio dipole

“…The CMB dipole was measured with high accuracy by the Planck collaboration (Akrami et al 2018): v = 369.82 ± 0.11 km/s towards (264.021 ± 0.011 , 48.253 ± 0.005) • , Measurements performed using these and other radio continuum catalogues have found that the radio dipole is compatible with the CMB dipole direction, but the dipole amplitude is 2 − 5 times larger than the signal observed in the CMB (Blake & Wall 2002;Singal 2011;Gibelyou & Huterer 2012;Rubart & Schwarz 2013;Tiwari et al 2014;Tiwari & Jain 2015;Tiwari & Nusser 2016;Colin et al 2017;Bengaly et al 2018b). However, no significant evidence for anomalous anisotropy in galaxy counts was reported at lower redshift ranges, such as those probed by visible and infrared catalogues (Itoh et al 2010;Gibelyou & Huterer 2012;Yoon et al 2014;Alonso et al 2015a;Yoon & Huterer 2015;Javanmardi & Kroupa 2017;Bengaly et al 2017Bengaly et al , 2018aRameez et al 2018).…”

Angular two-point correlation of NVSS galaxies revisited

“…If this rest frame is the same as the CMB rest frame, then the dipole measured in the radio catalogue should agree with the CMB dipole measured by WMAP and Planck (Jarosik et al 2011;Planck Collaboration XXVII 2014). Previous studies 6 (Blake & Wall 2002a;Singal 2011;Gibelyou & Huterer 2012;Rubart & Schwarz 2013;Tiwari et al 2015;Singal 2014) measured the radio dipole for the NVSS catalogue. It is actually significantly larger than expected, by a factor of two to four depending on the details of the analysis.…”

Rotation of the polarization vector from distant radio galaxies in the perturbed FRW metric

“…It has been tried to explain this preferred direction as a result of the peculiar motion of the solar system [10,11]. But the peculiar velocities determined from CMBR data and from radio data are not consistent with each other [11][12][13]. This suggests the existence of an intrinsic anisotropy in the large scale with the anisotropy axis roughly aligned along the CMBR dipole [12].…”

“…But the peculiar velocities determined from CMBR data and from radio data are not consistent with each other [11][12][13]. This suggests the existence of an intrinsic anisotropy in the large scale with the anisotropy axis roughly aligned along the CMBR dipole [12]. In recent years, single mode super-horizon perturbations have been considered [14,15] to explain the observed dipolar anisotropy as the wavevector corresponding to a single mode can readily introduce a special direction in the universe.…”