The Galaxy Power Spectrum from TGSS ADR1 and the Effect of Flux Calibration Systematics

Abstract

We explore the large to moderate scale anisotropy in distant radio sources using the TGSS ADR1 catalog. We use different measures, i.e. number counts, sky brightness and flux per source, for this study. In agreement with earlier results, we report a significant excess of clustering signal above the angular scale of roughly 10 degrees (i.e. l < 20). We find that some survey areas have a systematically low/high flux and argue this may be the cause of the observed signal of excess power at low multipoles. With mocks we demonstrate the effect of such large scale flux systematics and recover TGSS like excess clustering signal by assuming 20% flux uncertainties over ∼ 10 • × 10 • size patches. We argue that that TGSS at this stage, i.e. TGSS ADR1, is not suitable for large scale clustering measurements. We find that the measure, flux per source, shows evidence of isotropy for all multipoles l > 2 despite the presence of systematics in the data. Subject headings: cosmology: large-scale structure of universe -dark matter -galaxies: activehigh-redshift

Testing the cosmological principle in the radio sky

“…Therefore, there is no signal of isotropy violation in the NVSS source counts on smaller angular scales than the dipole. This result agrees with the findings of [28,29,30], using other methods of analysis. A more thorough investigation of the impact of clustering bias models (which can affect the large angular scales), as well as of magnification bias and redshift distribution modelling of the source counts, is left for future work.…”

“…In this work, we tested whether the NVSS catalogue is consistent with the statistical isotropy hypothesis on angular scales smaller than the dipole -whose amplitude is known to be larger than expected. We constructed a mask that extends the mask in [21] by excising a small region of anamolously high source counts, following [12,30]. We simulated radio sky maps that reproduce the NVSS specifications, as well as the radio source clustering and the power spectrum of the standard model, following [21].…”

“…This is a conservative choice in order to ensure sample completeness (lower limit) and elimination of very bright sources (upper limit). Since the NVSS flux densities are found to be statistically isotropic in the sky [30], no further flux cutoff is performed. We construct a mask which excises the following regions, as in [21]:…”

“…Furthermore, in order to systematically test isotropy, as opposed to a consistency test, we need to probe smaller scales. On smaller scales than the dipole, some isotropy tests have been performed on galaxy surveys, including infrared [25], optical [26] and radio [27,28,29,30] surveys. The results so far are consistent with isotropy.…”

Explaining excess dipole in NVSS data using superhorizon perturbation

“…However, the dipole signal from TGSS disagrees not only with the CMB predictions, but also with the dipole signal from NVSS. There might be some unknown systematic error present in the TGSS data [43,44], and so, in this paper, we focus on the higher-than-expected value of the velocity dipole found in the NVSS data and explain it using a particular extension of the ΛCDM cosmology.…”