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

We explore the large to moderate scale anisotropy in distant radio sources using the TIFR GMRT Sky Survey (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 less than or similar to 20-30). 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 a TGSS-like excess clustering signal by assuming 20% flux uncertainties over similar to 10 degrees x 10 degrees sized 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.