Radio spectral index from NVSS and TGSS

TL;DR

This study analyzes the radio spectral index from a large sample of sources observed in two major surveys, confirming its dependence on flux density and source size, and providing insights into the physical mechanisms of active galactic nuclei.

Contribution

It provides a large-scale, detailed analysis of the radio spectral index across a vast sample, including a parametric fit and the division into flux-based subsets to explore physical dependencies.

Findings

Spectral index steepens with increasing flux density.

Spectral index increases with source size, saturating at large sizes.

Confirmed dependence of spectral index on flux and size for AGN hosts.

Abstract

I extract the radio spectral index, $\alpha$, from 541,195 common sources observed in the 150 MHz TIFR GMRT Sky Survey (TGSS) and the 1.4 GHz NRAO VLA Sky Survey (NVSS). This large common source catalogue covers about $80\%$ of the sky. The flux density limits in these surveys are such that the observed galaxies are presumably hosts of active galactic nuclei (AGNs). I confirm the steepening of $\alpha$ with increasing flux density for this large sample and provide a parametric fit between $\alpha$ and flux density. Next, I divide the data into a low flux (LF) and a high flux (HF) density sample of roughly equal number of galaxies. The LF sample contains all galaxies below 100 mJy TGSS and 20 mJy NVSS flux density and the HF sample is all galaxies above 100 mJy TGSS and 20 mJy NVSS. I observe an increase in $\alpha$ with source size (TGSS measured), saturating for large sizes to $0.89\pm0.22$ and $0.76\pm 0.21$ for the LF and HF sources, respectively. I discuss the observed results and possible physical mechanisms to explain observed $\alpha$ dependence with source size for LF and HF samples.