The prevalence of core emission in faint radio galaxies in the SKA Simulated Skies

Abstract

Empirical simulations based on extrapolations from well-established low-frequency ($<5$ GHz) surveys fail to accurately model the faint, high frequency ($>10$~GHz) source population; they under-predict the number of observed sources by a factor of two below $S_{18\mathrm{GHz}}=10$ mJy and fail to reproduce the observed spectral index distribution. We suggest that this is because the faint radio galaxies are not modelled correctly in the simulations and show that by adding a flat-spectrum core component to the FRI sources in the SKA Simulated Skies, the observed 15-GHz source counts can be reproduced. We find that the observations are best matched by assuming that the fraction of the total 1.4-GHz flux density which originates from the core varies with 1.4-GHz luminosity; sources with 1.4-GHz luminosities $<10^{25}\mathrm{W}\mathrm{H}{\mathrm{z}}^{-1}$ require a core fraction $\sim 0.3$, while the more luminous sources require a much smaller core fraction of $5\times 10^{-4}$. The low luminosity FRI sources with high core fractions which were not included in the original simulation may be equivalent to the compact `FR0' sources found in recent studies.

6 pages, 5 figures, accepted for publication in MNRAS

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• An insight into the extragalactic transient and variable microJy radio sky across multiple decades
• The faint radio source population at 15.7 GHz -- IV. The dominance of core emission in faint radio galaxies
• AMI-LA Observations of the SuperCLASS Super-cluster
• Observations of compact sources in galaxy clusters using MUSTANG2
• Confusion noise due to clustered extragalactic point sources. Application of logarithmic cumulants for parameter estimation
• MIGHTEE: total intensity radio continuum imaging and the COSMOS / XMM-LSS Early Science fields