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 \leavevmode \nobreak 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 } W Hz ^ { -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 .