The faint source population at 15.7 GHz - I. The radio properties

TL;DR

This study investigates the radio spectral properties of faint 15.7 GHz sources, revealing a significant population of flat spectrum sources emerging below 1 mJy, which are underrepresented in lower frequency selected samples and simulations.

Contribution

It provides new insights into the spectral characteristics of faint high-frequency radio sources and highlights the limitations of existing simulations in reproducing these populations.

Findings

Increase in flat spectrum sources below 1 mJy flux density

Flat spectrum sources are underrepresented in 1.4 GHz selected samples

Simulations underestimate the number of faint, flat spectrum sources

Abstract

We have studied a sample of 296 faint (> 0.5 mJy) radio sources selected from an area of the Tenth Cambridge (10C) survey at 15.7 GHz in the Lockman Hole. By matching this catalogue to several lower frequency surveys (e.g. including a deep GMRT survey at 610 MHz, a WSRT survey at 1.4 GHz, NVSS, FIRST and WENSS) we have investigated the radio spectral properties of the sources in this sample; all but 30 of the 10C sources are matched to one or more of these surveys. We have found a significant increase in the proportion of flat spectrum sources at flux densities below approximately 1 mJy - the median spectral index between 15.7 GHz and 610 MHz changes from 0.75 for flux densities greater than 1.5 mJy to 0.08 for flux densities less than 0.8 mJy. This suggests that a population of faint, flat spectrum sources is emerging at flux densities below 1 mJy. The spectral index distribution of this sample of sources selected at 15.7 GHz is compared to those of two samples selected at 1.4 GHz from FIRST and NVSS. We find that there is a significant flat spectrum population present in the 10C sample which is missing from the samples selected at 1.4 GHz. The 10C sample is compared to a sample of sources selected from the SKADS Simulated Sky by Wilman et al. and we find that this simulation fails to reproduce the observed spectral index distribution and significantly underpredicts the number of sources in the faintest flux density bin. It is likely that the observed faint, flat spectrum sources are a result of the cores of FRI sources becoming dominant at high frequencies. These results highlight the importance of studying this faint, high frequency population.