Radio continuum spectra of SFGs in the XMM-LSS Field below-threshold

Abstract

This study investigates the radio spectral properties of \textit{K}$_{S}$-selected star-forming galaxies (SFGs) in the XMM-LSS field using extensive multiwavelength data. By employing various diagnostics, SFGs are distinguished from quiescent galaxies and AGN across seven redshift bins ($\rm{0.1\leq\,\textit{z}\,\leq\,3.0}$). The broadband radio frequency spectral energy distribution is analysed at observer-frame frequencies from 144 to 1500 MHz using median stacking techniques correcting for median flux boosting. We investigate the relationship between the radio spectral index, $\alpha$ (where $S\propto\nu^{\alpha}$) and redshift ($z$). Our analysis reveals no significant inverse correlation between $\alpha$ and $z$, indicating that the radio spectrum remains independent with varying redshift. We fit the stacked median radio SEDs with a power law (\textit{PL}), curved power law (\textit{CPL}) and double power law (\textit{DPL}) models. For the \textit{DPL} and \textit{CPL} models, we observe a consistent steepening of the low-frequency spectral index across all redshift bins. For the \textit{CPL} model, the curvature term $q$ is greater than zero in all redshift bins. Model comparisons indicate that spectra are generally well fitted by all the models considered. At 1500 MHz, SFGs display both a steep synchrotron component and a flat free-free emission component, with a thermal fraction consistently around 11$\%$ to 18$\%$. Further deep radio observations, with higher resolution to better deal with source blending and confusion noise and wider frequency coverage to better separate non-thermal and thermal radio emission, are required to reveal the detailed physical processes, thus clarifying the nature of radio sources.