Speaker
Description
Studying the radio spectral energy distribution (SED) of distant galaxies is essential for understanding their assembly and evolution over cosmic time. We present rest-frame mid-radio ($\simeq 1-16$\, GHz) SEDs of a sample of starburst galaxies at 1.5 < z < 4.5 in the COSMOS field as part of the MeerKAT International GHz Tiered Extragalactic Exploration (MIGHTEE) project. MeerKAT observations combined with archival VLA and GMRT data also allow us to measure the total energy budget of high-redshift galaxies emitting in the mid-radio continuum (MRC bolometric luminosities), the equipartition magnetic field, and star formation rate (SFR). A Bayesian Markov Chain Monte Carlo (MCMC) method was used to model the radio SEDs and to separate the thermal free-free and non-thermal synchrotron emission. We find that almost 94\% of the sample possess fully nonthermal mid-radio SEDs. On average, the nonthermal spectral index ($\alpha_{nt} \simeq 0.7$) is flatter than that of normal star-forming galaxies in the nearby universe (with $\alpha_{nt} \sim 0.9$). We also find that $\alpha_{nt}$ flattens with redshift and specific star formation rate (sSFR) indicates that cosmic rays are more energetic in the early universe due to higher star formation activity. With a mean of $65\pm12\;\mu$G, the equipartition magnetic field strength increases with redshift, $B\propto (1 + z)^{(0.8 \pm 0.1)}$, and with SFR ($B\propto {\rm SFR}^{0.3}$) indicating a small-scale dynamo as their amplification mechanism. Considering the evolution of the SEDs, we show that the rest-frame radio-infrared correlation is invariant with redshift.
