The Radio Spectral Energy Distribution and Star Formation Calibration in MIGHTEE-COSMOS Highly Star-Forming Galaxies at 1.5 < z < 3.5

TL;DR

This study analyzes the radio spectral energy distributions of high-redshift starburst galaxies to calibrate star formation rates and investigate magnetic field evolution, revealing that magnetic fields strengthen with redshift and SFR, and the IR-radio correlation remains consistent over cosmic time.

Contribution

It provides new insights into the evolution of radio SEDs, magnetic fields, and star formation calibration in distant galaxies at 1.5 < z < 3.5 using combined radio data and Bayesian modeling.

Findings

Synchrotron spectral index flattens with redshift and specific SFR.

Magnetic field strength increases with redshift and SFR, following specific proportionalities.

IR-radio correlation remains redshift-invariant and does not vary with stellar mass.

Abstract

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 radio SEDs of a sample of 160 starburst galaxies at redshifts 1.5 to 3.5 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, allow us to determine the integrated mid-radio (1-10 GHz) continuum (MRC) luminosity and magnetic field strength. A Bayesian method is used to model the SEDs and separate free-free and synchrotron emission. We calibrate the star formation rate (SFR) in radio both directly through SED analysis and indirectly via the infrared-radio correlation (IRRC). With a mean synchrotron spectral index of approximately 0.7, we find that the index flattens with redshift and specific SFR, suggesting that cosmic rays are more energetic in the early universe due to higher star formation activity. The magnetic field strength increases with redshift (B is proportional to (1 + z)^0.7) and with star formation rate (B is proportional to SFR^0.3), indicating a small-scale dynamo as the dominant amplification mechanism. Accounting for SED evolution, the IRRC remains redshift-invariant and does not vary with stellar mass at 1.5 < z < 3.5, though the correlation deviates from linearity. Similarly, we show that SFR estimates based on integrated MRC luminosity are also redshift-invariant.