Average radio spectral energy distribution of highly star-forming galaxies

TL;DR

This study constrains the radio spectral energy distribution of highly star-forming galaxies, revealing a steepening spectral index at higher frequencies, which impacts the interpretation of the infrared-radio correlation across redshifts.

Contribution

It provides the first detailed measurement of the radio SED of highly star-forming galaxies over a broad frequency range, showing a frequency-dependent spectral index.

Findings

Spectral index steepens from 0.51 to 0.98 above 4.5 GHz.

Radio SED is better described by a double power law than a single power law.

K-corrections assuming a single power-law are unlikely to cause IRRC trends.

Abstract

The infrared-radio correlation (IRRC) offers a way to assess star formation from radio emission. Multiple studies found the IRRC to decrease with increasing redshift. This may in part be due to the lack of knowledge about the possible radio spectral energy distributions (SEDs) of star-forming galaxies. We constrain the radio SED of a complete sample of highly star-forming galaxies ($SFR>100\,\mathrm{M_{\odot}}/\,\mathrm{yr}$) based on the VLA-COSMOS $1.4\,\mathrm{GHz}$ Joint and $3\,\mathrm{GHz}$ Large Project catalogs. We reduce archival GMRT $325\,\mathrm{MHz}$ and $610\,\mathrm{MHz}$ observations, broadening the rest-frame frequency range to $0.3-15\,\mathrm{GHz}$. Employing survival analysis and fitting a double power law SED, we find that the slope steepens from a spectral index of $\alpha_1=0.51\pm 0.04$ below $4.5\,\mathrm{GHz}$ to $\alpha_2=0.98\pm0.07$ above $4.5\,\mathrm{GHz}.$ Our results suggest that the use of a K-correction assuming a single power-law radio SED for star forming galaxies is likely not the root cause of the IRRC trend.