Radio emission in star-forming galaxies: connection to restarted or relic AGN activity

Increasing evidence shows that active galactic nuclei (AGNs) with radio detections have more perturbed ionized gas kinematics and higher outflow detection rates, suggesting a link between radio emission and these processes. In galaxies with weak or ambiguous AGN signatures, some studies attribute the radio emission to star formation, while others propose AGN-driven winds or weak, unresolved jets as the dominant mechanism. To investigate this connection, we take a step back and analyze a sample of star-forming (SF) galaxies with no clear current AGN signatures. Using low-frequency (LOFAR, 144 MHz) and high-frequency (FIRST, 1.4 GHz) radio surveys, combined with spatially resolved spectroscopy from the SDSS-IV MaNGA survey, we compare SF galaxies with 144 MHz detections that either do or do not have gigahertz detections. Despite being matched in stellar mass, redshift, and radio (megahertz) luminosity, gigahertz-detected SF galaxies systematically differ from their non-GHz-detected counterparts. The former display enhanced ionized gas-emission line widths, higher central outflow fractions, redder colors, increased central obscuration, and offset emission-line ratios that shift towards (or closer to) the AGN regime (in the [N II] BPT diagram). Furthermore, the non-gigahertz galaxies are likely undetected due to their extended radio morphologies, while the gigahertz-detected ones are significantly more radio compact. Most of the properties from the GHz-detected (compared to non-detected) remarkably resemble the behavior found in many studies of radio-detected AGNs. This suggests that the underlying physical mechanisms shaping GHz-detected SF galaxies’ properties are fundamentally similar. This raises intriguing questions about whether some compact SF galaxies represent a precursor phase of AGN evolution or a form of low-power AGN activity. The radio compact characteristic sizes of GHz-detected SF galaxies also suggest a connection between AGNs and old starburst galaxies.