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

TL;DR

This study investigates the link between radio emission and AGN activity in star-forming galaxies, revealing that GHz-detected galaxies exhibit properties similar to AGN hosts, suggesting a possible evolutionary connection or low-power AGN presence.

Contribution

The paper provides a comparative analysis of star-forming galaxies with and without GHz radio detections, highlighting similarities to AGN-hosting galaxies and proposing a potential evolutionary or physical connection.

Findings

GHz-detected SF galaxies show AGN-like emission line properties.

GHz-detected galaxies are more radio compact and have higher outflow fractions.

Properties of GHz-detected SF galaxies resemble those of radio-detected AGNs.

Abstract

Increasing evidence shows that AGN 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-(LOFAR, 144MHz) and high-frequency (FIRST, 1.4GHz) surveys, combined with spatially resolved spectroscopy from the MaNGA survey, we compare SF galaxies with 144 MHz detections that either do or do not have GHz detections. Despite being matched in stellar mass, redshift, and radio (MHz) luminosity, GHz-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 [NII] BPT diagram). Furthermore, the non-GHz galaxies are likely undetected due to their extended radio morphologies, while the GHz-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. The radio compact characteristic sizes of GHz-detected SF galaxies also suggest a connection between AGN and old starburst galaxies.