A Multiwavelength Evaluation of AGN in the Post-starburst Phase

Abstract The quenching of star formation is a crucial phase in galaxy evolution. Although feedback from active galactic nuclei (AGN) has been proposed as a key driver of this transition, the lack of strong AGN in nearby quenching galaxies raises questions about its effectiveness. In this study, we investigate AGN activity in post-starburst galaxies (PSBs), star-forming galaxies (SFGs), and quiescent galaxies (QGs) at z < 0.2, using multiwavelength data from eROSITA Final Equatorial Depth Survey (X-ray), Wide-field Infrared Survey Explorer (mid-infrared), and Faint Images of the Radio Sky at Twenty cm (radio). We assess AGN incidence and strength across different stages and apply stacking techniques to undetected galaxies to recover average AGN properties. Comparisons between observed luminosity and that expected from star formation ( L obs / L SF ) show that PSBs are consistent with star formation dominating their radio and X-ray emission. Although PSBs exhibit a mid-infrared (MIR) AGN incidence rate twice that of SFGs, their estimated AGN luminosities are small compared to those of MIR AGN in the literature. PSBs overall do not display significantly enhanced AGN emission relative to mass- and redshift-matched SFGs and QGs. While the presence of obscured, low-luminosity AGN in PSBs cannot be excluded, such AGN, if present, could be fueled by residual gas from the preceding starburst and may not play a dominant role in quenching. Our findings suggest that the role of AGN in quenching at low redshift is more subtle than violently removing the gas—the feedback is likely more “preventive” than “ejective.”