A Multiwavelength Evaluation of AGN in the Post-Starburst Phase

TL;DR

This study investigates whether AGN activity drives quenching in the local universe by examining post-starburst galaxies (PSBs) with a multiwavelength census (X-ray from eROSITA/eFEDS, MIR from WISE/unWISE, radio from FIRST) and by stacking undetected sources to probe average properties. It finds that, while PSBs show a higher MIR AGN incidence than star-forming galaxies, the inferred AGN luminosities are modest compared to typical MIR-identified AGN in the literature, and PSBs do not exhibit a significant enhancement of AGN emission relative to mass- and redshift-matched SFGs and QGs. Obscured, low-luminosity AGN, if present, could be fueled by residual gas from the prior starburst but are unlikely to be the dominant quenching mechanism at $z<0.2$. Overall, the results support a quenching picture in which AGN feedback is more preventive than ejective, suppressing future gas cooling or accretion rather than expelling existing gas from PSB hosts.

Abstract

The quenching of star formation is a crucial phase in galaxy evolution. Although active galactic nuclei (AGN) feedback 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/eFEDS (X-ray), WISE (mid-infrared), and FIRST (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$_{\rm obs}$/L$_{\rm SF}$) show that PSBs are consistent with star formation dominating their radio and X-ray emission. Although PSBs exhibit a 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 AGN's role in quenching at low redshift is more subtle than violently removing the gas -- the feedback is likely more "preventive" than "ejective".

Figures (3)

• Figure 1: Left: The distribution of SFGs, PSBs and QGs on the H$\delta_A$ vs. Dn4000 plot. Middle: [O$\;$]/H$\beta$ vs. [N$\;$]/H$\alpha$ BPT diagram. The dashed and solid lines are demarcation lines for SF and AGN ionization, from Kauffmann_2003 and Kewley_2001, respectively. Only galaxies with SNR $\geqslant$ 3 for all four emission lines are shown. QGs are selected to have minimal emission (Section \ref{['sec:control sample']}) and therefore not plotted here. Right: Stellar mass vs. redshift for the PSB sample and mass- and redshift-matched comparison samples of SFGs and QGs. The vertical and diagonal aggregations of QGs in this panel are not biases introduced by the matching process. See Section \ref{['sec:control sample']} and Appendix \ref{['appendix control sample']} for more details.
• Figure 2: Median stacked radio images (2$^\prime$$\times$ 2$^\prime$, 1.8$^{\prime\prime}$/pixel) in mJy for undetected SFGs, PSBs, and QGs. The white circle represents the 10$^{\prime\prime}$ diameter aperture used to measure the stacked signal (Table \ref{['tab:radio_stack']}). All three samples show SNR $>$ 2 signals in the stacks.
• Figure 3: Open points are galaxies detected in the FIRST catalog and filled points are measurements from stacking only undetected galaxies. The horizontal bar on the stacked points shows the 16th and 84th percentile range of the M$_*$ and the vertical bar shows the luminosity error from the bootstrapping method. Luminosity errors for individually detected galaxies (from the RMS in the catalog) are plotted though the error bars are smaller than the marker size.