A Multiwavelength Evaluation of AGN in the Post-Starburst Phase

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”.

💡 Research Summary

This paper investigates whether active galactic nuclei (AGN) play a dominant role in quenching star formation during the post‑starburst (PSB) phase of nearby galaxies (z < 0.2). The authors assemble a multi‑wavelength data set comprising X‑ray observations from eROSITA/eFEDS, mid‑infrared (MIR) photometry from WISE, and 1.4 GHz radio measurements from the FIRST survey. A parent sample of over 6 000 PSBs identified in SDSS is cross‑matched with the three surveys, and after imposing a stellar‑mass cut (M_* > 10⁹ M⊙) and requiring coverage in all bands, 73 PSBs remain. For each PSB, five star‑forming galaxies (SFGs) and five quiescent galaxies (QGs) are selected as control samples, matched in stellar mass (Δlog M_* < 0.4 dex) and redshift (Δz ≲ 0.01) using the Hungarian algorithm, yielding 346 SFGs and 333 QGs.

The analysis proceeds in two steps: (1) direct detection of individual sources in each band, and (2) stacking of non‑detected objects to recover average emission. X‑ray stacking of the 0.5–2 keV eFEDS images provides an average observed X‑ray luminosity (L_X,obs) for each galaxy class. This is compared to the expected X‑ray output from star formation (L_X,SF) derived from SFR estimates and the established L_X–SFR relation. All three populations show L_X,obs/L_X,SF ≈ 1, indicating that star formation fully accounts for the X‑ray signal and that there is no excess AGN‑related X‑ray emission in PSBs.

A similar stacking approach is applied to the FIRST radio data. The observed radio power (L_R,obs) again matches the star‑formation‑predicted radio power (L_R,SF) for PSBs, SFGs, and QGs (L_R,obs/L_R,SF ≈ 1). Thus, radio emission in PSBs is also dominated by star‑forming processes rather than AGN jets.

MIR diagnostics use WISE colour cuts (e.g., W1–W2 > 0.8 mag) to identify AGN candidates. The incidence of MIR‑selected AGN in PSBs is ≈ 12 %, roughly twice the ≈ 6 % found in the matched SFG sample, while QGs show an even lower rate. However, the MIR luminosities of the PSB AGN are modest (L_MIR ≈ 10⁴² erg s⁻¹), well below the typical luminosities of luminous MIR AGN reported in the literature. Stacked MIR fluxes are consistent with the contribution expected from star formation, suggesting that any AGN present are low‑luminosity and possibly heavily obscured.

Overall, the study finds no evidence that PSBs host a population of powerful AGN capable of driving rapid gas expulsion (“ejective” feedback). Instead, the data support a scenario where any AGN activity in PSBs is weak, perhaps fueled by residual gas left over from the preceding starburst, and likely contributes to “preventive” feedback—suppressing cooling and future gas accretion rather than removing existing gas reservoirs.

The authors discuss the implications for galaxy evolution models, noting that the prevalence of low‑luminosity, possibly obscured AGN aligns with recent simulations that emphasize maintenance‑mode feedback at low redshift. They also compare their results with earlier work that reported occasional high‑ionization lines or X‑ray detections in individual PSBs, arguing that those cases represent rare outliers rather than the norm. The paper concludes that, at least in the local universe, AGN are not the primary agents of quenching; instead, subtle, long‑term heating or suppression of gas inflow appears to be the dominant mechanism. Future deeper X‑ray and radio observations, together with high‑resolution molecular gas studies, are recommended to further clarify the interplay between low‑luminosity AGN and the gas reservoirs in post‑starburst systems.