No hidden monsters: Probing recently-quenched galaxies for obscured AGN with JWST-PRIMER MIRI and NIRCam

Guillaume Hewitt, Omar Almaini, David Maltby, Emma Chapman, Thomas de Lisle, Pallavi Patil, Kate Rowlands, Maya Skarbinski, Elizabeth Taylor, Vivienne Wild, Adam C. Carnall, James S. Dunlop, Norman Grogin, Anton M. Koekemoer, Derek J. McLeod, Pablo G. Pérez-González

First listed 2026-06-23 | Last updated 2026-06-18

Abstract

We investigate the role of obscured active galactic nuclei (AGN) in recently quenched post-starburst galaxies (PSBs), using a sample of 65 photometrically selected PSBs in the PRIMER-UDS field at $1 < z < 2$. Combining JWST/MIRI 7.7 $μ$m and 18 $μ$m (F770W and F1800W) imaging with eight NIRCam and three HST/ACS bands, we probe hot dust emission to test for hidden AGN or dust-enshrouded star formation. We find strong differences between the low- and high-mass PSBs. Most high-mass PSBs ($ > 10^{10}\textrm{M}_\odot$) show no excess infrared emission (consistent with the quiescent population), indicating little or no dust-obscured activity, while low-mass PSBs display enhanced emission at 18 $μ$m, which we attribute to residual star formation. AGN template modelling indicates that the absence of mid-IR excess in massive PSBs limits any dust-enshrouded AGN to Eddington ratios of $ < 1\%$. In addition, we show that the F770W--F1800W colour alone is a highly effective diagnostic for separating passive and star-forming galaxies, particularly at high stellar masses. Overall, our results provide further evidence for distinct quenching pathways within the PSB population, and confirm that massive PSBs show no evidence for excess AGN activity relative to older passive galaxies.

Short digest

This paper uses JWST-PRIMER MIRI F770W and F1800W imaging, together with eight NIRCam bands and three HST/ACS bands, to test whether 65 photometrically selected post-starburst galaxies in PRIMER-UDS at 1 < z < 2 hide dust-obscured AGN. The main result is strongly mass-dependent: most massive PSBs above 10^10 Msun have mid-IR colours consistent with ordinary quiescent galaxies and show no excess hot-dust emission, while lower-mass PSBs are brighter at 18 um in a way the authors attribute to residual star formation. AGN template experiments then turn that non-detection into a quantitative limit, implying any obscured AGN in the massive PSBs must be accreting below about 1 percent of Eddington. The paper therefore argues against a substantial hidden-AGN population in massive recently quenched galaxies and highlights F770W-F1800W as a simple, powerful passive-versus-star-forming diagnostic in this redshift range.

Key figures to inspect

• Figure 1. Use this for the sample definition. It shows the stellar-mass versus redshift distribution of the PRIMER-UDS galaxies, the 65 PSBs within the F200W-selected parent sample, and the mass-completeness limits that frame where the high-mass and low-mass conclusions are reliable.
• Figure 2. This is the clearest single statement of the paper’s observational result. It shows that F770W-F1800W cleanly separates passive and star-forming systems over the targeted redshift range and that the PSBs split by mass, with massive PSBs overlapping the quiescent locus while low-mass PSBs show a redder mid-IR excess.
• Figure 4. Recommend this as the main diagnostic figure because it defines the MIR-excess versus MIR-non-excess populations in the full colour-colour space. It also shows where the PSBs sit relative to star-forming, quiescent, and X-ray-detected sources, making the case that the massive PSBs do not occupy an obviously AGN-like mid-IR regime.
• Figure 6. This figure is central to the paper’s quantitative claim about hidden AGN. By overlaying obscured-AGN template tracks and probability contours on the PSB colour distribution, it visualizes why the MIR-non-excess massive PSBs are inconsistent with anything but very weak AGN activity and motivates the sub-1-percent Eddington-ratio limit.
• Figure 7. Include this for the physical interpretation of the mass split. The rest-frame SED comparison shows that MIR-excess and MIR-non-excess PSBs have different mid-IR behaviour, supporting the paper’s conclusion that the excess seen in low-mass PSBs is better explained by residual star formation than by a buried AGN population.