Modeling the JWST MIRI Counts, Insights Into the Source Properties and Role of Dust-Obscured AGN

Edgar P. Vidal, Anna Sajina, Amber R. Banks, Matthieu Béthermin, Carl Ferkinhoff, Andreea Petric, Alexandra Pope, Jianwei Lyu, Vivian U, L. Y. Aaron Yung, Pallavi Patil

First listed 2025-09-18 | Last updated 2026-03-29

Abstract

Understanding the co-evolution of galaxies and active galactic nuclei (AGN) requires accurate modeling of dust-obscured systems. Recent surveys using the Mid Infrared Instrument (MIRI) onboard the James Webb Space Telescope (JWST) have uncovered a large population of dust obscured AGN, challenging current theoretical frameworks. We present an updated version of the Simulated Infrared Extragalactic Dusty Sky (SIDES) simulation framework. Our updates include modified star-forming and starburst galaxy spectral energy distribution (SED) templates as well as quiescent and AGN templates. We also incorporate a probabilistic assignment of the fraction of the IR emission that is due to an AGN. Our simulations successfully reproduce the observed MIRI source number counts, redshift distributions, and AGN population fractions. We find that AGN dominate at bright flux densities $(S_ν\gtrsim 20\, μ\rm Jy$ while main sequence galaxies dominate at the faint end. We also quantify the effects of cosmic variance, showing that surveys with areas below $25\, \rm arcmin^2$ suffer from $\sim 30 \% $ uncertainty in bright AGN counts. Finally, we provide diagnostic color-color diagrams and joint Near Infrared Camera (NIRCam) and MIRI flux distributions to aid interpretation of current and upcoming JWST surveys.

Short digest

The authors update the SIDES framework with revised star-forming, starburst, quiescent, and AGN SED templates plus a probabilistic assignment of the AGN contribution to the IR, applying it within pySIDES and the SC SAM lightcone. The model reproduces JWST/MIRI number counts, redshift distributions, and AGN fractions across the filters, finding AGN dominate at bright fluxes (Sν ≳ 20 μJy) while main-sequence galaxies set the faint-end counts. They quantify field-to-field variance, showing ≈30% uncertainty in bright AGN counts for survey areas <25 arcmin², and provide MIRI color–color and joint NIRCam+MIRI flux diagnostics for selecting obscured AGN. This delivers a practical forward model for interpreting MIRI-selected dusty AGN and for designing future mid/far-IR surveys.

Key figures to inspect

• Figure 1: Check how SC SAM galaxies separate in the sSFR–M⋆ plane; this sets the main-sequence/starburst/quenched assignments that drive which SEDs and AGN prescriptions feed the MIRI counts.
• Figure 2: Inspect the redshift-dependent 1.22 μm mass-to-light fit for quiescent systems and its residuals; this controls the Ell5 stellar normalization and impacts short-wavelength MIRI fluxes of old populations.
• Figure 3: Compare modified Magdis et al. SEDs with added stellar templates across radiation field U; overlay of MIRI (and PRIMA) bands shows which filters sample PAHs vs continuum for MS vs starbursts, informing the color diagnostics.
• Figure 4: Review the Kirkpatrick mid-IR template grid binned by AGN fraction (SF/composite/AGN); note continuum slope and PAH behavior changes with fAGN that explain rising AGN dominance above ~20 μJy and guide color cuts.