A Census of the Most Obscured Galaxy Nuclei over Cosmic Time to be revealed by PRIMA

Fergus R. Donnan, Dimitra Rigopoulou, Ismael García-Bernete, Laura Bisigello, Susanne Aalto

First listed 2025-03-14 | Last updated 2025-03-14

Abstract

Characterizing the growth of supermassive Black Holes (SMBHs) is critical to the evolution of galaxies, however the majority of this activity is obscured, rendering traditional tracers of active SMBHs, such as in the restframe optical/UV, ineffective. The mid-infrared has been particularly successful in revealing obscured AGN activity however much of this work is confined to the local universe due to the lack of a far-IR telescope with the required sensitivity and wavelength coverage. In this work we demonstrate the effectiveness of PRIMA (PRobe far-Infrared Mission for Astrophysics), a concept 1.8m far-IR observatory, to detect and characterize deeply obscured galaxy nuclei over cosmic time. With the PRIMAger instrument covering 25 - 235 $μ$m, we find that we can accurately detect obscured nuclei via the deep silicate absorption at restframe $9.8 μ$m between $z=2-7$. Additionally, the FIRESS spectrograph can produce R$\sim$100 spectra of obscured nuclei out to $z\sim7$, detecting Polycyclic Aromatic Hydrocarbons (PAHs), ices, ionized and molecular gas. With the large number of deeply obscured nuclei PRIMA can detect and characterize, such a mission is critical to understanding the growth of SMBHs.

Short digest

Concept study for the 1.8 m far‑IR PRIMA mission shows how to census the most buried galaxy nuclei across cosmic time. PRIMAger (25–235 um) photometry captures rest‑frame 9.8 um silicate absorption from z=2–7, enabling color selections that recover local CONs and scale to high z. FIRESS low‑resolution (R~100) spectroscopy to z~7 would detect PAHs, ice bands, and warm‑gas diagnostics even when high‑ionization lines are quenched, yielding redshifts and column densities. Simulations plus local (U)LIRG templates (NGC 4418 vs NGC 7714) suggest large, confusion‑mitigated yields in 1500 h/deg^2 surveys, making PRIMA pivotal for the obscured phase of SMBH growth.

Key figures to inspect

• Figure 1 — Check how the PRIMAger filter set tracks the 9.8 um silicate trough with redshift and separates an obscured template (NGC 4418) from a PAH‑dominated star‑former (NGC 7714), foreshadowing photometric redshift leverage.
• Figure 2 — Inspect the mock FIRESS R~100 spectrum (5 h) for an obscured nucleus in a HLIRG: which PAH bands, ice absorptions, and the silicate depth are recovered, and how the continuum shape enables column‑density estimates when high‑ionization lines are weak.
• Figure 3 — Follow the redshift‑binned color–color cuts: where GOALS/HERUS objects with known deep obscuration land relative to pure star‑forming galaxies, and how the NGC 4418–NGC 7714 mixing track maps increasing nuclear fraction into the selection box.
• Figure 4 — Read off predicted PRIMAger detections versus L_IR from the SPRITZ simulation for a 1500 h/deg^2 field; note how confusion‑mitigated sensitivities affect the counts only marginally at long wavelengths, guiding survey design.