AGN Selection and Demographics: A New Age with JWST/MIRI

TL;DR

The study tackles the incomplete census of AGN across cosmic time by exploiting JWST/MIRI data in SMILES to perform deep mid-infrared AGN selection in GOODS-S/HUDF. It develops a comprehensive SED-fitting framework that jointly models stellar populations, AGN emission, and dust, leveraging updated templates and dual extinction laws. Applying this to 3273 MIRI-detected sources yields 217 AGN candidates across massive, low-mass, and high-redshift regimes, with the majority emerging as new discoveries thanks to JWST’s sensitivity to obscured activity. The work underscores how obscuration and SED variations shape AGN demographics, compares selection techniques across wavelengths, and provides a foundation for the most complete AGN sample to date.

Abstract

Understanding the co-evolution of supermassive black holes (SMBHs) and their host systems requires a comprehensive census of active galactic nuclei (AGN) behavior across a wide range of redshift, luminosity, obscuration level and galaxy properties. We report significant progress with JWST towards this goal from the Systematic Mid-infrared Instrument Legacy Extragalactic Survey (SMILES). Based on comprehensive SED analysis of 3273 MIRI-detected sources, we identify 217 AGN candidates over a survey area of $\sim$34 arcmin$^2$, including a primary sample of 111 AGNs in normal massive galaxies ($M_{*}>10^{9.5}~M_\odot$) at $z\sim$0--4, an extended sample of 86 AGN {\it candidates} in low-mass galaxies ($M_{*}<10^{9.5}~M_\odot$) and a high-$z$ sample of 20 AGN {\it candidates} at $z\sim$4--8.4. Notably, about 80\% of our MIRI-selected AGN candidates are new discoveries despite the extensive pre-JWST AGN searches. Even among the massive galaxies where the previous AGN search is believed to be thorough, 34\% of the MIRI AGN identifications are new, highlighting the impact of obscuration on previous selections. By combining our results with the efforts at other wavelengths, we build the most complete AGN sample to date and examine the relative performance of different selection techniques. We find the obscured AGN fraction increases from $L_{\rm AGN, bol}\sim10^{10}~L_\odot$ to $10^{11}~L_\odot$ and then drops towards higher luminosity. Additionally, the obscured AGN fraction gradually increases from $z\sim0$ to $z\sim4$ with most high-$z$ AGNs obscured. We discuss how AGN obscuration, intrinsic SED variations, galaxy contamination, survey depth and selection techniques complicate the construction of a complete AGN sample.

Figures (4)

• Figure 1: Survey layout from X-ray to radio (left) and the 5$\sigma$ flux limits of photometric bands at 0.3 -- 26 $\mu$m (right) used for SED fittings in GOODS-S/HUDF. On the left, the footprint of our MIRI survey is shown as the red-shaded region with JADES NIRCam deep (solid green line) and medium (dashed green line) observations, FRESCO NIRCam/grism coverage (solid blue line), HST ACS GOODS-S coverage (thick gray line), Chandra X-ray coverage (light blue shaded region) and JVLA radio observations at 3 GHz (dark green lines) and 6 GHz (dark orange lines). On the right, we denote the pre-JWST filters in gray, JWST/NIRCam filters in green and JWST/MIRI filters in red. Besides the flux limits of these filters as a function of wavelength, we also show some representative SEDs of obscured AGNs (orange), unobscured AGNs (blue) and starburst galaxies (magenta) at $z$=2 and $z$=4, where JWST observations are expected to bring major advances to identify and characterize these objects.
• Figure 2: Showcase of multi-wavelength data used for AGN searches in SMILES with a field of view 3.5 arcmin$\times$1.0 arcmin. The yellow circles indicate the AGNs or AGN candidates identified from this work. The JWST NIRCam and MIRI data are shown as three-color images with the long wavelength in red, intermediate wavelength in green and short wavelength in blue.
• Figure 3: Redshift distribution of MIRI sources. We highlight the sources of the final adopted redshifts with different colors.
• Figure 4: Illustration of how the AGN SED shape is changed by different levels of attenuation as well as the templates for the galaxy stellar and dust components in our model. The upper panel shows the AGN SED obscured by the SMC extinction curve, mainly used for UV-optical reddened AGNs. The lower panel presents the AGN SEDs modified by the empirical IR attenuation law. We show a galaxy stellar SED with a stellar age of 500 Myr (light green thick line), and the pure dust emission templates for normal star-forming galaxies (light orange line) and low-metallicity dwarf galaxies (light magenta line) to compare with the AGN SEDs in both panels. See text for details.