Constraining the AGN tori at cosmic noon using high-resolution JWST imaging and simultaneous SED fitting

TL;DR

This study leverages high-resolution JWST CEERS imaging to decompose AGN torus and host galaxy emission at cosmic noon ($z\sim2$) by fitting both central (nuclear) and integrated SEDs within a Bayesian framework. The authors introduce a two-step SED fitting approach using SKIRTOR torus models, BC03 stellar templates, and Dale14 dust templates, enabling simultaneous constraints on torus geometry (including the covering fraction $CF$) and host properties. Across 88 X-ray–selected AGN, they find a torus $CF$ distribution peaking near $0.25$ with a long high-$CF$ tail and no strong evidence for evolution of $CF$ with redshift when controlling for luminosity, though results are sensitive to torus-model definitions and sample selection. The work demonstrates the power of combining central JWST photometry with wide-field archival data to yield tighter AGN characterizations and sets the stage for larger, more definitive studies with upcoming JWST surveys. The methodology and findings have significant implications for understanding the co-evolution of SMBHs and their hosts across cosmic time and for refining torus models in population studies.

Abstract

There is evidence for significant evolution in the gaseous and dust properties of galaxies since the era of cosmic noon ($1\lesssim z\lesssim 2.5$). The well known co-evolution of supermassive black holes with their host galaxies suggests a constant connection between the small-scale (nuclear) and large-scale regions of galaxies. A fundamental component of Active Galactic Nuclei (AGN) is the "torus", a dense, dusty structure that acts as the interface between the accretion disc and the ISM of the host galaxy. The transitional nature of the torus makes it a prime subject to search for evolution since cosmic noon. We use high-resolution near- and mid-IR imaging from the JWST CEERS program to disentangle the emission from the torus in unprecedented detail for 88 X-ray selected AGN at $z\sim2$. We employ a novel SED fitting technique that combines archival low-resolution multi-band photometry at UV to FIR wavelengths with the new high-resolution JWST photometry to constrain essential AGN and torus parameters, such as accretion disc luminosity, torus opening angle, and inclination angle. We demonstrate that this SED fitting approach leads to better AGN characterisation and tighter constraints on AGN parameters. The population-level analysis finds that the Covering Fraction ($CF$) distribution peaks at $\approx0.25$ with a long tail towards higher $CF$. Despite the well-known evolution of the ISM and structural properties of AGN hosts to these redshifts, the $CF$ distribution of our sample does not show any strong statistical difference with that found in local AGN of equivalent luminosity, or with those at intermediate redshifts.

Figures (15)

• Figure 1: The 2-10 keV X-ray luminosity ($L_{X}$) plotted against redshift of all 143 AEGIS-X X-ray sources observed in CEERS (smaller gray circles in the background). The 88 sources in our final sample after filtering are shown by larger circles whose colour gives the equivalent Hydrogen column density $N_H$ (determined using X-ray spectral fitting by Buchner_AEGISX) whenever available. These circles are black if the $N_H$ is not available. The dashed lines show $L_{X}$ cut applied while selecting the final sample (see Section \ref{['subsec:sample']} for more details).
• Figure 2: The position of the 88 sources in the final sample used in this study along with the HST/WFC3 F160W mosaic from CANDELS in the background. Different symbols show whether a particular source was observed using at least one filter in NIRCam (red triangle), MIRI (blue star), or both (black circle). Note that the region shown in the figure is entirely covered by the HELP and the AEGIS-X surveys.
• Figure 3: A flowchart describing the workflow of the analysis in this work. The blue boxes represent data product acquired from other studies (e.g. source catalogues, images, etc.), yellow rounded rectangles represent the processing steps, and the green rectangles represent the fitting steps. The arrows indicate the flow of products from one step to others and the section numbers given in the bracket describe those steps in more detail in the main text.
• Figure 4: $3"\times 3"$ images of one of our objects (XID471) in different NIRCam and MIRI filters. The large left-most panel shows the image in NIRCam/F277W filter, where the PSF has a 80% enclosed energy radius of $0\farcs 179$ that was adopted as the aperture size to measure the "central SED" (Section \ref{['subsub:centralSED']}). This aperture was fixed for all filters and is shown by a red circle in each image. The black dotted line in the left-most panel gives the Kron radius of this source used to measure the flux reported in the CANDELS catalogues that form our "integrated SED" (Section \ref{['subsub:integratedSED']}). The black star shows the coordinates of the optical counterpart given by AEGIS-X, which is generally not centred on the galaxy nucleus. We have estimated the centres of all our sources using JWST images as described in Section \ref{['subsec:recentring']}. This new centre is shown by a red star in the left-most panel and used as source co-ordinate throughout this work.
• Figure 5: This figure illustrates the concept of simultaneous SED fitting used in this work. Solid and dashed black lines show the total model SEDs for integrated fluxes and central fluxes respectively. These observed fluxes are shown as black and red points without error bars for readability. The stellar population (blue) and dust illuminated by stars (green) models are scaled by a factor of 0.23 (specific to this source) for central SED as compared to the integrated SED, while the AGN model (red) remains same across these two SEDs as indicated by lack of dashed red line. This is mathematically represented by Equation \ref{['eq:SEDfit']}. Note that for the sake of decluttering we drop all the dashed lines from other similar SED fitting figures in this work.