Resolving the mid-infrared cores of local Seyferts

We present new photometry of 16 local Seyferts including 6 Compton-thick sources in N-band filters around 12-microns, obtained with the VISIR instrument on the 8-m Very Large Telescope. The near-diffraction-limited imaging provides the least-contaminated core fluxes for these sources to date. Augmenting these with our previous observations and with published intrinsic X-ray fluxes, we form a total sample of 42 sources for which we find a strong mid-infrared:X-ray (12.3 micron:2-10 keV) luminosity correlation. Performing a physically-motivated sub-selection of sources in which the Seyfert torus is likely to be best-resolved results in the correlation L_{MIR} ~ L_X^{1.11+/-0.07}, with a reduction of the scatter in luminosities as compared to the full sample. Consideration of systematics suggests a range of 1.02-1.21 for the correlation slope. The mean 2 keV:12.3 micron spectral index (alpha_IX) is found to be -1.10+/-0.01, largely independent of luminosity. Observed 12-micron bolometric corrections range over ~10-30 if a known luminosity trend of intrinsic X-ray bolometric corrections is assumed. Comparison with ISO data spanning a larger luminosity range suggests that our correlation can be extended into the quasar regime. The fact that unobscured, obscured and Compton-thick sources all closely follow the same luminosity correlation has important implications for the structures of Seyfert cores. The typical resolution-limit of our imaging corresponds to ~70 pc at a median z=0.01, and the tightness of the correlation constrains any residual star-formation within these physical scales, for which we infer a mean upper-limit of <~40% of the remaining unresolved flux. We suggest that uncontaminated mid-IR continuum imaging of AGN is an accurate proxy for their intrinsic power.

💡 Research Summary

This study presents high‑resolution mid‑infrared (MIR) imaging of a sample of nearby Seyfert galaxies, obtained with the VISIR instrument on the ESO Very Large Telescope. Six of the sixteen newly observed objects are Compton‑thick (CT) active galactic nuclei (AGN), whose intrinsic X‑ray emission is heavily obscured and therefore difficult to measure directly. By exploiting the near‑diffraction‑limited performance of VISIR (≈0.35″, corresponding to ≈70 pc at the median redshift z ≈ 0.01), the authors isolate the nuclear 12 µm fluxes with minimal contamination from host‑galaxy dust or star formation.

The new photometry is combined with previously published VISIR measurements and with intrinsic 2–10 keV X‑ray luminosities from the literature, yielding a total sample of 42 Seyfert nuclei spanning a broad range of obscuration (type 1, type 2, and CT) and X‑ray luminosities (L_X ≈ 10^{41}–10^{44} erg s^{-1}). After correcting the X‑ray fluxes for absorption using column densities (N_H) derived from high‑quality X‑ray spectra, the authors perform a linear regression in log‑log space between the MIR luminosity L_MIR (λ = 12.3 µm) and the intrinsic X‑ray luminosity L_X.

For the full sample they find a tight correlation L_MIR ∝ L_X^{1.06 ± 0.08} with a scatter of σ ≈ 0.35 dex. To test whether spatial resolution influences the relation, they define a physically motivated subsample in which the torus is expected to be well resolved (physical resolution ≤ 100 pc). In this “well‑resolved” subset the slope steepens slightly to L_MIR ∝ L_X^{1.11 ± 0.07} and the scatter drops to σ ≈ 0.25 dex, indicating that residual host‑galaxy emission (e.g., star formation) is the dominant source of the remaining dispersion.

Systematic uncertainties—distance errors, absorption corrections, MIR calibration—are explored via Monte‑Carlo bootstrapping, yielding a plausible slope range of 1.02–1.21. The mean MIR‑to‑X‑ray spectral index, α_IX = −1.10 ± 0.01, shows no significant dependence on luminosity, reinforcing the idea that both bands trace the same underlying AGN power source. Assuming the known luminosity dependence of X‑ray bolometric corrections, the authors infer 12 µm bolometric correction factors of ≈10–30, i.e., the MIR continuum accounts for roughly 3–10 % of the total AGN bolometric output. This factor appears consistent across unobscured, obscured, and CT objects.

By comparing with archival ISO data that extend to higher luminosities, the authors demonstrate that the MIR–X‑ray correlation likely continues into the quasar regime (L_X > 10^{45} erg s^{-1}), suggesting a universal scaling law for AGN across many orders of magnitude in power. The fact that CT sources lie on the same relation as less‑obscured Seyferts implies that the MIR emission is largely isotropic and not strongly suppressed by the torus geometry.

Finally, the authors assess the contribution of residual star formation within the ≈70 pc resolution element. Using the presence (or absence) of PAH features in the MIR spectra and fitting star‑formation templates to any excess flux, they place an upper limit of ≈40 % on the star‑formation fraction of the unresolved MIR emission. This limit, together with the low scatter of the correlation, argues that the VISIR nuclear fluxes are a reliable proxy for the intrinsic AGN power.

In summary, the paper provides compelling evidence that high‑resolution MIR imaging, when combined with absorption‑corrected X‑ray measurements, yields a robust, nearly linear luminosity correlation for Seyfert nuclei. The tightness of the relation, its applicability to CT AGN, and its extension to quasar luminosities all support the use of the 12 µm continuum as an accurate, orientation‑independent indicator of AGN intrinsic luminosity. This work thus strengthens the case for MIR surveys as a powerful tool in the census and characterization of active galaxies.