Resolving the mid-infrared cores of local Seyferts
P. Gandhi, H. Horst, A. Smette, S. Hönig, A. Comastri, R. Gilli, C. Vignali, and W. Duschl
Astronomy & Astrophysics, Vol.502, pg.457-472 (2009)
Abstract
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.
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