24 Micron Properties of X-ray Selected Active Galactic Nuclei

J. R. Rigby, G. H. Rieke, R. Maiolino, R. Gilli, C. Papovich, P. G. Perez-Gonzalez, A. Alonso-Herrero, E. Le Floc'h, C. W. Engelbracht, K. Gordon, D. C. Hines, J. L. Hinz, J. E. Morrison, J. Muzerolle, M. J. Rieke, K. Y. Su 2004, ApJS, 154, 160

We examine the 24 micron to X-ray color of 157 X-ray--selected AGN as a function of X-ray obscuration and optical classification in the Chandra Deep Field South. The sample consists of the Chandra hard--band detections with hard flux >10^-15 erg/s/cm². A deep 24 micron mosaic obtained with Spitzer provides mid--infrared fluxes for the sample. Since obscured AGN locally have higher 24 micron / 2-8 keV ratios than unobscured AGN, and since X-ray background models predict a large population of obscured AGN, we expect to find many X-ray--hard, IR--bright AGN. Instead, we find that the 24 micron to X-ray flux ratio does not depend on X-ray hardness in the full sample, nor does it differ between narrow--line and broad--line AGN. We identify 5 nearly Compton--thick AGN, and find they have similar 24 micron to X-ray flux ratios compared to the full sample. We consider AGN in the narrow redshift spikes at z approx. 0.7; for these AGN, there is some evidence that the flux ratio increases with X-ray hardness. The redshift slice also shows an odd trend, which is also prominent in the full sample: a group of X-ray--hard AGN with very low 24 micron to X-ray flux ratios.

[XXX/astro-ph Preprint] [ADS Entry]

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