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N. A. Levenson (U KY)
The detected soft X-rays of obscured active galactic nuclei (AGN) are generally not the continuum of the buried central engines. Line emission typically dominates these spectra, the result of either photoionization or collisional excitation. Current data obtained with Chandra distinguish these two phenomena even at low spectral resolution, although they do not provide direct measurements of the physical conditions through diagnostics such as emission line intensity ratios. AGN can produce the photoionizing continuum, while strong thermal emission is characteristic of intense star formation. Starbursts are in fact common in the nuclear regions of active galaxies, but these "starburst/AGN composite" galaxies are too faint to observe with the current generation of high spectral resolution instruments. Future observations with Constellation-X, however, will allow for unbiased, detailed study of the soft X-ray emission of all AGN.
Constellation-X will also detect the elusive highly obscured and distant quasars, which may be significant in contributing to the cosmic X-ray background. When obscuration is Compton thick, having column density NH > 1.5\times 1024 {\rm \, cm-2}, the emergent luminosity at energy E < 10 keV is only about 1% of the intrinsic AGN output. Constellation-X's sensitive soft instruments will identify the signature large equivalent width Fe K\alpha fluorescence line, and the hard X-ray telescope will directly measure the higher-energy continuum of these deeply buried AGN.
This work is supported by NSF award AST-0237291, NASA grant G04-5117, and NASA's SFRO.
The author(s) of this abstract have provided an email address for comments about the abstract: levenson@pa.uky.edu
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Bulletin of the American Astronomical Society, 37 #4
© 2005. The American Astronomical Soceity.