Spitzer Space Telescope Infrared Spectra of Xray Bright, Optically Normal Galaxies

Astronomy and Astrophysics – Astronomy

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Scientific paper

In 1982 Elvis et al called attention to a pair of galaxies with AGN-like x-ray emission but no AGN features in their visible spectra [referred to here as XBONGS]. Since then, deep x-ray surveys have revealed large numbers of galaxies with these characteristics: luminous in the x-ray but without the high excitation lines, specific line ratios, or broad emission lines which characterize Type I and/or Type II AGN. Thus understanding XBONGS is an important step towards defining the accretion history of the Universe. For brevity, we continue to refer to these objects as XBONGS, while recognizing that they are not a homogeneous group, and in fact a variety of explanations have been proposed. It has been suggested that XBONGs are powered by AGN which are hidden at visible wavelengths by (i) dilution by the bright emission from the galaxy, by (ii) extinction, or by (iii) the absence of visible and ultraviolet radiation due to modifications in the character of the accretion disk which surrounds the central black hole. We tested these possibilities by taking Spitzer spectra of 4 XBONGs carefully selected from 26 XBONGs identified in the XBootes survey of the NOAO Deep Wide-Field Survey. We present the spectra and analyze the features to see which of the above three possibilities is more likely. What is most striking is that these spectra show none of the characteristic AGN features in the IR like high excitation [Ne] lines or a rising flux density towards longer wavelengths.

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