Astron. Astrophys. 355, L47-L51 (2000)

4. Conclusions

We presented new near-IR (J and Ks band) observations of a sample of 10 objects selected in the hard X-rays (5-10 keV). These sources were discovered in a large survey (HELLAS) performed by the BeppoSAX satellite, which resolves % of the hard X-ray background. The sample includes 4 blue broad line QSOs and 6 AGNs with redder continua whose optical emission line spectra range from broad line objects (red QSOs), to intermediate type 1.9 AGNs, to LINER.

The B-K color ranges from the standard value of (typical of U-B color selected QSOs) up to 6, similar to the color of red QSOs found in radio surveys.

The red AGNs show extended near-IR images. Model fitting of the photometry and spectral data shows that all and only the red AGNs are dominated by the emission of the host galaxy (with an age of 10⁹-10¹⁰ yr). Red AGNs amount to about a third of the total HELLAS sources, and their fraction is expected to increase significantly at fainter X-ray fluxes, where most of the hard X-ray background is produced. Therefore, our result suggests that a significant fraction of the counterparts of the sources making the hard X-ray background appear as "normal" galaxies at optical and near-IR wavelengths. Chandra and XMM are expected to discover a large number of this class of objects.

© European Southern Observatory (ESO) 2000

Online publication: March 21, 2000
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