Very Large Telescope near-infrared spectra of hard serendipitous Chandra sources

We present near-infrared long-slit spectra of eight optically dim X-ray sources obtained with ISAAC on the Very Large Telescope. Six of the sources have hard X-ray emission with a significant fraction of the counts emerging above 2 keV All were discovered serendipitously in the fields of three nearby galaxy clusters observed with Chandra, and identified through near-infrared imaging. The X-ray fluxes lie close to the break in the source counts. Two of the sources show narrow emission lines and a third has a broad line. One of the narrow line-emitting sources has a clear redshift identification at z = 2.18, while the other has a tentative determination based on the highest-redshift detection of He I lambda10830 at z =1.26. The remainder have featureless spectra to deep limiting equivalent widths of similar to20-60Angstrom and line flux similar to5 x 10(-17) erg s(-1) cm(-2) in the K band. High-quality J-, H- and K-band images of the sources were combined with archival optical detections or limits to estimate a photometric redshift for six of them. Two sources show a complex double morphology. The hard sources have spectral count ratios consistent with heavily obscured active galactic nuclei, while the host galaxy emits much of the optical and near-infrared flux. The most likely explanation for the featureless continua is that the line photons are being scattered or destroyed by optically thick gas and associated dust with large covering fractions.