contribution of the host galaxy light. The latter can also be important for some redder X-ray type-1 AGN. There is a large population of EROs (R$-$K$>5$) as X-ray counterparts and their fraction strongly increases with decreasing optical flux, up to 25% for the R$ \geq 24$ sample. They cover the whole range of X-ray hardness ratios, comprise objects of various classes (in particular a high fraction of $z\gtrsim 1$ X-ray absorbed AGNs, but also elliptical and starburst galaxies) and more than half of them should be fairly bright X-ray sources ($L_{\rm X}$(0.5-10 keV)$>10^{42}$ erg s$^{-1}$). Photometric redshifts will be necessary to derive the properties and evolution of the X-ray selected EROs.'
author:
- 'G. P. Szokoly, J. Bergeron, G. Hasinger, I. Lehmann. L. Kewley, V. Mainieri, M. Nonino, P. Rosati, R. Giacconi, R. Gilli, R. Gilmozzi, C. Norman, M. Romaniello E. Schreier, P. Tozzi, J. X. Wang, W. Zheng and A. Zirm'
title: |
    The Chandra Deep Field South:\
    Optical Spectroscopy I. [^1] 
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INTRODUCTION
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Deep X-ray surveys indicate that the cosmic X-ray background (XRB) is largely due to accretion onto supermassive black holes, integrated over cosmic time. In the soft (0.5–2 keV) band more than 90% of the 