The sources discovered in deep hard X-ray surveys with 2-8 keV fluxes of S _ { 2 - 8 } \sim 10 ^ { -14 } erg cm ^ { -2 } s ^ { -1 } make up the bulk of the X-ray background at these energies .
We present here detailed multi-wavelength observations of three such sources from the ELAIS Deep X-ray Survey .
The observations include sensitive near-infrared spectroscopy with the Subaru Telescope and X-ray spectral information from the Chandra X-ray Observatory .
The sources observed all have optical-to-near-IR colours redder than an unobscured quasar and comprise a reddened quasar , a radio galaxy and an optically-obscured AGN .
The reddened quasar is at a redshift z = 2.61 and shows a very large X-ray absorbing column of N _ { H } \approx 3 \times 10 ^ { 23 } { cm } ^ { -2 } .
This contrasts with the relatively small amount of dust reddening , implying a gas-to-dust ratio along the line-of-sight a hundred times greater than that of the Milky Way .
The radio galaxy at z = 1.57 shows only narrow emission lines , but has a surprisingly soft X-ray spectrum .
The softness of this spectrum either indicates an unusually low gas-to-dust ratio for the absorbing medium or X-ray emission related to the young radio source .
The host galaxy is extremely red ( R - K = 6.4 ) and its optical/near-IR spectrum is best fit by a strongly reddened ( A _ { V } \approx 2 ) starburst .
The third X-ray source discussed is also extremely red ( R - K = 6.1 ) and lies in a close grouping of three other R - K > 6 galaxies .
No emission or absorption lines were detected from this object , but its redshift ( and that of one of the nearby galaxies ) are constrained by SED-fitting to be just greater than z = 1 .
The extremely red colours of these two galaxies can be accounted for by old stellar populations .
These observations illustrate the diverse properties of hard X-ray selected AGN at high redshift in terms of obscuration at optical and X-ray wavelengths and the evolutionary states of their host galaxies .