The ROSAT Deep Surveys in the direction of the Lockman Hole are the most sensitive X-ray surveys performed with the ROSAT satellite .
About 70-80 % of the X-ray background has been resolved into discrete sources at a flux limit of \sim 10 ^ { -15 } erg cm ^ { -2 } s ^ { -1 } in the 0.5-2.0 keV energy band .
A nearly complete optical identification of the ROSAT Deep Survey ( RDS ) has shown that the great majority of sources are AGNs .
We describe in this paper the ROSAT Ultra Deep Survey ( UDS ) , an extension of the RDS in the Lockman Hole .
The Ultra Deep Survey reaches a flux level of 1.2 \cdot 10 ^ { -15 } erg cm ^ { -2 } s ^ { -1 } in 0.5-2.0 keV energy band , a level \sim 4.6 times fainter than the RDS .
We present nearly complete spectroscopic identifications ( 90 % ) of the sample of 94 X-ray sources based on low-resolution Keck spectra . The majority of the sources ( 57 ) are broad emission line AGNs ( type I ) , whereas a further 13 AGNs show only narrow emission lines or broad Balmer emission lines with a large Balmer decrement ( type II AGNs ) indicating significant optical absorption .
The second most abundant class of objects ( 10 ) are groups and clusters of galaxies ( \sim 11 % ) .
Further we found five galactic stars and one ” normal ” emission line galaxy .
Eight X-ray sources remain spectroscopically unidentified .
We see no evidence for any change in population from the RDS survey to the UDS survey . The photometric redshift determination indicates in three out of the eight sources the presence of an obscured AGN .
Their photometric redshifts , assuming that the spectral energy distribution ( SED ) in the optical/near-infrared is due to stellar processes , are in the range of 1.2 \leq z \leq 2.7 .
These objects could belong to the long-sought population of type 2 QSOs , which are predicted by the AGN synthesis models of the X-ray background .
Finally , we discuss the optical and soft X-ray properties of the type I AGN , type II AGN , and groups and clusters of galaxies , and the implications to the X-ray backround . Tables 3 and 4 are also available in electronic form at the CDS via anonymous ftp to cdsarc.u-strasbg.fr ( 130.79.128.5 ) or via http : //cdsweb.u-strasbg.fr/cgi-bin/qcat ? J/A+A/