We report the detection of the 6.4 keV Iron K \alpha emission line in two infrared-luminous , massive , star-forming BzK galaxies at z = 2.578 and z = 2.90 in the CDF-S .
The Chandra 4 Ms spectra of BzK 4892 and BzK 8608 show a reflection dominated continuum with strong Iron lines , with rest-frame equivalent widths EW \sim 2.3 keV and 1.2 keV , respectively , demonstrating Compton thick obscuration of the central AGN .
For BzK 8608 the line identification closely matches the existing photometric redshift derived from the stellar emission .
We use the observed luminosities of the Iron K \alpha line , of the rest-frame mid-IR continuum and of the UV rest-frame narrow emission lines to infer intrinsic L _ { 2 - 10 ~ { } keV } \gtrsim 10 ^ { 44 } erg s ^ { -1 } , about 1.0–2.5 dex larger than the observed ones , hence confirming the presence of an absorber with N _ { H } > 10 ^ { 24 } cm ^ { -2 } .
The two BzK galaxies have stellar masses of 5 \times 10 ^ { 10 } M _ { \odot } and , based on VLA 1.4 GHz and submm 870 \mu m observations , they appear to host vigorous starburst activity with SFR \sim 300 -700 M _ { \odot } yr ^ { -1 } that is also optically thick .
We estimate that the AGN might also conceivably account for an important fraction of the bolometric far-IR emission of the galaxies .
The implied volume density of Compton thick ( CT ) AGN with L _ { 2 - 10 ~ { } keV } > 10 ^ { 44 } erg s ^ { -1 } is in agreement with predictions from X-ray background synthesis models .
These sources provide one of the first clearcut observations of the long-sought phase of simultaneous , heavily obscured quasar and star formation activity , predicted by models of massive galaxy evolution at high redshifts .