Paper detail

Discovery of strong Iron Kα emitting Compton thick quasars at z=2.5 and 2.9

We report the detection of the 6.4 keV Iron Kα 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 BzK4892 and BzK8608 show a reflection dominated continuum with strong Iron lines, with rest-frame equivalent widths EW-2.3 keV and 1.2 keV, respectively, demonstrating Compton thick obscuration of the central AGN. For BzK8608 the line identification closely matches the existing photometric redshift derived from the stellar emission. We use the observed luminosities of the Iron Kα line, of the rest-frame mid-IR continuum and of the UV rest-frame narrow emission lines to infer intrinsic L(2-10 keV)>1e44 erg/s, about 1.0-2.5 dex larger than the observed ones, hence confirming the presence of an absorber with N(H) > 1e24 cm-2. The two BzK galaxies have stellar masses of 5e10 M\odot and, based on VLA 1.4 GHz and submm 870μm observations, they appear to host vigorous starburst activity with SFR - 300-700 M\odot/yr 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)>1e44 erg/s 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.