DISCOVERY OF STRONG IRON Kα EMITTING COMPTON THICK QUASARS AT <i>z</i> = 2.5 AND 2.9

Abstract

We report the detection of the 6.4 keV Iron K{\\alpha} emission line in two\ninfrared-luminous, massive, star-forming BzK galaxies at z = 2.578 and z = 2.90\nin the CDF-S. The Chandra 4 Ms spectra of BzK4892 and BzK8608 show a reflection\ndominated continuum with strong Iron lines, with rest-frame equivalent widths\nEW-2.3 keV and 1.2 keV, respectively, demonstrating Compton thick obscuration\nof the central AGN. For BzK8608 the line identification closely matches the\nexisting photometric redshift derived from the stellar emission. We use the\nobserved luminosities of the Iron K{\\alpha} line, of the rest-frame mid-IR\ncontinuum and of the UV rest-frame narrow emission lines to infer intrinsic\nL(2-10 keV)&gt;1e44 erg/s, about 1.0-2.5 dex larger than the observed ones, hence\nconfirming the presence of an absorber with N(H) &gt; 1e24 cm-2. The two BzK\ngalaxies have stellar masses of 5e10 M\\odot and, based on VLA 1.4 GHz and submm\n870{\\mu}m observations, they appear to host vigorous starburst activity with\nSFR - 300-700 M\\odot/yr that is also optically thick. We estimate that the AGN\nmight also conceivably account for an important fraction of the bolometric\nfar-IR emission of the galaxies. The implied volume density of Compton thick\n(CT) AGN with L(2-10 keV)&gt;1e44 erg/s is in agreement with predictions from\nX-ray background synthesis models. These sources provide one of the first\nclearcut observations of the long-sought phase of simultaneous, heavily\nobscured quasar and star formation activity, predicted by models of massive\ngalaxy evolution at high redshifts.\n