Discovery of Strong Iron Kα Emitting Compton Thick Quasars at z = 2.5 and 2.9

Abstract

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 BzK 4892 and BzK 8608 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 (CT) obscuration of the central active galactic nucleus (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 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} >~ 10⁴⁴ erg s^-1, about 1.0-2.5 dex larger than the observed ones, hence confirming the presence of an absorber with N _H>10²⁴ cm^-2. The two BzK galaxies have stellar masses of 5 × 10¹⁰ M _sun and, based on Very Large Array 1.4 GHz and submillimeter 870 μm observations, they appear to host vigorous starburst activity with star formation rate ~ 300-700 M _sun 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 CT AGN with L _{2-10 keV} >10⁴⁴ erg s^-1 is in agreement with predictions from X-ray background synthesis models. These sources provide one of the first clear-cut 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.