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Discovery of strong Iron K{\alpha} emitting Compton thick quasars at z=2.5 and 2.9

Abstract : 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-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}$$\geq$10$^{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 x 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.
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Contributor : Bruno Savelli Connect in order to contact the contributor
Submitted on : Wednesday, January 6, 2021 - 11:19:01 AM
Last modification on : Friday, August 5, 2022 - 12:01:29 PM
Long-term archiving on: : Wednesday, April 7, 2021 - 7:18:14 PM


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C. Feruglio, Emanuele Daddi, F. Fiore, D. M. Alexander, E. Piconcelli, et al.. Discovery of strong Iron K{\alpha} emitting Compton thick quasars at z=2.5 and 2.9. The Astrophysical journal letters, Bristol : IOP Publishing, 2011, 729 (1), pp.L4. ⟨10.1088/2041-8205/729/1/L4⟩. ⟨cea-00995646⟩



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