We present a rest-frame spectral stacking analysis of ~1000 X-ray sources detected in the XMM-COSMOS field to investigate the iron-K line properties of active galaxies beyond redshift z ~ 1. In Type I AGN that have a typical X-ray luminosity of LX ~ 1.5 × 1044 (erg s-1) and z ~ 1.6 the cold Fe K at 6.4 keV is weak (EW ~ 0.05 keV), which agrees with the known trend. In contrast, high-ionization lines of Fe xxv and Fe xxvi are pronounced. These high-ionization Fe K lines appear to have a connection with high accretion rates. While no broad Fe emission is detected in the total spectrum, it might be present, albeit at low significance (~2σ), when the X-ray luminosity is restricted to the range below 3 × 1044 erg s-1, or when an intermediate range of Eddington ratio around λ ~ 0.1 is selected. In Type II AGN, both cold and high-ionzation lines become weak with increasing X-ray luminosity. However, we detected strong high-ionization Fe K (EW ~ 0.3 keV) in the spectrum of objects at z > 2, while we found no 6.4 keV line. We also found that the primary source of the high-ionization Fe K emission are those objects detected with Spitzer-MIPS at 24 μm. Given their median redshift of z ≃ 2.5, their bolometric luminosity is likely to reach 1013 L☉ and the MIPS-detected emission most likely originates from hot dust heated by embedded AGN, probably accreting at high Eddington ratio. These properties match those of rapidly growing black holes in ultra-luminous infrared galaxies at the interesting epoch (z ~ 2-3) of galaxy formation.
Fe K emission from active galaxies in the COSMOS field / Iwasawa K.; Mainieri V.; Brusa M.; Comastri A.; Gilli R.; Vignali C.; Hasinger G.; Sanders D.B.; Cappelluti N.; Impey C.D.; Koekemoer A.; Lanzuisi G.; Lusso E.; Merloni A.; Salvato M.; Taniguchi Y.; Trump J.R.. - In: ASTRONOMY & ASTROPHYSICS. - ISSN 0004-6361. - ELETTRONICO. - 537:(2012), pp. A86-0. [10.1051/0004-6361/201118203]
Fe K emission from active galaxies in the COSMOS field
Brusa M.;Gilli R.;Vignali C.;Lusso E.;
2012
Abstract
We present a rest-frame spectral stacking analysis of ~1000 X-ray sources detected in the XMM-COSMOS field to investigate the iron-K line properties of active galaxies beyond redshift z ~ 1. In Type I AGN that have a typical X-ray luminosity of LX ~ 1.5 × 1044 (erg s-1) and z ~ 1.6 the cold Fe K at 6.4 keV is weak (EW ~ 0.05 keV), which agrees with the known trend. In contrast, high-ionization lines of Fe xxv and Fe xxvi are pronounced. These high-ionization Fe K lines appear to have a connection with high accretion rates. While no broad Fe emission is detected in the total spectrum, it might be present, albeit at low significance (~2σ), when the X-ray luminosity is restricted to the range below 3 × 1044 erg s-1, or when an intermediate range of Eddington ratio around λ ~ 0.1 is selected. In Type II AGN, both cold and high-ionzation lines become weak with increasing X-ray luminosity. However, we detected strong high-ionization Fe K (EW ~ 0.3 keV) in the spectrum of objects at z > 2, while we found no 6.4 keV line. We also found that the primary source of the high-ionization Fe K emission are those objects detected with Spitzer-MIPS at 24 μm. Given their median redshift of z ≃ 2.5, their bolometric luminosity is likely to reach 1013 L☉ and the MIPS-detected emission most likely originates from hot dust heated by embedded AGN, probably accreting at high Eddington ratio. These properties match those of rapidly growing black holes in ultra-luminous infrared galaxies at the interesting epoch (z ~ 2-3) of galaxy formation.File | Dimensione | Formato | |
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