Fe K emission from active galaxies in the COSMOS field

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...

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Bibliographic Details
Published in:	Astronomy and astrophysics (Berlin) 2012-01, Vol.537, p.A86
Main Authors:	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.
Format:	Article
Language:	eng
Subjects:	Astronomy Earth, ocean, space Exact sciences and technology galaxies: active surveys X-rays: galaxies
Online Access:	Get full text
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Summary:	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.
ISSN:	0004-6361 1432-0746