Bolometric luminosities and Eddington ratios of X-ray selected Active Galactic Nuclei in the XMM-COSMOS survey

Abstract: Bolometric luminosities and Eddington ratios of both X-ray selected broad-line (Type-1) and narrow-line (Type-2) AGN from the XMM-Newton survey in the COSMOS field are presented. The sample is composed by 929 AGN (382 Type-1 AGN and 547 Type-2 AGN) and it covers a wide range of redshifts, X-ray luminosities and absorbing column densities. About 65% of the sources are spectroscopically identified as either Type-1 or Type-2 AGN (83% and 52% respectively), while accurate photometric redshifts are available for the rest of the sample. The study of such a large sample of X-ray selected AGN with a high quality multi-wavelength coverage from the far-infrared (now with the inclusion of Herschel data at 100 micron and 160 micron) to the optical-UV allows us to obtain accurate estimates of bolometric luminosities, bolometric corrections and Eddington ratios. The kbol-Lbol relations derived in the present work are calibrated for the first time against a sizable AGN sample, and rely on observed redshifts, X-ray luminosities and column density distributions. We find that kbol is significantly lower at high Lbol with respect to previous estimates by Marconi et al. (2004) and Hopkins et al. (2007). Black hole masses and Eddington ratios are available for 170 Type-1 AGN, while black hole masses for Type-2 AGN are computed for 481 objects using the black hole mass-stellar mass relation and the morphological information. We confirm a trend between kbol and lambda_Edd, with lower hard X-ray bolometric corrections at lower Eddington ratios for both Type-1 and Type-2 AGN. We find that, on average, Eddington ratio increases with redshift for all Types of AGN at any given Mbh, while no clear evolution with redshift is seen at any given Lbol.

• The paper presents a detailed measurement of bolometric luminosities and Eddington ratios for 929 X-ray selected AGNs, revealing revised bolometric correction trends.
• It employs a robust multi-wavelength methodology, including Herschel far-infrared data, to accurately calculate luminosities and identify predominantly sub-Eddington accretion states.
• Findings indicate evolving Eddington ratios with luminosity and redshift, challenging previous models and guiding future research on AGN evolution.

Overview of Bolometric Luminosities and Eddington Ratios in XMM-COSMOS AGN

This paper presents a detailed investigation of bolometric luminosities and Eddington ratios for a sample of 929 X-ray-selected Active Galactic Nuclei (AGN) from the XMM-COSMOS survey. The analysis covers both broad-line (Type-1) and narrow-line (Type-2) AGNs, emphasizing their bolometric output and the implications for accretion processes. The study leverages a comprehensive multi-wavelength dataset, including crucial far-infrared observations from Herschel, enabling precise calculations of bolometric luminosities and corrections.

Key Findings

1. Bolometric Correction Trends:
   • Both Type-1 and Type-2 AGN samples reveal an increasing trend of bolometric correction with rising bolometric luminosity in the X-ray bands. This research highlights a significant deviation from the previously established bolometric correction curves by Marconi et al. (2004) and Hopkins et al. (2007), presenting generally lower values across luminosities.
   • In particular, the Type-2 AGN sample provides a continuum at lower luminosities where the Type-1 sample leaves off, suggesting a consistent trend across AGN types.
2. Eddington Ratio Variation:
   • A salient result is the verification of higher Eddington ratios correlating with increased bolometric corrections, suggesting a lesser contribution of X-ray corona emissions at higher accretion rates.
   • The analysis discloses that the AGN sample is predominantly comprised of sub-Eddington accretion rate objects. This finding contrasts with previous assertions of near-Eddington dominance in certain AGN samples, possibly attributable to different luminosity scales.
3. Evolutionary Dynamics:
   • Eddington ratios evolve with bolometric luminosity, with trends indicating rising ratios at higher luminosities and redshifts. This evolution could be indicative of modifications in AGN fueling mechanisms or feedback processes across cosmic time.
   • Particularly for Type-2 AGN, findings allude to a recurring increase in Eddington ratio with redshift for a given black hole mass, propounding intrinsic growth patterns in massive systems.

Implications for Future Research

The implications of these findings extend to several domains in astrophysics. Primarily, the recalibrated bolometric corrections could influence the interpretations of black hole mass density estimates and AGN luminosity functions. Furthermore, the results challenge existing models of AGN energy distribution, urging for refinements that accommodate the demonstrated luminosity-dependent bolometric corrections.

The observed Eddington ratio distribution offers insights into the accretion history and AGN evolution, potentially informing simulations and theoretical models predicting black hole growth and galaxy co-evolution. Future studies might focus on investigating broader samples or incorporating different selection criteria to explore these trends further.

Conclusion

This work provides a substantial refinement over previous understandings of bolometric outputs and Eddington scaling laws for AGNs. By utilizing robust, multi-wavelength data and considering both Type-1 and Type-2 populations, it ushers in new benchmarks and challenges for astrophysical models, underscoring the intricate interplay between AGN accretion dynamics and cosmic evolution.