Citation:
Date Published:
September 1, 200Abstract:
We study the distribution of Eddington luminosity ratios,Lbol/LEdd, of active galactic nuclei (AGNs)discovered in the AGN and Galaxy Evolution Survey (AGES). We combineHβ, Mg II, and C IV line widths with continuum luminosities toestimate black hole (BH) masses in 407 AGNs, covering the redshift rangez~0.3-4 and the bolometric luminosity rangeLbol~1045-1047 ergs s-1. Thesample consists of X-ray or mid-infrared (24 μm) point sources withoptical magnitude R<=21.5 mag and optical emission-line spectracharacteristic of AGNs. For the range of luminosity and redshift probedby AGES, the distribution of estimated Eddington ratios is welldescribed as log-normal, with a peak atLbol/LEdd~=1/4 and a dispersion of 0.3 dex. Sinceadditional sources of scatter are minimal, this dispersion must accountfor contributions from the scatter between estimated and true BH massand the scatter between estimated and true bolometric luminosity.Therefore, we conclude that (1) neither of these sources of error cancontribute more than ~0.3 dex rms, and (2) the true Eddington ratios ofoptically luminous AGNs are even more sharply peaked. Because the massestimation errors must be smaller than ~0.3 dex, we can also investigatethe distribution of Eddington ratios at fixed BH mass. We show for thefirst time that the distribution of Eddington ratios at fixed BH mass ispeaked, and that the dearth of AGNs at a factor of ~10 below Eddingtonis real and not an artifact of sample selection. These results providestrong evidence that supermassive BHs gain most of their mass whileradiating close to the Eddington limit, and they suggest that thefueling rates in luminous AGNs are ultimately determined by BHself-regulation of the accretion flow rather than galactic-scaledynamical disturbances.Observations reported here were obtained at the MMT Observatory (MMTO),a joint facility of the University of Arizona and the SmithsonianInstitution.