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Register a new userEddington ratios of faint AGN at intermediate redshift: Evidence for a population of half-starved black holes
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We use one of the deepest spectroscopic samples of broad line active galactic nuclei (AGN) currently available, extracted from the VIMOS VLT Deep Survey (VVDS), to compute MgII and CIV virial masses estimate of 120 super-massive black holes in the redshift range 1.0<z<1.9 and 2.6<z<4.3. We find that the mass-luminosity relation shows considerably enhanced dispersion towards low AGN luminosities (log L_bol ~ 45). At these luminosities, there is a substantial fraction of black holes accreting far below their Eddington limit (L_bol/L_Edd < 0.1), in marked contrast to what is generally found for AGN of higher luminosities. We speculate that these may be AGN on the decaying branch of their light-curves, well past their peak activity. This would agree with recent theoretical predictions of AGN evolution. In the electronic Appendix of this paper we publish an update of the VVDS type-1 AGN sample, including the first and most of the second epoch observations. This sample contains 298 objects of which 168 are new.
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Black hole masses in Active Galactic Nuclei have been determined in 35 objects through reverberation mapping of the emission line region. I mention some uncertainties of the method, such as the ``scale factor relating the Virial Product to the mass, which depends on the unknown structure and dynamics of the Broad Line Region. When the black hole masses are estimated indirectly using the empirical size-luminosity relation deduced from this method, the uncertainties can be larger, especially when the relation is extrapolated to high and low masses and/or luminosities. In particular they lead to Eddington ratios of the order of unity in samples of Narrow Line Seyfert 1. As the optical-UV luminosity is provided by the accretion disk, the accretion rates can be determined and are found to be much larger than the Eddington rates. So, accretion must be performed at a super-critical rate through a slim disk, resulting in rapid growth of the black holes. The alternative is that the mass determination is wrong at this limit.
Intermediate-mass black holes (IMBHs), with masses in the range $100-10^{6}$ M$_{odot}$, are the link between stellar-mass BHs and supermassive BHs (SMBHs). They are thought to be the seeds from which SMBHs grow, which would explain the existence of quasars with BH masses of up to 10$^{10}$ M$_{odot}$ when the Universe was only 0.8 Gyr old. The detection and study of IMBHs has thus strong implications for understanding how SMBHs form and grow, which is ultimately linked to galaxy formation and growth, as well as for studies of the universality of BH accretion or the epoch of reionisation. Proving the existence of seed BHs in the early Universe is not yet feasible with the current instrumentation; however, those seeds that did not grow into SMBHs can be found as IMBHs in the nearby Universe. In this review I summarize the different scenarios proposed for the formation of IMBHs and gather all the observational evidence for the few hundreds of nearby IMBH candidates found in dwarf galaxies, globular clusters, and ultraluminous X-ray sources, as well as the possible discovery of a few seed BHs at high redshift. I discuss some of their properties, such as X-ray weakness and location in the BH mass scaling relations, and the possibility to discover IMBHs through high velocity clouds, tidal disruption events, gravitational waves, or accretion disks in active galactic nuclei. I finalize with the prospects for the detection of IMBHs with up-coming observatories.
We study a sample of $sim$50,000 dwarf starburst and late-type galaxies drawn from the COSMOS survey with the aim of investigating the presence of nuclear accreting black holes (BHs) as those seed BHs from which supermassive BHs could grow in the early Universe. We divide the sample into five complete redshift bins up to $z=1.5$ and perform an X-ray stacking analysis using the textit{Chandra} COSMOS-Legacy survey data. After removing the contribution from X-ray binaries and hot gas to the stacked X-ray emission, we still find an X-ray excess in the five redshift bins that can be explained by nuclear accreting BHs. This X-ray excess is more significant for $z<0.5$. At higher redshifts, these active galactic nuclei could suffer mild obscuration, as indicated by the analysis of their hardness ratios. The average nuclear X-ray luminosities in the soft band are in the range 10$^{39}-10^{40}$ erg s$^{-1}$. Assuming that the sources accrete at $geq$ 1% the Eddington rate, their BH masses would be $leq$ 10$^{5}$ M$_{odot}$, thus in the intermediate-mass BH regime, but their mass would be smaller than the one predicted by the BH-stellar mass relation. If instead the sources follow the correlation between BH mass and stellar mass, they would have sub-Eddington accreting rates of $sim$ 10$^{-3}$ and BH masses 1-9 $times$ 10$^{5}$ M$_{odot}$. We thus conclude that a population of intermediate-mass BHs exists in dwarf starburst galaxies, at least up to $z$=1.5, though their detection beyond the local Universe is challenging due to their low luminosity and mild obscuration unless deep surveys are employed.
We study the distribution of Eddington luminosity ratios, L_bol/L_edd, of active galactic nuclei (AGNs) discovered in the AGN and Galaxy Evolution Survey (AGES). We combine H-beta, MgII, and CIV line widths with continuum luminosities to estimate black hole (BH) masses in 407 AGNs, covering the redshift range z~0.3-4 and the bolometric luminosity range L_bol~10^45-10^47 erg/s. The sample consists of X-ray or mid-infrared (24 micron) point sources with optical magnitude R<=21.5 mag and optical emission line spectra characteristic of AGNs. For the range of luminosity and redshift probed by AGES, the distribution of estimated Eddington ratios is well described as log-normal with a peak at L_bol/L_edd ~ 1/4 and a dispersion of 0.3 dex. Since additional sources of scatter are minimal, this dispersion must account for contributions from the scatter between estimated and true BH mass and the scatter between estimated and true bolometric luminosity. Therefore, we conclude that: (1) neither of these sources of error can contribute more than ~0.3 dex rms; and (2) the true Eddington ratios of optically luminous AGNs are even more sharply peaked. Because the mass estimation errors must be smaller than ~0.3 dex, we can also investigate the distribution of Eddington ratios at fixed BH mass. We show for the first time that the distribution of Eddington ratios at fixed BH mass is peaked, and that the dearth of AGNs at a factor ~10 below Eddington is real and not an artifact of sample selection. These results provide strong evidence that supermassive BHs gain most of their mass while radiating close to the Eddington limit, and they suggest that the fueling rates in luminous AGNs are ultimately determined by BH self-regulation of the accretion flow rather than galactic scale dynamical disturbances.
We discuss a sample of 29 AGN (16 narrow-lined and 13 broad-lined) discovered in a spectroscopic survey of ~1000 star-forming Lyman-break galaxies (LBGs) at z~3. Reaching apparent magnitudes of R_{AB}=25.5, the sample includes broad-lined AGN approximately 100 times less UV-luminous than most surveys to date covering similar redshifts, and the first statistical sample of UV/optically-selected narrow-lined AGN at high redshift. The fraction of objects in our survey with clear evidence for AGN activity is ~3%. A substantial fraction, perhaps even most, of these objects would not have been detected in even the deepest existing X-ray surveys. We argue that these AGN are plausibly hosted by the equivalent of LBGs. The UV luminosities of the broad-lined AGN in the sample are compatible with Eddington-limited accretion onto black holes that satisfy the locally determined M_{BH} versus M_{bulge} relation given estimates of the stellar masses of LBGs. The clustering properties of the AGN are compatible with their being hosted by objects similar to LBGs. The implied lifetime of the active AGN phase in LBGs, if it occurs some time during the active star-formation phase, is ~10^7 years.
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