Suzy Collin

Growth of massive black holes by super-Eddington accretion

Narrow-Line Seyfert 1 galaxies (NLS1s) and Narrow-Line quasars (NLQs) seem to amount to ∼ 10–30% of active galactic nuclei (AGNs) in the local universe. Together with their average accretion rate, we argue that a black hole (BH) growth by factor of 8–800 happens in these super-Eddington accretion phase of AGNs. Moreover, there is a possible, systematic underestimation of accretion rates (in the Eddington unit) due to an overestimation of BH mass by massive accretion discs for super-Eddington objects. If it is true, the factor of BH growth above may be larger by order(s) of magnitude. In contrast, the growth factor expected in sub-Eddington phase is only ∼ 2. Therefore, the cosmic BH growth by accretion is likely dominated by super-Eddington phase, rather than sub-Eddington phase which is the majority among AGNs.
This analysis is based on the fraction and the average accretion rate of NLS1s and NLQs obtained for

Star formation and evolution in accretion disks around massive black holes

z \lesssim 0.5

Quasars and Galactic Nuclei, a Half‐Century Agitated Story

. If those numbers are larger at higher redshift (where BHs were probably less grown), super-Eddington accretion would be even more important in the context of cosmic BH growth history.

Star Formation in Accretion Disks Around Massive Black Holes and Pregalactic Enrichment

Broad Absorption Lines (BALs) prove the existence of a high velocity outflowing gas with metallicities larger than solar in the central few parsecs of high redshift quasars. At the same distance from the black hole, accretion disks in quasars and Active Galactic Nuclei (AGN) are locally gravitationally unstable, and clumps must form with a size of the order of the scale height of the disk. This is hardly a coincidence, and we have tried to link these two facts. We have assumed that the unstable clumps give rise to protostars, which become massive stars after a rapid stage of accretion, and explode as supernovae, producing strong outflows perpendicular to the disk and inducing outward transfer of angular momentum in the plane of the disk. As a consequence a self-regulated disk made of gas and stars where supernovae sustain the inflow mass rate required by the AGN is a viable solution in this region of the disk. This model could explain the BALs, and could also account for a pregalactic enrichment of the intergalactic medium and of the Galaxy, if massive black holes formed early in the Universe.We develop a model for the outer gravitationally unstable regions of accretion disks around massive black holes, for primeval or solar abundances. First we study star formation and evolution in a purely gaseous marginally unstable disk, and we show that unstable fragments should collapse rapidly and give rise to protostars which accrete at a high rate, acquire a mass of a few tens of Mo, and explode as supernovae, producing strong enriched outflows. Second we assume that the whole mass transport is ensured by the supernovae which induce a transfer of angular momentum. For a star formation rate proportional to the growth rate of gravitational instability, we find solutions for the gas and stellar densities where the gas is close to gravitational instability, between 0.1 and 10 pc M(BH) =10**6 Mo, and between 1 and 100 pc for M(BH)=10**8 Mo or larger, whatever the abundances, but for relatively low accretion rates. For larger accretion rates the number of stars becomes so large that they inhibit further star formation, and/or the rate of SN is so high that they distroy the homogeneity and the marginal stability of the disk. We postpone the study of this case. Besides the fact that the mechanism solves the problem of mass transport in a region of where global instabilities do not work, there are several consequences of this model: it can explain the high velocity metal enriched outflows implied by Broad Absorption Lines in quasars, it can account for a pregalactic enrichment, if black holes formed early, finally it can trigger starbursts in the central regions of galaxies. A check of the model would be to detect a supernova exploding within a few parsecs from the center of an AGN, an observation which can be performed in the near future.

Quasars in the Life of Astronomers

I recall how the discovery of quasars occurred more than forty years ago, and the strong debates marking out their story. It led to the discovery of Massive Black Holes, which are now known to be present in almost all galaxies, and it opened on a coherent physical model and on a new vision of galaxy evolution.

The Future of Quasar Studies

Broad Absorption Lines (BALs) prove the existence of a high velocity outflowing gas with metallicities larger than solar in the central few parsecs of high redshift quasars. At the same distance from the black hole, accretion disks in quasars and Active Galactic Nuclei (AGN) are locally gravitationally unstable, and clumps must form with a size of the order of the scale height of the disk. This is hardly a coincidence, and we have tried to link these two facts. We have assumed that the unstable clumps give rise to protostars, which become massive stars after a rapid stage of accretion, and explode as supernovae, producing strong outflows perpendicular to the disk and inducing outward transfer of angular momentum in the plane of the disk. As a consequence a self-regulated disk made of gas and stars where supernovae sustain the inflow mass rate required by the AGN is a viable solution in this region of the disk. This model could explain the BALs, and could also account for a pregalactic enrichment of the intergalactic medium and of the Galaxy, if massive black holes formed early in the Universe.

The accretion disk in NGC 4258

We are approaching the 50th anniversary of the discovery of quasars. Those old enough to have been cognizant of astronomy in 1962–1963 can remember the sense of excitement connected with this finding. There was talk of a major new constituent of the universe. The excitement of the discovery was palpable even to one of us (the most senior of the editors) who was then a high school teenager.

X‐ray diagnostics with full radiative transfer (ALI method)

How will quasar studies be pursued in the near future? Starting from a summary of past achievement, we ask questions on expected instrumental and computational advancements that can shape quasar research in a foreseeable way. There is excitement due to the planned large new-generation telescopes, ground-based and in space. New surveys will lead to an order-of-magnitude increase of known quasars.

Non-LTE Radiation Processes: Application to the Solar Corona

In the framework of α-disk models, we show that the accretion disk in NGC 4258 is self-gravitating at the distance where the masing regions are observed, whatever the accretion rate. This conclusion holds in the presence of the external irradiation. As a result, the maser region cannot involve the whole disk thickness as proposed by Neufeld & Maloney (1995).

Systematic Effects in Measurement of Black Hole Masses by Emission-Line Reverberation of Active Galactic Nuclei: Eddington Ratio and Inclination

With the advent of the present and future spatial X‐ray missions, it becomes crucial to model correctly the line spectrum of X‐ray emitting/absorbing media. We have built a photoionization code, TITAN, solving the transfer of a thousand lines and of the continuum with the “Accelerated Lambda Iteration” method (ALI), which is most reliable for line transfer. We give some details about this method and a justification for its use as a complement to usual approximations (e.g., escape probability or two‐stream) made at present in other codes. We show that the escape probability approximation leads to a wrong estimation of the emitted X‐ray line intensities, especially in the soft X‐ray range. The errors can exceed one order of magnitude in the case of thick media (Thomson thickness of the order of unity). It also happens, but for different reasons, in the case of moderately thin media (Thomson thickness of 0.001 to 0.1), characteristic of the Warm Absorber in Seyfert 1 or of the X‐ray emitting medium in Seyfer...