Dormant Black Holes Research Articles

The cosmological evolution of quasar black hole masses

Virial black hole mass estimates are presented for 12 698 quasars in the redshift interval 0.1 ≤z≤ 2.1, based on modelling of spectra from the Sloan Digital Sky Survey (SDSS) first data release. The black hole masses of the SDSS quasars are found to lie between ≃107 M⊙ and an upper limit of ≃3 × 109 M⊙, entirely consistent with the largest black hole masses found to date in the local Universe. The estimated Eddington ratios of the broad-line quasars (full width at half-maximum ≥ 2000 km s−1) show a clear upper boundary at Lbol/LEdd≃ 1, suggesting that the Eddington luminosity is still a relevant physical limit to the accretion rate of luminous broad-line quasars at z≤ 2. By combining the black hole mass distribution of the SDSS quasars with the two degree field (2dF) quasar luminosity function, the number density of active black holes at z≃ 2 is estimated as a function of mass. In addition, we independently estimate the local black hole mass function for early-type galaxies using the Mbh–σ and Mbh–Lbulge correlations. Based on the SDSS velocity dispersion function and the Two Micron All Sky Survey (2MASS) K-band luminosity function, both estimates are found to be consistent at the high-mass end (Mbh≥ 108 M⊙). By comparing the estimated number density of active black holes at z≃ 2 with the local mass density of dormant black holes, we set lower limits on the quasar lifetimes and find that the majority of black holes with mass ≥108.5 M⊙ are in place by ≃2.

A Physical Model for the Luminosity Function of High‐Redshift Quasars

We provide a simple theoretical model for the quasar luminosity function at high redshifts that naturally reproduces the statistical properties of the luminous Sloan Digital Sky Survey (SDSS) quasar sample at redshifts z ~ 4.3 and z 5.7. Our model is based on the assumptions that quasar emission is triggered by galaxy mergers and that the black hole mass is proportional to a power law in the circular velocity of the host galactic halo, vc. We assume that quasars shine at their Eddington luminosity over a time proportional to the mass ratio between the small and final galaxies in the merger. This simple model fits the quasar luminosity function at z ~ 2-3, reproduces the normalization and logarithmic slope (β ~ -2.58) at z ~ 4.3, explains the space density of bright SDSS quasars at z ~ 6.0, reproduces the black hole-halo mass relation for dormant black holes in the local universe, and matches the inferred duty cycle of quasar activity (~107 yr) in Lyman break galaxies at z ~ 3. An acceptable fit to all of these constraints requires 0.7 σ8 1.0. Based on the derived luminosity function, we predict the resulting gravitational lensing rates for high-redshift quasars. The lens fractions in the SDSS samples are predicted to be ~2% at z ~ 4.3 and ~8% at z 5.7. It is interesting to note that the limiting quasar luminosity in our best-fit relation L ∝ v/G scales as the binding energy of the host galaxy divided by its dynamical time, implying that feedback is the mechanism that regulates black hole growth in galactic potential wells.

Acceleration of Ultra High Energy Cosmic Rays

Some general features of cosmic ray acceleration are summarized along with some inferences that can be drawn concerning the origin of the UHE component. The UHE luminosity density is found to be similar to that derived for GeV cosmic rays and its slope suggests a distinct origin. Reports of clustering on small angular scale, if confirmed, would rule out most proposed source models. More generally, it is argued that the highest energy particles can only be accelerated in sites that can sustain an EMF ℰ ≳ 3 × 1020 V and an associated power Lmin ≳ ℰ2/Z ~ 1039 W, where Z is the characteristic, electrical impedance, typically ≲ 100 Ω. Shock acceleration, unipolar induction and magnetic flares are the three most potent, observed, acceleration mechanisms and radio jet termination shocks, γ-ray blast waves, dormant black holes in galactic nuclei and magnetars are the least implausible, “conventional” manifestations of these mechanisms that have been invoked to explain the UHE cosmic rays. Each of these models presents problems and deciding between these and “exotic” origins for UHE cosmic rays, including those involving new particles or defects will require improved statistical information on the energies, arrival times and directions, as should be provided by the AUGER project.

Mass function of dormant black holes and the evolution of active galactic nuclei

Under the assumption that accretion onto massive black holes powers active galactic nuclei (AGN), the mass function (MF) of the BHs responsible for their past activity is estimated. For this, we take into account not only the activity related to the optically selected AGN, but also that required to produce the Hard X–Ray Background (HRXB). The MF of the Massive Dark Objects (MDOs) in nearby quiescent galaxies is computed by means of the most recent results on their demography. The two mass functions match well under the assumption that the activity is concentrated in a single significant burst with � = L/LEdd being a weakly increasing function of luminosity. This behaviour may be indicative of some level of recurrence and/or of accretion rates insufficient to maintain the Eddington rates in low luminosity/low redshift objects. Our results support the scenario in which the early phase of intense nuclear activity occurred mainly in early type galaxies (E/S0) during the relatively short period in which they had still an abundant interstellar medium. Only recently, with the decline of the QSO luminosities, did the activity in late type galaxies (Sa/Sab) become statistically significant.

Joint cosmological formation of QSOs and bulge-dominated galaxies

Older and more recent pieces of observational evidence suggest a strong connection between QSOs and galaxies; in particular, the recently discovered correlation between black hole and galactic bulge masses suggests that QSO activity is directly connected to the formation of galactic bulges. The cosmological problem of QSO formation is analysed in the framework of an analytical model for galaxy formation; for the first time a joint comparison with galaxy and QSO observables is performed. In this model it is assumed that the same physical variable that determines galaxy morphology is able to modulate the mass of the black hole responsible for QSO activity. Both halo spin and the occurrence of a major merger are considered as candidates for this role. The predictions of the model are compared with available data for the type-dependent galaxy mass functions, the star formation history of elliptical galaxies, the QSO luminosity function and its evolution (including the obscured objects contributing to the hard-X-ray background), the mass function of dormant black holes and the distribution of black hole-to-bulge mass ratios. A good agreement with observations is obtained if the halo spin modulates the efficiency of black hole formation, and if the galactic haloes at z = 0 have shone in an inverted order with respect to the hierarchical one (i.e., stars and black holes in bigger galactic haloes have formed before those in smaller ones). This inversion of hierarchical order for galaxy formation, which reconciles galaxy formation with QSO evolution, is consistent with many pieces of observational evidence.