T. Di Matteo

ACCRETION ONTO THE SUPERMASSIVE BLACK HOLE IN M87

Chandra X-ray observations of the giant elliptical galaxy M87 resolve the thermal state of the hot interstellar medium into the accretion (Bondi) radius of its central 3 × 109 M☉ black hole. We measure the X-ray gas temperature and density profiles and calculate the Bondi accretion rate, Bondi ~ 0.1 M☉ yr-1. The X-ray luminosity of the active nucleus of M87 observed with Chandra is LX,0.5-7 keV ~ 7 × 1040 ergs s-1. This value is much less than the predicted nuclear luminosity, LBondi ~ 5 × 1044 ergs s-1, for accretion at the Bondi rate with a canonical accretion radiative efficiency of 10%. If the black hole in M87 accretes at this rate it must do so at a much lower radiative efficiency than the canonical value. The multiwavelength spectrum of the nucleus is consistent with that predicted by an advection-dominated flow. However, as is likely, the X-ray nucleus is dominated by jet emission then the properties of flow must be modified, possibly by outflows. We show that the overall energetics of the system are just consistent with the predicted Bondi nuclear power. This suggests that either most of the accretion energy is released in the relativistic jet or the central engine of M87 undergoes on-off activity cycles. We show that, at present, the energy dumped into the ISM by the jet may reduce the accretion rate onto the black hole by a factor ∝ (vj/cs)-2, where vj is the jet velocity and cs the ISM sound speed, and that this is sufficient to account for the low nuclear luminosity.

COLD FLOWS AND THE FIRST QUASARS

Observations of the most distant bright quasars imply that billion solar mass supermassive black holes (SMBHs) have to be assembled within the first 800 million years. Under our standard galaxy formation scenario such fast growth implies large gas densities providing sustained accretion at critical or supercritical rates onto an initial black hole seed. It has been a long standing question whether and how such high black hole accretion rates can be achieved and sustained at the centers of early galaxies. Here we use our new MassiveBlack cosmological hydrodynamic simulation covering a volume (0.75 Gpc)3 appropriate for studying the rare first quasars to show that steady high density cold gas flows responsible for assembling the first galaxies produce the high gas densities that lead to sustained critical accretion rates and hence rapid growth commensurate with the existence of ~109 M ☉ black holes as early as z ~ 7. We find that under these conditions quasar feedback is not effective at stopping the cold gas from penetrating the central regions and hence cannot quench the accretion until the host galaxy reaches . This cold-flow-driven scenario for the formation of quasars implies that they should be ubiquitous in galaxies in the early universe and that major (proto)galaxy mergers are not a requirement for efficient fuel supply and growth, particularly for the earliest SMBHs.

The "quiescent" black hole in M87

It is believed that most giant elliptical galaxies possess nuclear black holes with masses in excess of 10 8 M⊙. Bondi accretion from the interstellar medium might then be expected to produce quasar-like luminosities from the nuclei of even quiescent elliptical galaxies. It is a puzzle that such luminosities are not observed. Motivated by this problem, Fabian & Rees have recently suggested that the final stages of accretion in these objects occurs in an advection-dominated mode with a correspondingly small radiative efficiency. Despite possessing a long-known active nucleus and dynamical evidence for a black hole, the low radiative and kinetic luminosities of the core of M87 provide the best illustration of this problem. We examine an advection-dominated model for the nucleus of M87 and show that accretion at the Bondi rate is compatible with the best known estimates for the core flux from radio through to X-ray wavelengths. The success of this model prompts us to propose that FR-I radio galaxies and quiescent elliptical galaxies accrete in an advection dominated mode whereas FR-II type radio-loud nuclei possess radiatively efficient thin accretion disks.

Low‐radiative‐efficiency accretion in the nuclei of elliptical galaxies

ABSTRA C T The discovery of hard X-ray emission from a sample of six nearby elliptical galaxies, including the dominant galaxies of the Virgo, Fornax and Centaurus clusters (M87, NGC 1399 and NGC 4696, respectively), and NGC 4472, 4636 and 4649 in the Virgo cluster, has important implications for the study of quiescent supermassive black holes. We describe how the broad-band spectral energy distributions for these galaxies, which accrete from their hot gaseous haloes at rates comparable to their Bondi rates, can be explained by low-radiativeefficiency accretion flows in which a significant fraction of the mass, angular momentum and energy is removed from the flows by winds. The observed suppression of the synchrotron components in the radio band (excluding the case of M87) and the systematically hard X-ray spectra, which are interpreted as thermal bremsstrahlung emission, support the conjecture that significant mass outflow is a natural consequence of systems accreting at low radiative efficiencies. We briefly discuss an alternative model for the observed X-ray emission, in which it is due to non-thermal synchrotron-self-Compton processes in the accretion flows, or outflows. This appears to require implausibly weak magnetic fields. Emission from a collimated jet viewed off-axis should be distinguishable from the bremsstrahlung model by variability and thermal line emission studies. We argue that the difference in radiative efficiency between the nuclei of spiral and elliptical galaxies may arise from the different manner in which interstellar gas is fed into the nuclei. In ellipticals, matter fed from the hot (slowly cooling) interstellar medium (ISM) is likely to be highly magnetized and have low specific angular momentum, which favours low-radiativeefficiency accretion solutions and, possibly, the formation of the observed jets.

Black hole growth and activity in a Lambda cold dark matter universe

The observed properties of supermassive black holes suggest a fundamental link between their assembly and the formation of their host spheroids. We model the growth and activity of black holes in galaxies using Λ cold dark matter cosmological hydrodynamic simulations by following the evolution of the baryonic mass component in galaxy potential wells. We find that the observed steep relation between black hole mass and spheroid velocity dispersion, MBH σ4, is reproduced if the gas mass in bulges is linearly proportional to the black hole mass. To a good approximation, this is equivalent to assuming the conversion of a fixed fraction of gas mass into black hole mass. In this model, star formation and supernova feedback in the gas are sufficient for regulating and limiting the growth of the central black hole and of its gas supply. Black hole growth saturates because of the competition with star formation and, in particular, feedback, both of which determine the gas fraction available for accretion. Unless other processes also operate, we predict that the MBH-σ relation is not set in primordial structures but is fully established at low redshifts, z 2, and is shallower at earlier times. Once this relation is established, we find that central black hole masses are related to their dark matter halos simply via MBH ≈ M. We assume that galaxies undergo a quasar phase with a typical lifetime, tQ ~ 2 × 107 yr, the only free parameter of the model, and show that star formation-regulated depletion of gas in spheroids is sufficient to explain, for the most part, the decrease of the quasar population at redshift z < 3 in the optical blue band. However, with the simplest assumption of a redshift-independent quasar lifetime, the model slightly overpredicts optical quasar numbers at high redshifts, although it yields the observed evolution of number density of X-ray-faint quasars over the whole redshift range 1 < z < 6. Finally, we find that the majority of black hole mass is assembled in galaxies by z ~ 3 and that the black hole accretion rate density peaks in rough correspondence to the star formation rate density at z ~ 4-5.

Low‐Luminosity States of the Black Hole Candidate GX 339−4. I. ASCA and Simultaneous Radio/RXTE Observations

We discuss a series of observations of the black hole candidate GX 339(4 in low-luminosity, spec- trally hard states. We present spectral analysis of three separate archival Advanced Satellite for Cosmol- ogy and Astrophysics (ASCA) data sets and eight separate Rossi X-Ray T iming Explorer (RXT E) data sets. Three of the RXT E observations were strictly simultaneous with 843 MHz and 8.3¨9.1 GHz radio observations. All of these observations have (3¨9 keV) —ux ergs s~1 cm~2. The ASCA data show (10~9 evidence for an B6.4 keV Fe line with equivalent width B40 eV, as well as evidence for a soft excess that is well modeled by a power law plus a multicolor blackbody spectrum with peak temperature B150¨200 eV. The RXT E data sets also show evidence of an Fe line with equivalent widths B20¨140 eV. Re—ection models show a hardening of the RXT E spectra with decreasing X-ray —ux; however, these models do not exhibit evidence of a correlation between the photon index of the incident power law —ux and the solid angle subtended by the re—ector. ii Sphere)disk ˇˇ Comptonization models and advection- dominated accretion —ow (ADAF) models also provide reasonable descriptions of the RXT E data. The former models yield coronal temperatures in the range 20¨50 keV and optical depths of qB 3. The model —ts to the X-ray data, however, do not simultaneously explain the observed radio properties. The most likely source of the radio —ux is synchrotron emission from an extended out—ow of size greater than O(107GM/c2). Subject headings: binaries: closeblack hole physicsradiation mechanisms: nonthermal ¨ stars: individual (GX 339(4) ¨ X-rays: stars

Resolving the composite Fe K alpha emission line in the Galactic black hole Cygnus X-1 with Chandra

We observed the Galactic black hole Cyg X-1 with the Chandra High Energy Transmission Grating Spectrometer for 30 ks on 2001 January 4. The source was in an intermediate state, with a flux that was approximately twice that commonly observed in its persistent low/hard state. Our best-fit model for the X-ray spectrum includes narrow Gaussian emission line (E = 6.415 ? 0.007 keV, FWHM= 80 eV, W = 16 eV) and broad-line (E = 5.82 keV, FWHM= 1.9 keV, W = 140 eV) components, and a smeared edge at 7.3 ? 0.2 keV (? ~ 1.0). The broad-line profile is not as strongly skewed as those observed in some Seyfert galaxies. We interpret these features in terms of an accretion disk with irradiation of the inner disk producing a broad Fe K? emission line and edge, and irradiation of the outer disk producing a narrow Fe K? emission line. The broad line is likely shaped predominantly by Doppler shifts and gravitational effects, and to a lesser degree by Compton scattering due to reflection. We discuss the underlying continuum X-ray spectrum and these line features in the context of diagnosing the accretion flow geometry in Cyg X-1 and other Galactic black holes.

Magnetic flares in accretion disc coronae and the spectral states of black hole candidates: the case of GX339‐4

We examine the constraints that the observations of different spectral states displayed by Galactic black hole candidates impose on the properties of magnetic flares resulting from the reconnection of flux tubes that rise from the accretion disc into a corona because of the magnetic buoyancy (Parker) instability. Using observations of one of the best-studied examples, GX339-4, we identify the geometry and physical conditions characterizing each of these states. We find that, if in the soft state flaring occurs at small scaleheights above the accretion disc, a soft thermal-like spectrum, characteristic of this state, can result from the heating and consequent reradiation of the hard X-rays produced by such flares. The hard tail can then be produced by Comptonization of the soft radiation. Conversely, the hard state may result from a phase in which flares are triggered high above the underlying accretion disc and produce X-rays via Comptonization of either internal cyclo-synchrotron radiation or soft disc photons. The spectral characteristics of the different states are naturally accounted for by the choice of geometry: when flares are triggered high above the disc the system is photon-starved, hence the hard Comptonized spectrum of the hard state. Intense flaring close to the disc greatly enhances the local soft-photon field with the result that the spectrum softens. We interpret these two states as being related to two different phases of magnetic energy dissipation. We speculate that, in the soft state, Parker instability in the disc may favour the emergence of large numbers of relatively low-magnetic-field flux tubes. In the hard state, only intense magnetic fields become buoyant and magnetic loops are able to rise and expand in the coronal atmosphere. This possibility can also qualitatively account for the observed short time-scale variability and the characteristics of the X-ray-reflected component of the hard state.

Magnetic reconnection: flares and coronal heating in active galactic nuclei

ABSTRA C T A magnetically structured accretion disc corona, generated by buoyancy instability in the disc, can account for observations of flare-like events in active galactic nuclei. We examine how Petschek magnetic reconnection, associated with MHD turbulence, can result in a violent release of energy and heat the magnetically closed regions of the corona up to canonical X-ray emitting temperatures. X-ray magnetic flares, the after effect of the energy released in slow shocks, can account for the bulk of the X-ray luminosity from Seyfert galaxies and consistently explain the observed short-time-scale variability. process directly into the corona outside the disc. Magnetic recon- nection can be responsible for the rapid dissipation of magnetic energy though a field-aligned electric potential in thin current sheets. We, here, describe how the observationally required large heating rates per unit mass, in the optically thin gas of a hot coronal region, can be accounted for by an ion-acoustic instability in the context of slow shocks associated with Petschek-type reconnection and give rise to flare-like events. We estimate that such a process may be responsible for the heating of the coronal plasma in the active regions of an AGN to a level where it emits X-rays at a temperature of ,10 9 K even in the presence of inverse Compton and synchro- tron cooling processes. Magnetic buoyancy, which drives magnetic flux out of the disc, naturally decreases the plasma density to the point where the active region can reconnect very efficiently. X-ray observations imply that the coronal plasma is very tenuous with a density much lower than that of the underlying disc. We show that the energetics of such flares are consistent with the X-ray lumin- osities of typical Seyfert galaxies. The onset of magnetic flares through slow shocks associated with Petschek reconnection, should occur over time-scales comparable to those required to explain

Strong observational constraints on advection-dominated accretion in the cores of elliptical galaxies

ABSTRA C T The growing evidence for supermassive black holes in the centres of relatively nearby galaxies has brought into sharper focus the question of why elliptical galaxies, rich in hot gas, do not possess quasar-like luminosities. Recent studies suggest that the presence of advectiondominated accretion flows (ADAFs), with their associated low radiative efficiency, might provide a very promising explanation for the observed quiescence of these systems. Although ADAF models have been applied to a number of low-luminosity systems, compelling observational evidence for their existence is still required. Here, we examine new highfrequency radio observations of the three giant, low-luminosity elliptical galaxies NGC 4649, NGC 4472 and NGC 4636 obtained using the Very Large Array (VLA) and the Submillimetre Common-User Bolometer Array (SCUBA) on the James Clerk Maxwell Telescope (JCMT). At these frequencies the predictions are very precise and an ADAF is unequivocally characterized by a slowly rising spectrum with a sharp spectral cut-off produced by thermal synchrotron radiation. Although X-ray analysis of these galaxy cores provides very strong clues for their extreme quiescence (and makes the case of advective accretion plausible), the new radio limits disagree severely with the canonical ADAF predictions which significantly overestimate the observed flux. While the present observations do not rule out the presence of an ADAF in the systems considered here, they do place strong constraints on the model. If the accretion in these objects occurs in an advection-dominated mode, then our radio limits imply that the emission from their central regions must be suppressed. We examine the possibility that the magnetic field in the flow is extremely low, or that synchrotron emission is free‐free absorbed by cold material in the accretion flow. We also discuss whether slow non-radiating accretion flows may drive winds/outflows to remove energy, angular momentum and mass so that the central densities, pressures and emissivities are much smaller than in a standard ADAF.