Massimo Dotti

Supermassive black hole binaries in gaseous and stellar circumnuclear discs: orbital dynamics and gas accretion

The dynamics of two massive black holes in a rotationally supported nuclear disc of mass M disc = 10 8 M⊙ is explored using N-body/smoothed particle hydrodynamics simulations. Gas and star particles are copresent in the disc. Described by a Mestel profile, the disc has a vertical support provided by turbulence of the gas, and by stellar velocity dispersion. A primary black hole of mass 4 x 10 6 M⊙ is placed at the centre of the disc, while a secondary black hole is set initially on an eccentric corotating orbit in the disc plane. Its mass is in a 1:1, 1:4, and 1:10 ratio, relative to the primary. With this choice, we mimic the dynamics of black hole pairs released in the nuclear region at the end of a gas-rich galaxy merger. It is found that, under the action of dynamical friction, the two black holes form a close binary in ∼10 Myr. The inspiral process is insensitive to the mass fraction in stars and gas present in the disc and is accompanied by the circularization of the orbit. We detail the gaseous mass profile bound to each black hole that can lead to the formation of two small Keplerian discs, weighing ≈2 per cent of the black hole mass, and of size ∼0.01 pc. The mass of the tightly (loosely) bound particles increases (decreases) with time as the black holes spiral into closer and closer orbits. Double active galactic nucleus activity is expected to occur on an estimated time-scale of ≤10 Myr, comparable to the inspiral time-scale. The double nuclear point-like sources that may appear during dynamical evolution will have typical separations of ≤10 pc.

Limiting eccentricity of subparsec massive black hole binaries surrounded by self-gravitating gas discs

We study the dynamics of supermassive black hole binaries embedded in circumbinary gaseous discs, with the SPH code GADGET-2. The sub-parsec binary (of total mass M and mass ratio q = 1/3) has excavated a gap and transfers its angular momentum to the self‐ gravitating disc (Mdisc = 0.2M ). We explore the changes of the binary eccentricity e, by simulating a sequence of binary models that differ in the initial eccentricity e0, only. In initially low-eccentric binaries, the eccentricity increase s with time, while in high-eccentric binaries e declines, indicating the existence of a limiting eccentric ity ecrit that is found to fall in the interval [0.6,0.8]. We also present an analytical interpretation for this satu ration limit. An important consequence of the existence of ecrit is the detectability of a significant residual eccentricity eLISA by the proposed gravitational wave detector LISA. It is found that at the moment of entering the LISA frequency domain eLISA � 10 3 10 2 ; a signature of its earlier coupling with the massive circumbinary disc. We also observe large periodic inflows across the gap, occurring on the binary and disc dynamical time scales rather than on the viscous time. These periodic changes in the accretion rate (with amplitudes up to � 100%, depending on the binary eccentricity) can be considered a fin gerprint of eccentric sub-parsec binaries migrating inside a circumbinary disc.

Dual black holes in merger remnants – II. Spin evolution and gravitational recoil

Using high-resolution hydrodynamical simulations, we explore the spin evolution of massive dual black holes orbiting inside a circumnuclear disc, relic of a gas-rich galaxy merger. The black holes spiral inwards from initially eccentric co- or counter-rotating coplanar orbits relative to the disc’s rotation, and accrete gas that is carrying a net angular momentum. As the black hole mass grows, its spin changes in strength and direction due to its gravito-magnetic coupling with the small-scale accretion disc. We find that the black hole spins loose memory of their initial orientation, as accretion torques suffice to align the spins with the angular momentum of their orbit on a short time-scale (1–2 Myr). A residual off-set in the spin direction relative to the orbital angular momentum remains, at the level of 10 ◦ for the case of a cold disc, and 30 ◦ for a warmer disc. Alignment in a cooler disc is more effective due to the higher coherence of the accretion flow near each black hole that reflects the largescale coherence of the disc’s rotation. If the massive black holes coalesce preserving the spin directions set after formation of a Keplerian binary, the relic black hole resulting from their coalescence receives a relatively small gravitational recoil. The distribution of recoil velocities inferred from a simulated sample of massive black hole binaries has median 70 km s −1 ,

Evolution of binary black holes in self gravitating discs Dissecting the torques

Context. Massive black hole binaries, formed in galaxy mergers, are expected to evolve in dense circumbinary discs. Understanding of the disc-binary coupled dynamics is vital to assess both t he final fate of the system and the potential observable featu res that may be tested against observations. Aims. Aimed at understanding the physical roots of the secular evolution of the binary, we study the interplay between gas accretion and gravity torques in changing the binary elements (semi-major axis and eccentricity) and its total angular momentum budget. We pay special attention to the gravity torques, by analysing t heir physical origin and location within the disc. Methods. We analyse three-dimensional smoothed particle hydrodynamics simulations of the evolution of initially quasi-circu lar massive black hole binaries (BHBs) residing in the central hollow (cavity) of massive self-gravitating circumbinary discs. We perfo rm a set of simulations adopting different thermodynamics for the gas within the cavity and for the ’numerical size’ of the black holes. Results. We show that (i) the BHB eccentricity growth found in our previous work is a general result, independent of the accretion a nd the adopted thermodynamics; (ii) the semi-major axis decay depends not only on the gravity torques but also on their subt le interplay with the disc-binary angular momentum transfer due to accretion; (iii) the spectral structure of the gravity torques is predominately caused by disc edge overdensities and spiral arms developing in the body of the disc and, in general, does not reflect direc tly the period of the binary; (iv) the net gravity torque changes sig n across the BHB corotation radius (positive inside vs negative outside) We quantify the relative importance of the two, which appear to depend on the thermodynamical properties of the instreaming gas, and which is crucial in assessing the disc‐binary angular momentum transfer; (v) the net torque manifests as a purely kin ematic (non-resonant) effect as it stems from the low density cavity, where the material flows in and out in highly eccentric orbits. Conclusions. Both accretion onto the black holes and the interaction with gas streams inside the cavity must be taken into account to assess the fate of the binary. Moreover, the total torque exe rted by the disc affects the binary angular momentum by changing all the elements (mass, mass ratio, eccentricity, semimajor axis) of the black hole pair. Commonly used prescriptions equating tidal torque to semi-major axis shrinking might therefore be poor approximations for real astrophysical systems.

Laser Interferometer Space Antenna double black holes: dynamics in gaseous nuclear discs

We study the inspiral of double black holes, with masses in the LISA window of detectability, orbiting inside a massive circum-nuclear disc. Using high-resolution SPH simulations, we follow the black hole dynamics in the early phase when gas-dynamical friction acts on the black holes individually, and continue our simulation until they form a close binary. We find that in the early sinking the black holes lose memory of their initial orbital eccentricity if they co-rotate with the gaseous disc, forming a binary with a low eccentricity, consistent with zero within our numerical resolution limit. The cause of circularization resides in the rotation present in the gaseous background where dynamical friction operates. Circularization may hinder gravitational waves from taking over and leading the binary to coalescence. In the case of counter-rotating orbits the initial eccentricity does not decrease, and the black holes may bind forming an eccentric binary. When dynamical friction has subsided, for equal mass black holes and regardless their initial eccentricity, angular momentum loss, driven by the gravitational torque exerted on the binary by surrounding gas, is nevertheless observable down to the smallest scale probed. In the case of unequal masses, dynamical friction remains efficient down to our resolution limit, and there is no sign of formation of any ellipsoidal gas distribution that may further harden the binary. During inspiral, gravitational capture of gas by the black holes occurs mainly along circular orbits: eccentric orbits imply high relative velocities and weak gravitational focusing. Thus, AGN activity may be excited during the black hole pairing process and double active nuclei may form when circularization is completed, on distance-scales of tens of pcs.

Black hole mass estimate for a sample of radio-loud narrow-line Seyfert 1 galaxies

We discuss the relationship between a standard Shakura & Sunyaev (1973) accretion disk model and the Big Blue Bump (BBB) observed in Type 1 AGN, and propose a new method to estimate black hole masses. We apply this method to a sample of 23 radio-loud narrow-line Seyfert 1 (RL-NLS1) galaxies, using data from WISE (Wide-field Infrared Survey Explorer), SDSS (Sloan Digital Sky Survey) and GALEX. Our black hole mass estimates are at least a factor

SDSSJ092712.65+294344.0: a candidate massive black hole binary

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A SYSTEMATIC SEARCH FOR MASSIVE BLACK HOLE BINARIES IN THE SLOAN DIGITAL SKY SURVEY SPECTROSCOPIC SAMPLE

6 above previous results based on single epoch virial methods, while the Eddington ratios are correspondingly lower. Hence, the black hole masses of RL-NLS1 galaxies are typically above

Are the black hole masses in narrow‐line Seyfert 1 galaxies actually small?

10^8 M_{\sun}

On the Orientation and Magnitude of the Black Hole Spin in Galactic Nuclei

, in agreement with the typical black hole mass of blazars.