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Laporte C.F.P.,Max Planck Institute for Astrophysics | White S.D.M.,Max Planck Institute for Astrophysics | Naab T.,Max Planck Institute for Astrophysics | Ruszkowski M.,University of Michigan | And 3 more authors.
Monthly Notices of the Royal Astronomical Society | Year: 2012

We study the evolution of the stellar and dark matter components in a galaxy cluster of 10 15M ⊙ from z= 3 to the present epoch using the high-resolution collisionless simulations of Ruszkowski & Springel. At z= 3 the dominant progenitor haloes were populated with spherical model galaxies with and without accounting for adiabatic contraction. We apply a weighting scheme which allows us to change the relative amount of dark and stellar material assigned to each simulation particle in order to produce luminous properties which agree better with abundance-matching arguments and observed bulge sizes at z= 3. This permits the study of the effect of initial compactness on the evolution of the mass-size relation. We find that for more compact initial stellar distributions the size of the final brightest cluster galaxy grows with mass according to r∝M 2, whereas for more extended initial distributions, r∝M. Our results show that collisionless mergers in a cosmological context can reduce the strength of inner dark matter cusps with changes in logarithmic slope of 0.3-0.5 at fixed radius. Shallow cusps such as those found recently in several strong lensing clusters thus do not necessarily conflict with cold dark matter, but may rather reflect on the initial structure of the progenitor galaxies, which was shaped at high redshift by their formation process. © 2012 The Authors Monthly Notices of the Royal Astronomical Society © 2012 RAS.

Morsony B.J.,University of Wisconsin - Madison | Heinz S.,University of Wisconsin - Madison | Bruggen M.,Jacobs University Bremen | Ruszkowski M.,University of Michigan | Ruszkowski M.,The Michigan Center for Theoretical Physics
Monthly Notices of the Royal Astronomical Society | Year: 2010

We present a series of three-dimensional hydrodynamical simulations of central active galactic nuclei (AGN)-driven jets in a dynamic, cosmologically evolved galaxy cluster. Extending previous work, we study jet powers ranging from L jet = 10 44 erg s -1 to L jet = 10 46 erg s -1 and in duration from 30 to 200 Myr. We find that large-scale motions of cluster gas disrupt the AGN jets, causing energy to be distributed throughout the centre of the cluster, rather than confined to a narrow angle around the jet axis. Disruption of the jet also leads to the appearance of multiple disconnected X-ray bubbles from a long-duration AGN with a constant luminosity. This implies that observations of multiple bubbles in a cluster are not necessarily an expression of the AGN duty cycle. We find that the 'sphere of influence' of the AGN, the radial scale within which the cluster is strongly affected by the jet, scales as R ∝ L jet 1/3. Increasing the duration of AGN activity does not increase the radius affected by the AGN significantly, but does change the magnitude of the AGN's effects. How an AGN delivers energy to a cluster will determine where that energy is deposited: a high luminosity is needed to heat material outside the core of the cluster, while a low-luminosity, long-duration AGN is more efficient at heating the inner few tens of kpc. © 2010 The Authors. Journal compilation © 2010 RAS.

Gaspari M.,Max Planck Institute for Astrophysics | Ruszkowski M.,University of Michigan | Ruszkowski M.,The Michigan Center for Theoretical Physics | Oh S.P.,University of California at Santa Barbara
Monthly Notices of the Royal Astronomical Society | Year: 2013

Bondi theory is often assumed to adequately describe the mode of accretion in astrophysical environments. However, the Bondi flow must be adiabatic, spherically symmetric, steady, unperturbed, with constant boundary conditions. Using 3D adaptive mesh refinement simulations, linking the 50 kpc to the sub-parsec (sub-pc) scales over the course of 40 Myr, we systematically relax the classic assumptions in a typical galaxy hosting a supermassive black hole. In the more realistic scenario, where the hot gas is cooling, while heated and stirred on large scales, the accretion rate is boosted up to two orders of magnitude compared with the Bondi prediction. The cause is the non-linear growth of thermal instabilities, leading to the condensation of cold clouds and filaments when tcool/tff≲10. The clouds decouple from the hot gas, 'raining' on to the centre. Subsonic turbulence of just over 100 km s-1 (M > 0.2) induces the formation of thermal instabilities, even in the absence of heating, while in the transonic regime turbulent dissipation inhibits their growth (tturb/tcool≲1). When heating restores global thermodynamic balance, the formation of the multiphase medium is violent, and the mode of accretion is fully cold and chaotic. The recurrent collisions and tidal forces between clouds, filaments and the central clumpy torus promote angular momentum cancellation, hence boosting accretion. On sub-pc scales the clouds are channelled to the very centre via a funnel. In this study, we do not inject a fixed initial angular momentum, though vorticity is later seeded by turbulence. A good approximation to the accretion rate is the cooling rate, which can be used as subgrid model, physically reproducing the boost factor of 100 required by cosmological simulations, while accounting for the frequent fluctuations. Since our modelling is fairly general (turbulence/heating due to AGN feedback, galaxy motions, mergers, stellar evolution), chaotic cold accretion may be common in many systems, such as hot galactic haloes, groups and clusters. In this mode, the black hole can quickly react to the state of the entire host galaxy, leading to efficient self-regulated AGN feedback and the symbiotic Magorrian relation. Chaotic accretion can generate high-velocity clouds, likely leading to strong variations in the AGN luminosity, and the deflection or mass-loading of jets. During phases of overheating, the hot mode becomes the single channel of accretion, though strongly suppressed by turbulence. High-resolution data could determine the current mode of accretion: assuming quiescent feedback, the cold mode results in a quasi-flat-temperature core as opposed to the cuspy profile of the hot mode. © 2013 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society.

Ruszkowski M.,University of Michigan | Ruszkowski M.,The Michigan Center for Theoretical Physics | Oh S.P.,University of California at Santa Barbara
Monthly Notices of the Royal Astronomical Society | Year: 2011

Unopposed radiative cooling in clusters of galaxies results in excessive mass deposition rates on to the central brightest cluster galaxy. However, the cool cores of galaxy clusters are continuously heated by thermal conduction and turbulent heat diffusion due to minor mergers or the galaxies orbiting the cluster centre. These processes can either reduce the energy requirements for active galactic nucleus heating of cool cores, or they can prevent overcooling altogether. We perform three-dimensional magnetohydrodynamics simulations including field-aligned thermal conduction and self-gravitating particles to model this in detail. Turbulence is not confined to the wakes of galaxies but is instead volume filling, due to the excitation of large-scale g-modes. We systematically probe the parameter space of galaxy masses and numbers to assess when the cooling catastrophe is prevented. For a wide range of observationally motivated galaxy parameters, we find that the magnetic field is randomized by stirring motions, restoring the conductive heat flow to the core. The cooling catastrophe either does not occur or it is sufficiently delayed to allow the cluster to experience a major merger that could reset the conditions in the intracluster medium. Whilst dissipation of turbulent motions (and hence dynamical friction heating) is negligible as a heat source, turbulent heat diffusion is extremely important; it predominates in the cluster centre. However, thermal conduction becomes important at larger radii, and simulations without thermal conduction suffer a cooling catastrophe. Conduction is important both as a heat source and to reduce stabilizing buoyancy forces, enabling more efficient diffusion. Turbulence enables conduction, and conduction enables turbulence. In these simulations, the gas vorticity - which is a good indicator of trapped g-modes - increases with time. The vorticity growth is approximately mirrored by the growth of the magnetic field, which is amplified by turbulence. © 2011 The Authors Monthly Notices of the Royal Astronomical Society © 2011 RAS.

Dotti M.,University of Michigan | Dotti M.,University of Insubria | Volonteri M.,University of Michigan | Perego A.,University of Basel | And 6 more authors.
Monthly Notices of the Royal Astronomical Society | Year: 2010

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 large-scale 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 much smaller than the median resulting from an isotropic distribution of spins. © 2009 The Authors. Journal compilation © 2009 RAS.

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