arXiv subjects

Davide Fiacconi

Publications and source records attributed to Davide Fiacconi.

13 recordsLinked to original sources

arXiv2023

Dynamics and spin alignment in massive, gravito-turbulent circumbinary discs around supermassive black hole binaries

Parsec-scale separation supermassive black hole binaries in the centre of gas-rich galaxy merger remnants could be surrounded by massive circumbinary discs (CBDs). Black hole mass and spin evolution during the gas-rich binary inspiral are crucial in determining the direction and power of relativistic jets that radio observations with LOFAR and SKAO will probe, and for predicting gravitational wave (GW) emission that IPTA and LISA will measure. We present 3D hydrodynamic simulations capturing gas-rich, self-gravitating CBDs around a $2\times 10^6$ M$_{\odot}$ supermassive black hole binary, that probe different mass ratios, eccentricities and inclinations. We employ a sub-grid Shakura-Sunyaev accretion disc to self-consistently model black hole mass and spin evolution together with super-Lagrangian refinement techniques to resolve gas flows, streams and mini-discs within the cavity, which play a fundamental role in torquing and feeding the binary. We find that higher mass ratio and eccentric binaries result in larger cavities, while retrograde binaries result in smaller cavities. All of the simulated binaries are expected to shrink with net gravitational torques being negative. Unlike previous simulations, we do not find preferential accretion onto the secondary black hole. This implies smaller chirp masses at coalescence and hence a weaker GW background. Critically this means that spin-alignment is faster than the binary inspiral timescale even for low mass ratios. When considering initially misaligned systems, the orientation of the mini-discs around each black hole can vary significantly. We discuss the implications of this behaviour for black hole spin alignment and highlight the need for broader parameter space studies of misaligned systems to understand the impact on black hole recoil velocities.

astro-ph.HE↗

arXiv2020

Stellar and Weak Lensing Profiles of Massive Galaxies in the Hyper-Suprime Cam Survey and in Hydrodynamic Simulations

We perform a consistent comparison of the mass and mass profiles of massive ($M_\star > 10^{11.4}M_{\odot}$) central galaxies at z~0.4 from deep Hyper Suprime-Cam (HSC) observations and from the Illustris, TNG100, and Ponos simulations. Weak lensing measurements from HSC enable measurements at fixed halo mass and provide constraints on the strength and impact of feedback at different halo mass scales. We compare the stellar mass function (SMF) and the Stellar-to-Halo Mass Relation (SHMR) at various radii and show that the radius at which the comparison is performed is important. In general, Illustris and TNG100 display steeper values of $α$ where $M_{\star}\propto M_{\rm vir}^α$. These differences are more pronounced for Illustris than for TNG100 and in the inner rather than outer regions of galaxies. Differences in the inner regions may suggest that TNG100 is too efficient at quenching in-situ star formation at $M_{\rm vir}\simeq10^{13} M_{\odot}$ but not efficient enough at $M_{\rm vir}\simeq10^{14} M_{\odot}$. The outer stellar masses are in excellent agreement with our observations at $M_{\rm vir}\simeq10^{13} M_{\odot}$, but both Illustris and TNG100 display excess outer mass as $M_{\rm vir}\simeq10^{14} M_{\odot}$ (by ~0.25 and ~0.12 dex, respectively). We argue that reducing stellar growth at early times in $M_\star \sim 10^{9-10} M_{\odot}$ galaxies would help to prevent excess ex-situ growth at this mass scale. The Ponos simulations do not implement AGN feedback and display an excess mass of ~0.5 dex at $r<30$ kpc compared to HSC which is indicative of over-cooling and excess star formation in the central regions. Joint comparisons between weak lensing and galaxy stellar profiles are a direct test of whether simulations build and deposit galaxy mass in the correct dark matter halos and thereby provide powerful constraints on the physics of feedback and galaxy growth.

astro-ph.GA↗

arXiv2018

Galactic nuclei evolution with spinning black holes: method and implementation

Supermassive black holes at the centre of galactic nuclei mostly grow in mass through gas accretion over cosmic time. This process also modifies the angular momentum (or spin) of black holes, both in magnitude and in orientation. Despite being often neglected in galaxy formation simulations, spin plays a crucial role in modulating accretion power, driving jet feedback, and determining recoil velocity of coalescing black hole binaries. We present a new accretion model for the moving-mesh code {\sc arepo} that incorporates (i) mass accretion through a thin $α$-disc, and (ii) spin evolution through the Bardeen-Petterson effect. We use a diverse suite of idealised simulations to explore the physical connection between spin evolution and larger scale environment. We find that black holes with mass $\lesssim 10^{7}$ M$_{\odot}$ experience quick alignment with the accretion disc. This favours prolonged phases of spin-up, and the spin direction evolves according to the gas inflow on timescales as short as $\lesssim 100$ Myr, which might explain the observed jet direction distribution in Seyfert galaxies. Heavier black holes ($\gtrsim 10^{8}$ M$_{\odot}$) are instead more sensitive to the local gas kinematic. Here we find a wider distribution in spin magnitudes: spin-ups are favoured if gas inflow maintains a preferential direction, and spin-downs occur for nearly isotropic infall, while the spin direction does not change much over short timescales $\sim 100$ Myr. We therefore conclude that supermassive black holes with masses $\gtrsim 5 \times 10^{8}$ M$_{\odot}$ may be the ideal testbed to determine the main mode of black hole fuelling over cosmic time.

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arXiv2017

Constraining the mass of accreting black holes in ultraluminous X-ray sources with ultrafast outflows

The nature of ultraluminous X-ray sources (ULXs) -- off-nuclear extra-galactic sources with luminosity, assumed isotropic, $\gtrsim 10^{39}$ erg s$^{-1}$ -- is still debated. One possibility is that ULXs are stellar black holes accreting beyond the Eddington limit. This view has been recently reinforced by the discovery of ultrafast outflows at $\sim 0.1$-$0.2c$ in the high resolution spectra of a handful of ULXs, as predicted by models of supercritical accretion discs. Under the assumption that ULXs are powered by super-Eddington accretion onto black holes, we use the properties of the observed outflows to self-consistently constrain their masses and accretion rates. We find masses $\lesssim 100$ M$_{\odot}$ and typical accretion rates $\sim 10^{-5}$ M$_{\odot}$ yr$^{-1}$, i.e. $\approx 10$ times larger than the Eddington limit calculated with a radiative efficiency of 0.1. However, the emitted luminosity is only $\approx 10\%$ beyond the Eddington luminosity, because most of the energy released in the inner part of the accretion disc is used to accelerate the wind, which implies radiative efficiency $\sim 0.01$. Our results are consistent with a formation model where ULXs are black hole remnants of massive stars evolved in low-metallicity environments.

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arXiv2017

Young and turbulent: the early life of massive galaxy progenitors

We present results from the "Ponos" simulation suite on the early evolution of a massive, $M_{\rm vir}(z=0)=1.2\times 10^{13}$ M$_{\odot}$ galaxy. At $z\gtrsim6$, before feedback from a central supermassive black hole becomes dominant, the main galaxy has a stellar mass $\sim 2\times 10^{9}$ M$_{\odot}$ and a star formation rate $\sim 20$ M$_{\odot}$ yr$^{-1}$. The galaxy sits near the expected main sequence of star-forming galaxies at those redshifts, and resembles moderately star-forming systems observed at $z>5$. The high specific star formation rate results in vigorous heating and stirring of the gas by supernovae feedback, and the galaxy develops a thick and turbulent disc, with gas velocity dispersion $\sim 40$ km s$^{-1}$, rotation to dispersion ratio $\sim 2$, and with a significant amount of gas at $\sim 10^5$ K. The Toomre parameter always exceeds the critical value for gravito-turbulence, $Q\sim 1.5-2$, mainly due to the contribution of warm/hot gas inside the disc. Without feedback, a nearly gravito-turbulent regime establishes with similar gas velocity dispersion and lower $Q$. We propose that the "hot and turbulent" disc regime seen in our simulations, unlike the "cold and turbulent" gravito-turbulent regime of massive clumpy disc galaxies at $z\sim 1-2$, is a fundamental characterisation of main sequence galaxies at $z\gtrsim 6$, as they can sustain star formation rates comparable to those of low-mass starbursts at $z=0$. This results in no sustained coherent gas inflows through the disc, and in fluctuating and anisotropic mass transport, possibly postponing the assembly of the bulge and causing the initial feeding of the central black hole to be highly intermittent.

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arXiv2016

Light or heavy supermassive black hole seeds: the role of internal rotation in the fate of supermassive stars

Supermassive black holes are a key ingredient of galaxy evolution. However, their origin is still highly debated. In one of the leading formation scenarios, a black hole of $\sim100$ M$_{\odot}$ results from the collapse of the inner core of a supermassive star ($\gtrsim 10^{4-5}$ M$_{\odot}$), created by the rapid accumulation ($\gtrsim 0.1 $ M$_{\odot}$ yr$^{-1}$) of pristine gas at the centre of newly formed galaxies at $z\sim 15$. The subsequent evolution is still speculative: the remaining gas in the supermassive star can either directly plunge into the nascent black hole, or part of it can form a central accretion disc, whose luminosity sustains a surrounding, massive, and nearly hydrostatic envelope (a system called a "quasi-star"). To address this point, we consider the effect of rotation on a quasi-star, as angular momentum is inevitably transported towards the galactic nucleus by the accumulating gas. Using a model for the internal redistribution of angular momentum that qualitative matches results from simulations of rotating convective stellar envelopes, we show that quasi-stars with an envelope mass greater than a few $10^{5}$ M$_{\odot} \times (\rm black~hole~mass/100 M_{\odot})^{0.82}$ have highly sub-keplerian gas motion in their core, preventing gas circularisation outside the black hole's horizon. Less massive quasi-stars could form but last for only $\lesssim 10^4$ years before the accretion luminosity unbinds the envelope, suppressing the black hole growth. We speculate that this might eventually lead to a dual black hole seed population: (i) massive ($>10^{4}$ M$_{\odot}$) seeds formed in the most massive ($> 10^{8}$ M$_{\odot}$) and rare haloes; (ii) lighter ($\sim 10^{2}$ M$_{\odot}$) seeds to be found in less massive and therefore more common haloes.

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arXiv2016

Swift coalescence of supermassive black holes in cosmological mergers of massive galaxies

Supermassive black holes (SMBHs) are ubiquitous in galaxies with a sizable mass. It is expected that a pair of SMBHs originally in the nuclei of two merging galaxies would form a binary and eventually coalesce via a burst of gravitational waves. So far theoretical models and simulations have been unable to predict directly the SMBH merger timescale from ab-initio galaxy formation theory, focusing only on limited phases of the orbital decay of SMBHs under idealized conditions of the galaxy hosts. The predicted SMBH merger timescales are long, of order Gyrs, which could be problematic for future gravitational wave searches. Here we present the first multi-scale $Λ$CDM cosmological simulation that follows the orbital decay of a pair of SMBHs in a merger of two typical massive galaxies at $z\sim3$, all the way to the final coalescence driven by gravitational wave emission. The two SMBHs, with masses $\sim10^{8}$ M$_{\odot}$, settle quickly in the nucleus of the merger remnant. The remnant is triaxial and extremely dense due to the dissipative nature of the merger and the intrinsic compactness of galaxies at high redshift. Such properties naturally allow a very efficient hardening of the SMBH binary. The SMBH merger occurs in only $\sim10$ Myr after the galactic cores have merged, which is two orders of magnitude smaller than the Hubble time.

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arXiv2016

Cold Dark Matter Substructures in Early-Type Galaxy Halos

We present initial results from the "Ponos" zoom-in numerical simulations of dark matter substructures in massive ellipticals. Two very highly resolved dark matter halos with $M_{\rm vir}=1.2\times 10^{13}$ $M_{\odot}$ and $M_{\rm vir}=6.5\times 10^{12}$ $M_{\odot}$ and different ("violent" vs. "quiescent") assembly histories have been simulated down to $z=0$ in a $Λ$CDM cosmology with a total of 921,651,914 and 408,377,544 particles, respectively. Within the virial radius, the total mass fraction in self-bound $M_{\rm sub}>10^6$ $M_{\odot}$ subhalos at the present epoch is 15% for the violent host and 16.5% for the quiescent one. At $z=0.7$, these fractions increase to 19 and 33%, respectively, as more recently accreted satellites are less prone to tidal destruction. In projection, the average fraction of surface mass density in substructure at a distance of $R/R_{\rm vir}=0.02$ ($\sim 5-10$ kpc) from the two halo centers ranges from 0.6% to $\gtrsim 2$%, significantly higher than measured in simulations of Milky Way-sized halos. The contribution of subhalos with $M_{\rm sub} < 10^9$ $M_{\odot}$ to the projected mass fraction is between one fifth and one third of the total, with the smallest share found in the quiescent host. We assess the impact of baryonic effects via twin, lower-resolution hydrodynamical simulations that include metallicity-dependent gas cooling, star formation, and a delayed-radiative-cooling scheme for supernova feedback. Baryonic contraction produces a super-isothermal total density profile and increases the number of massive subhalos in the inner regions of the main host. The host density profiles and projected subhalo mass fractions appear to be broadly consistent with observations of gravitational lenses.

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arXiv2015

Bright vigorous winds as signposts of supermassive black hole birth

The formation of supermassive black holes is still an outstanding question. In the quasi-star scenario, black hole seeds experience an initial super-Eddington growth, that in less than a million years may leave a $10^4-10^5$ M$_{\odot}$ black hole at the centre of a protogalaxy at $z \sim 20-10$. Super-Eddington accretion, however, may be accompanied by vigorous mass loss that can limit the amount of mass that reaches the black hole. In this paper, we critically assess the impact of radiative driven winds, launched from the surface of the massive envelopes from which the black hole accretes. Solving the full wind equations coupled with the hydrostatic structure of the envelope, we find mass outflows with rates between a few tens and $10^4$ M$_{\odot}$ yr$^{-1}$, mainly powered by advection luminosity within the outflow. We therefore confirm the claim by Dotan, Rossi & Shaviv (2011) that mass losses can severely affect the black hole seed early growth within a quasi-star. In particular, seeds with mass $>10^4$ M$_{\odot}$ can only form within mass reservoirs $ \gtrsim 10^7$ M$_{\odot}$, unless they are refilled at huge rates ($ \gtrsim 100$ M$_{\odot}$ yr$^{-1}$). This may imply that only very massive halos ($>10^9$ M$_{\odot}$) at those redshifts can harbour massive seeds. Contrary to previous claims, these winds are expected to be relatively bright ($10^{44}-10^{47}$ erg s$^{-1}$), blue ($T_{\rm eff} \sim 8000$ K) objects, that while eluding the Hubble Space Telescope, could be observed by the James Webb Space Telescope.

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arXiv2014

Direct formation of supermassive black holes in metal-enriched gas at the heart of high-redshift galaxy mergers

We present novel 3D multi-scale SPH simulations of gas-rich galaxy mergers between the most massive galaxies at $z \sim 8 - 10$, designed to scrutinize the direct collapse formation scenario for massive black hole seeds proposed in \citet{mayer+10}. The simulations achieve a resolution of 0.1 pc, and include both metallicity-dependent optically-thin cooling and a model for thermal balance at high optical depth. We consider different formulations of the SPH hydrodynamical equations, including thermal and metal diffusion. When the two merging galaxy cores collide, gas infall produces a compact, optically thick nuclear disk with densities exceeding $10^{-10}$ g cm$^3$. The disk rapidly accretes higher angular momentum gas from its surroundings reaching $\sim 5$ pc and a mass of $\gtrsim 10^9$ $M_{\odot}$ in only a few $10^4$ yr. Outside $\gtrsim 2$ pc it fragments into massive clumps. Instead, supersonic turbulence prevents fragmentation in the inner parsec region, which remains warm ($\sim 3000-6000$ K) and develops strong non-axisymmetric modes that cause prominent radial gas inflows ($> 10^4$ $M_{\odot}$ yr$^{-1}$), forming an ultra-dense massive disky core. Angular momentum transport by non-axisymmetric modes should continue below our spatial resolution limit, quickly turning the disky core into a supermassive protostar which can collapse directly into a massive black hole of mass $10^8-10^9$ $M_{\odot}$ via the relativistic radial instability. Such a "cold direct collapse"' explains naturally the early emergence of high-z QSOs. Its telltale signature would be a burst of gravitational waves in the frequency range $10^{-4} - 10^{-1}$ Hz, possibly detectable by the planned eLISA interferometer.

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arXiv2014

The Argo Simulation: II. The Early Build-up of the Hubble Sequence

The Hubble sequence is a common classification scheme for the structure of galaxies. Despite the tremendous usefulness of this diagnostic, we still do not fully understand when, where, and how this morphological ordering was put in place. Here, we investigate the morphological evolution of a sample of 22 high redshift ($z\geq3$) galaxies extracted from the Argo simulation. Argo is a cosmological zoom-in simulation of a group-sized halo and its environment. It adopts the same high resolution ($\sim10^4$ M$_\odot$, $\sim100$ pc) and sub-grid physical model that was used in the Eris simulation but probes a sub-volume almost ten times bigger with as many as 45 million gas and star particles in the zoom-in region. Argo follows the early assembly of galaxies with a broad range of stellar masses ($\log M_{\star}/{\rm M}_{\odot}\sim8-11$ at $z\simeq3$), while resolving properly their structural properties. We recover a diversity of morphologies, including late-type/irregular disc galaxies with flat rotation curves, spheroid dominated early-type discs, and a massive elliptical galaxy, already established at $z\sim3$. We identify major mergers as the main trigger for the formation of bulges and the steepening of the circular velocity curves. Minor mergers and non-axisymmetric perturbations (stellar bars) drive the bulge growth in some cases. The specific angular momenta of the simulated disc components fairly match the values inferred from nearby galaxies of similar $M_{\star}$ once the expected redshift evolution of disc sizes is accounted for. We conclude that morphological transformations of high redshift galaxies of intermediate mass are likely triggered by processes similar to those at low redshift and result in an early build-up of the Hubble sequence.

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arXiv2013

Massive black hole pairs in clumpy, self-gravitating circumnuclear disks: stochastic orbital decay

We study the dynamics of massive black hole pairs in clumpy gaseous circumnuclear disks. We track the orbital decay of the light, secondary black hole $M_{\bullet2}$ orbiting around the more massive primary at the center of the disk, using $N$-body/smoothed particle hydrodynamic simulations. We find that the gravitational interaction of $M_{\bullet2}$ with massive clumps $M_{\rm cl}$ erratically perturbs the otherwise smooth orbital decay. In close encounters with massive clumps, gravitational slingshots can kick the secondary black hole out of the disk plane. The black hole moving on an inclined orbit then experiences the weaker dynamical friction of the stellar background, resulting in a longer orbital decay timescale. Interactions between clumps can also favor orbital decay when the black hole is captured by a massive clump which is segregating toward the center of the disk. The stochastic behavior of the black hole orbit emerges mainly when the ratio $M_{\bullet2}/M_{\rm cl}$ falls below unity, with decay timescales ranging from $\sim1$ to $\sim50$ Myr. This suggests that describing the cold clumpy phase of the inter-stellar medium in self-consistent simulations of galaxy mergers, albeit so far neglected, is important to predict the black hole dynamics in galaxy merger remnants.

astro-ph.CO↗

arXiv2012

Adaptive mesh refinement simulations of collisional ring galaxies: effects of the interaction geometry

Collisional ring galaxies are the outcome of nearly axisymmetric high-speed encounters between a disc and an intruder galaxy. We investigate the properties of collisional ring galaxies as a function of the impact parameter, the initial relative velocity and the inclination angle. We employ new adaptive mesh refinement simulations to trace the evolution with time of both stars and gas, taking into account star formation and supernova feedback. Axisymmetric encounters produce circular primary rings followed by smaller secondary rings, while off-centre interactions produce asymmetric rings with displaced nuclei. We propose an analytical treatment of the disc warping induced by an inclination angle greater then zero. The star formation history of our models is mainly influenced by the impact parameter: axisymmetric collisions induce impulsive short-lived starburst episodes, whereas off-centre encounters produce long-lived star formation. We compute synthetic colour maps of our models and we find that rings have a B-V colour typically ~0.2 mag bluer than the inner and outer disc, in agreement with observations.

astro-ph.CO↗