arXiv subjects

Manuel Arca Sedda

Publications and source records attributed to Manuel Arca Sedda.

61 records · Page 4Linked to original sources

arXiv2019

Direct N-body simulation of the Galactic centre

We study the dynamics and evolution of the Milky Way nuclear star cluster performing a high resolution direct one-million-body simulation. Focusing on the interactions between such stellar system and the central supermassive black hole, we find that different stellar components adapt their overall distribution differently. After 5 Gyr, stellar mass black holes are characterized by a spatial distribution with power-slope $-1.75$, fully consistent with the prediction of Bahcall-Wolf pioneering work. Using the vast amount of data available, we infer the rate for tidal disruption events, being $4 \times 10^{-6}$ per yr, and estimate the number of objects that emit gravitational waves during the phases preceding the accretion onto the super-massive black hole, $\sim 270$ per Gyr. We show that some of these sources could form extreme mass-ratio inspirals. We follow the evolution of binary stars population, showing that the initial binary fraction of $5\%$ drops down to $2.5\%$ inside the inner parsec. Also, we explored the possible formation of binary systems containing a compact object, discussing the implications for millisecond pulsars formation and the development of Ia Supernovae.

astro-ph.GA↗

arXiv2018

Using final black hole spins and masses to infer the formation history of the observed population of gravitational wave sources

In this paper we propose a novel technique to constrain the progenitor binary black hole (BBH) formation history using the remnant masses and spins of merged black holes (BHs). Exploring different models, we found that dynamically formed BBHs are distributed differently in the mass-spin plane than those that have formed in isolation. Stellar evolution recipes crucially affect the remnant mass distribution, suggesting that future efforts should be devoted to finding a common way of modelling the evolutionary phases of single and binary stars. Our simple approach has allowed us to place weak constraints on the origin of the presently observed population of merged BHs, although with high uncertainties. Our results show that the fingerprints of different BBH formation channels will emerge as soon as LIGO detects more than $\sim 10^2$ merger events. This work provides a way of thinking that can be easily used both by people working on isolated and on dynamical BBH formation and evolution.

astro-ph.GA↗

arXiv2018

Supermassive black holes coalescence mediated by massive perturbers: implications for gravitational waves emission and nuclear cluster formation

A large fraction of galactic nuclei is expected to host supermassive black hole binaries (BHB), likely formed during the early phase of galaxies assembly and merging. In this paper, we use a large set of state-of-art numerical models to investigate the interplay between a BHB and a massive star cluster (GCs) driven toward the galactic centre by dynamical friction. Varying the BHB mass and mass ratio and the GC orbit, we show that the reciprocal feedback exerted between GCs and the BHB shapes their global properties. We show that, at GC-to-BHB mass ratios above 0.1, the GC affects notably the BHB orbital evolution, possibly boosting its coalescence. This effect is maximized if the GC moves on a retrograde orbit, and for a non-equal mass BHB. We show that the GC debris dispersed around the galactic nucleus can lead to the formation of a nuclear cluster, depending on the BHB tidal field, and that the distribution of compact remnants resulting from the GC disruption can carry information about the BHB orbital properties. We find that red giant stars delivered by the spiralling GC can be disrupted at a rate of $\simeq (0.7-7)\times 10^{-7}$ yr$^{-1}$ for BHB masses $\sim 10^7{\rm M}_\odot$, while tens to hundreds of stars can be possibly observed in the galactic halo as high-velocity stars, with velocities up to $\sim 2000$ km s$^{-1}$, depending on the BHB orbital properties.

astro-ph.GA↗

arXiv2018

MOCCA-SURVEY Database I: Galactic Globular Clusters Harbouring a Black Hole Subsystem

There have been increasing theoretical speculations and observational indications that certain globular clusters (GCs) could contain a sizeable population of stellar mass black holes (BHs). In this paper, we shortlist at least 29 Galactic GCs that could be hosting a subsystem of BHs (BHS). In a companion paper, we analysed results from a wide array of GC models (simulated with the MOCCA code for cluster simulations) that retained few tens to several hundreds of BHs at 12 Gyr and showed that the properties of the BHS in those GCs correlate with the GC's observable properties. Building on those results, we use available observational properties of 140 Galactic GCs to identify 29 GCs that could potentially be harbouring up to a few hundreds of BHs. Utilizing observational properties and theoretical scaling relations, we estimate the density, size and mass of the BHS in these GCs. We also calculate the total number of BHs and the fraction of BHs contained in a binary system for our shortlisted Galactic GCs. Additionally, we mention other Galactic GCs that could also contain significant number of single BHs or BHs in binary systems.

astro-ph.GA↗

arXiv2018

Gravitational wave sources from inspiralling globular clusters in the Galactic Centre and similar environments

We model the inspiral of globular clusters (GCs) towards a galactic nucleus harboring a supermassive black hole (SMBH), a leading scenario for the formation of nuclear star clusters. We consider the case of GCs containing either an intermediate-mass black hole (IMBH) or a population of stellar mass black holes (BHs), and study the formation of gravitational wave (GW) sources. We perform direct summation $N$-body simulations of the infall of GCs with different orbital eccentricities in the live background of a galaxy with either a shallow or steep density profile. We find that the GC acts as an efficient carrier for the IMBH, facilitating the formation of a bound pair. The hardening and evolution of the binary depends sensitively on the galaxy's density profile. If the host galaxy has a shallow profile the hardening is too slow to allow for coalescence within a Hubble time, unless the initial cluster orbit is highly eccentric. If the galaxy hosts a nuclear star cluster, the hardening leads to coalescence by emission of GWs within $3-4$ Gyr. In this case, we find a IMBH-SMBH merger rate of $Γ_{\rm IMBH-SMBH} = 2.8\times 10^{-3}$ yr$^{-1}$ Gpc$^{-3}$. If the GC hosts a population of stellar BHs, these are deposited close enough to the SMBH to form extreme-mass-ratio-inspirals with a merger rate of $Γ_{\rm EMRI} = 0.25$ yr$^{-1}$ Gpc$^{-3}$. Finally, the SMBH tidal field can boost the coalescence of stellar black hole binaries delivered from the infalling GCs. The merger rate for this merging channel is $Γ_{\rm BHB} = 0.4-4$ yr$^{-1}$ Gpc$^{-3}$.

astro-ph.GA↗

arXiv2018

MOCCA-SURVEY Database I. Unravelling black hole subsystems in globular clusters

In this paper, we discuss how globular clusters (GCs) structural and observational properties can be used to infer the presence of a black hole system (BHS) inhabiting their inner regions. We propose a novel way to identify the BHS size, defined as the GC radius containing a mass contributed equally from stars and stellar BHs. Using this definition, similar to the well-known concept of "influence radius", we found a "fundamental plane" connecting the BHS typical density with the GC central surface density profile, total luminosity and observational half-mass radius. Our approach allows us to define a unique way to connect the observational GCs parameters with their dark content. Comparing our results with observed Milky Way GCs, we found that many of them likely host, at the present time, as many as several hundreds of BHs. These BHS are characterized by a relatively low typical density, $ρ_\bhs \sim 10-10^5\Ms$ pc$^{-3}$ and composed of relatively massive BHs, with average masses in the range $m_\bhs = 14-22\Ms$. We also show that a similar approach can be used to find Milky Way GCs potentially hosting an intermediate-mass black hole.

astro-ph.GA↗

arXiv2016

Formation and evolution of heavy sub-structures in the centre of galaxy clusters: the local effect of dark energy

We discuss how the centres of galaxy clusters evolve in time, showing the results of a series of direct N-body simulations. In particular, we followed the evolution of a galaxy cluster with a mass $M_{clus} \simeq 10^{14} $M$_{\odot}$ in different configurations. The dynamical evolution of the system leads in all the cases to the formation of dense and massive sub-structures in the cluster centre, that form in consequence of a series of collisions and merging among galaxies travelling in the cluster core. We investigate how the structural properties of the main merging product depends on the characteristics of those galaxies that contributed to its formation.

astro-ph.CO↗