arXiv subjects

Michela Mapelli

Publications and source records attributed to Michela Mapelli.

At least 127 records · Page 7Linked to original sources

arXiv2019

Merging black holes in young star clusters

Searching for distinctive signatures, which characterize different formation channels of binary black holes (BBHs), is a crucial step towards the interpretation of current and future gravitational wave detections. Here, we investigate the demography of merging BBHs in young star clusters (SCs), which are the nursery of massive stars. We performed $4\times{} 10^3$ N-body simulations of SCs with metallicity $Z=0.002$, initial binary fraction $0.4$ and fractal initial conditions, to mimic the clumpiness of star forming regions. Our simulations include a novel population-synthesis approach based on the code MOBSE. We find that SC dynamics does not affect the merger rate significantly, but leaves a strong fingerprint on the properties of merging BBHs. More than 50 % of merging BBHs in young SCs form by dynamical exchanges in the first few Myr. Dynamically formed merging BBHs are significantly heavier than merging BBHs in isolated binaries: merging BBHs with total mass up to $\sim{}120$ M$_\odot$ form in young SCs, while the maximum total mass of merging BBHs in isolated binaries with the same metallicity is only $\sim{}70$ M$_\odot$. Merging BBHs born via dynamical exchanges tend to have smaller mass ratios than BBHs in isolated binaries. Furthermore, SC dynamics speeds up the merger: the delay time between star formation and coalescence is significantly shorter in young SCs. In our simulations, massive systems such as GW170729 form only via dynamical exchanges. Finally $\sim{}2$ % of merging BBHs in young SCs have mass in the pair-instability mass gap ($\sim{}60-120$ M$_\odot$). This represents a unique fingerprint of merging BBHs in SCs.

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arXiv2018

The impact of electron-capture supernovae on merging double neutron stars

Natal kicks are one of the most debated issues about double neutron star (DNS) formation. Several observational and theoretical results suggest that some DNSs have formed with low natal kicks ($\lesssim{}50$ km s$^{-1}$), which might be attributed to electron-capture supernovae (ECSNe). We investigate the impact of ECSNe on the formation of DNSs by means of population synthesis simulations. In particular, we assume a Maxwellian velocity distribution for the natal kick induced by ECSNe with one dimensional root-mean-square $σ_{\rm ECSN} = 0,7,15,26,265$ km s$^{-1}$. The total number of DNSs scales inversely with $σ_{\rm ECSN}$ and the number of DNS mergers is higher for relatively low kicks. This effect is particularly strong if we assume low efficiency of common-envelope ejection (described by the parameter $α=1$), while it is only mild for high efficiency of common-envelope ejection ($α=5$). In most simulations, more than 50 per cent of the progenitors of merging DNSs undergo at least one ECSN and the ECSN is almost always the first SN occurring in the binary system. Finally, we have considered the extreme case in which all neutron stars receive a low natal kick ($\lesssim{}50$~km~s$^{-1}$). In this case, the number of DNSs increases by a factor of ten and the percentage of merging DNSs which went through an ECSN is significantly suppressed ($<40$ per cent).

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arXiv2018

The High Mass X-ray Binaries in star-forming galaxies

The high mass X-ray binaries (HMXBs) provide an exciting framework to investigate the evolution of massive stars and the processes behind binary evolution. HMXBs have shown to be good tracers of recent star formation in galaxies and might be important feedback sources at early stages of the Universe. Furthermore, HMXBs are likely the progenitors of gravitational wave sources (BH--BH or BH--NS binaries that may merge producing gravitational waves). In this work, we investigate the nature and properties of HMXB population in star-forming galaxies. We combine the results from the population synthesis model MOBSE (Giacobbo et al. 2018) together with galaxy catalogs from EAGLE simulation (Schaye et al. 2015). Therefore, this method describes the HMXBs within their host galaxies in a self-consistent way. We compute the X-ray luminosity function (XLF) of HMXBs in star-forming galaxies, showing that this methodology matches the main features of the observed XLF.

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arXiv2018

Astrophysics of stellar black holes

On September 14 2015, the LIGO interferometers captured a gravitational wave (GW) signal from two merging black holes (BHs), opening the era of GW astrophysics. Five BH mergers have been reported so far, three of them involving massive BHs ($\gtrsim{}30$ M$_\odot$). According to stellar evolution models, such massive BHs can originate from massive, relatively metal-poor stars. The formation channels of merging BH binaries are still an open question: a plethora of uncertainties affect the evolution of massive stellar binaries (e.g. the process of common envelope) and their dynamics. This review aims to discuss the open questions about BH binaries, and to present the state-of-the-art knowledge about the astrophysics of BHs to non-specialists, in light of the first LIGO-Virgo detections.

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arXiv2018

The host galaxies of double compact objects merging in the local Universe

We investigate the host galaxies of compact objects merging in the local Universe, by combining the results of binary population-synthesis simulations with the Illustris cosmological box. Double neutron stars (DNSs) merging in the local Universe tend to form in massive galaxies (with stellar mass $>10^{9}$ M$_\odot$) and to merge in the same galaxy where they formed, with a short delay time between the formation of the progenitor stars and the DNS merger. In contrast, double black holes (DBHs) and black hole $-$ neutron star binaries (BHNSs) form preferentially in small galaxies (with stellar mass $<10^{10}$ M$_\odot$) and merge either in small or in larger galaxies, with a long delay time. This result is an effect of metallicity: merging DBHs and BHNSs form preferentially from metal-poor progenitors ($Z\leq{}0.1$ Z$_\odot$), which are more common in high-redshift galaxies and in local dwarf galaxies, whereas merging DNSs are only mildly sensitive to progenitor's metallicity and thus are more abundant in massive galaxies nowadays. The mass range of DNS hosts we predict in this work is consistent with the mass range of short gamma-ray burst hosts.

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arXiv2018

Weighing the IMBH candidate CO-0.40-0.22* in the Galactic Centre

The high velocity gradient observed in the compact cloud CO-0.40-0.22, at a projected distance of 60 pc from the centre of the Milky Way, has led its discoverers to identify the closeby mm continuum emitter, CO-0.40-0.22*, with an intermediate mass black hole (IMBH) candidate. We describe the interaction between CO-0.40-0.22 and the IMBH, by means of a simple analytical model and of hydrodynamical simulations. Through such calculation, we obtain a lower limit to the mass of CO-0.40-0.22* of few $10^4 \times \; M_{\odot}$. This result tends to exclude the formation of such massive black hole in the proximity of the Galactic Centre. On the other hand, CO-0.40-0.22* might have been brought to such distances in cosmological timescales, if it was born in a dark matter halo or globular cluster around the Milky Way.

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arXiv2018

Lecture notes on black hole binary astrophysics

We describe some key astrophysical processes driving the formation and evolution of black hole binaries of different nature, from stellar-mass to supermassive systems. In the first part, we focus on the mainstream channels proposed for the formation of stellar mass binaries relevant to ground-based gravitational wave detectors, namely the {\it field} and the {\it dynamical} scenarios. For the field scenario, we highlight the relevant steps in the evolution of the binary, including mass transfer, supernovae explosions and kicks, common envelope and gravitational wave emission. For the dynamical scenario, we describe the main physical processes involved in the formation of star clusters and the segregation of black holes in their centres. We then identify the dynamical processes leading to binary formation, including three-body capture, exchanges and hardening. The second part of the notes is devoted to massive black hole formation and evolution, including the physics leading to mass accretion and binary formation. Throughout the notes, we provide several step-by-step pedagogical derivations, that should be particularly suited to undergraduates and PhD students, but also to gravitational wave physicists interested in approaching the subject of gravitational wave sources from an astrophysical perspective.

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arXiv2018

Black hole demography at the dawn of gravitational-wave astronomy: state of the art and future perspectives

The first LIGO-Virgo detections have confirmed the existence of massive black holes (BHs), with mass $30-40$ M$_\odot$. Such BHs might originate from massive metal-poor stars ($Z<0.3$ Z$_\odot$) or from gravitational instabilities in the early Universe. The formation channels of merging BHs are still poorly constrained. The measure of mass, spin and redshift distribution of merging BHs will give us fundamental clues to distinguish between different models. Also, a better understanding of several astrophysical processes (e.g. common envelope, core-collapse supernovae, and dynamical evolution of BHs) is decisive, to shed light on the formation channels of merging BHs.

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arXiv2018

Forming circumnuclear disks and rings in galactic nuclei: a competition between supermassive black hole and nuclear star cluster

We investigate the formation of circumnuclear gas structures from the tidal disruption of molecular clouds in galactic nuclei, by means of smoothed particle hydrodynamics simulations. We model galactic nuclei as composed of a supermassive black hole (SMBH) and a nuclear star cluster (NSC) and consider different mass ratios between the two components. We find that the relative masses of the SMBH and the NSC have a deep impact on the morphology of the circumnuclear gas. Extended disks form only inside the sphere of influence of the SMBH. In contrast, compact rings naturally form outside the SMBH's sphere of influence, where the gravity is dominated by the NSC. This result is in agreement with the properties of the Milky Way's circumnuclear ring, which orbits outside the SMBH sphere of influence. Our results indicate that compact circumnuclear rings can naturally form outside the SMBH sphere of influence.

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arXiv2018

The Maxwell's demon of star clusters

Stellar binaries are the most important energy reservoir of star clusters. Via three-body encounters, binaries can reverse the core collapse and prevent a star cluster from reaching equipartition. Moreover, binaries are essential for the formation of stellar exotica, such as blue straggler stars, intermediate-mass black holes and massive ($\gtrsim{}30$ M$_\odot$) black hole binaries.

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arXiv2018

Unravelling the progenitors of merging black hole binaries

The recent detection of gravitational waves has proven the existence of massive stellar black hole binaries (BBHs), but the formation channels of BBHs are still an open question. Here, we investigate the demography of BBHs by using our new population-synthesis code MOBSE. MOBSE is an updated version of the widely used binary population-synthesis code BSE (Hurley et al. 2000, Hurley et al. 2002) and includes the key ingredients to determine the fate of massive stars: up-to-date stellar wind prescriptions and supernova models. With MOBSE, we form BBHs with total mass up to $\sim{}120$ M$_\odot$ at low metallicity, but only systems with total mass up to $\sim{}80$ M$_\odot$ merge in less than a Hubble time. Our results show that only massive metal-poor stars ($Z\lesssim 0.002$) can be the progenitors of gravitational wave events like GW150914. Moreover, we predict that merging BBHs form much more efficiently from metal-poor than from metal-rich stars.

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arXiv2018

The cosmic merger rate of neutron stars and black holes

Six gravitational wave detections have been reported so far, providing crucial insights on the merger rate of double compact objects. We investigate the cosmic merger rate of double neutron stars (DNSs), neutron star-black hole binaries (NSBHs) and black hole binaries (BHBs) by means of population-synthesis simulations coupled with the Illustris cosmological simulation. We have performed six different simulations, considering different assumptions for the efficiency of common envelope (CE) ejection and exploring two distributions for the supernova (SN) kicks. The current BHB merger rate derived from our simulations spans from $\sim{}150$ to $\sim{}240$ Gpc$^{-3}$ yr$^{-1}$ and is only mildly dependent on CE efficiency. In contrast, the current merger rates of DNSs (ranging from $\sim{}20$ to $\sim{}600$ Gpc$^{-3}$ yr$^{-1}$) and NSBHs (ranging from $\sim{}10$ to $\sim{}100$ Gpc$^{-3}$ yr$^{-1}$) strongly depend on the assumptions on CE and natal kicks. The merger rate of DNSs is consistent with the one inferred from the detection of GW170817 only if a high efficiency of CE ejection and low SN kicks (drawn from a Maxwellian distribution with one dimensional root mean square $σ=15$ km s$^{-1}$) are assumed.

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arXiv2018

The X-Ray Luminosity Function of Ultra Luminous X-Ray Sources in Collisional Ring Galaxies

Ring galaxies are fascinating laboratories: a catastrophic impact between two galaxies (one not much smaller than the other) has produced fireworks especially in the larger one, when hit roughly perpendicularly to the plane. We analyze the point sources, produced by the starburst episode following the impact, in the rings of seven galaxies and determine their X-ray luminosity function (XLF). In total we detect 63 sources, of which 50 have luminosity L$_X \geq 10^{39}$ erg s$^{-1}$, classifying them as ultra luminous X-ray sources (ULXs). We find that the total XLF is not significantly different from XLFs derived for other kinds of galaxies, with a tendency of having a larger fraction of high X-ray luminosity objects. Both the total number of ULXs and the number of ULXs per unit star formation rate are found in the upper envelope of the more normal galaxies distribution. Further analysis would be needed to address the issue of the nature of the compact component in the binary system.

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arXiv2018

Star Cluster Disruption by a Supermassive Black Hole Binary

Supermassive black hole binaries (BHBs) are expected to be one of the most powerful sources of low-frequency gravitational waves (GWs) for future space-borne detectors. Prior to the GW emission stage, BHBs evolving in gas-poor nuclei shrink primarily through the slingshot ejection of stars approaching the BHB from sufficiently close distances. Here we address the possibility that the BHB shrinking rate is enhanced through the infall of a star cluster (SC) onto the BHB. We present the results of direct summation N-body simulations exploring different orbits for the SC infall, and we show that SCs reaching the BHB on non-zero angular momentum orbits (with eccentricity 0.75) fail to enhance the BHB hardening, while SCs approaching the BHB on radial orbits reduce the BHB separation by ~10% in less than 10 Myr, effectively shortening the BHB path towards GWs.

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arXiv2017

Merging black hole binaries: the effects of progenitor's metallicity, mass-loss rate and Eddington factor

The first four gravitational wave events detected by LIGO were all interpreted as merging black hole binaries (BHBs), opening a new perspective on the study of such systems. Here we use our new population-synthesis code MOBSE, an upgraded version of BSE (Hurley et al. 2002), to investigate the demography of merging BHBs. MOBSE includes metallicity-dependent prescriptions for mass loss of massive hot stars. It also accounts for the impact of the electron-scattering Eddington factor on mass loss. We perform >10^8 simulations of isolated massive binaries, with 12 different metallicities, to study the impact of mass loss, core-collapse supernovae and common envelope on merging BHBs. Accounting for the dependence of stellar winds on the Eddington factor leads to the formation of black holes (BHs) with mass up to 65 Msun at metallicity Z~0.0002. However, most BHs in merging BHBs have masses <40 Msun. We find merging BHBs with mass ratios in the 0.1 - 1.0 range, even if mass ratios >0.6 are more likely. We predict that systems like GW150914, GW170814 and GW170104 can form only from progenitors with metallicity Z<=0.006, Z<=0.008 and Z<=0.012, respectively. Most merging BHBs have gone through a common envelope phase, but up to ~17 per cent merging BHBs at low metallicity did not undergo any common envelope phase. We find a much higher number of mergers from metal-poor progenitors than from metal-rich ones: the number of BHB mergers per unit mass is ~10^-4 Msun^-1 at low metallicity (Z = 0.0002 - 0.002) and drops to ~10^-7 Msun^-1 at high metallicity (Z ~ 0.02).

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arXiv2017

Star cluster disruption by a massive black hole binary

Massive black hole binaries (BHBs) are expected to form as the result of galaxy mergers; they shrink via dynamical friction and stellar scatterings, until gravitational waves (GWs) bring them to the final coalescence. It has been argued that BHBs may stall at a parsec scale and never enter the GW stage if stars are not continuously supplied to the BHB loss cone. Here we perform several N-body experiments to study the effect of an 80,000 solar masses stellar cluster (SC) infalling on a parsec-scale BHB. We explore different orbital elements for the SC and we perform runs both with and without accounting for the influence of a rigid stellar cusp (modelled as a rigid Dehnen potential). We find that the semi-major axis of the BHB shrinks by more than 10 per cent if the SC is on a nearly radial orbit; the shrinking is more efficient when a Dehnen potential is included and the orbital plane of the SC coincides with that of the BHB. In contrast, if the SC orbit has non-zero angular momentum, only a few stars enter the BHB loss cone and the resulting BHB shrinking is negligible. Our results indicate that SC disruption might significantly contribute to the shrinking of a parsec-scale BHB only if the SC approaches the BHB on a nearly radial orbit.

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arXiv2017

The cosmic merger rate of stellar black hole binaries from the Illustris simulation

The cosmic merger rate density of black hole binaries (BHBs) can give us an essential clue to constraining the formation channels of BHBs, in light of current and forthcoming gravitational wave detections. Following a Monte Carlo approach, we couple new population-synthesis models of BHBs with the Illustris cosmological simulation, to study the cosmic history of BHB mergers. We explore six population-synthesis models, varying the prescriptions for supernovae, common envelope, and natal kicks. In most considered models, the cosmic BHB merger rate follows the same trend as the cosmic star formation rate. The normalization of the cosmic BHB merger rate strongly depends on the treatment of common envelope and on the distribution of natal kicks. We find that most BHBs merging within LIGO's instrumental horizon come from relatively metal-poor progenitors (<0.2 Zsun). The total masses of merging BHBs span a large range of values, from ~6 to ~82 Msun. In our fiducial model, merging BHBs consistent with GW150914, GW151226 and GW170104 represent ~6, 3, and 12 per cent of all BHBs merging within the LIGO horizon, respectively. The heavy systems, like GW150914, come from metal-poor progenitors (<0.15 Zsun). Most GW150914-like systems merging in the local Universe appear to have formed at high redshift, with a long delay time. In contrast, GW151226-like systems form and merge all the way through the cosmic history, from progenitors with a broad range of metallicities. Future detections will be crucial to put constraints on common envelope, on natal kicks, and on the BHB mass function.

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arXiv2017

The formation and coalescence sites of the first gravitational wave events

We present a novel theoretical model to characterize the formation and coalescence sites of compact binaries in a cosmological context. This is based on the coupling between the binary population synthesis code SeBa with a simulation following the formation of a Milky Way-like halo in a well resolved cosmic volume of 4 cMpc, performed with the GAMESH pipeline. We have applied this technique to investigate when and where systems with properties similar to the recently observed LIGO/VIRGO events are more likely to form and where they are more likely to reside when they coalesce. We find that more than 70% of GW151226 and LVT151012-like systems form in galaxies with stellar mass M* > 10^8 Msun in the redshift range [0.06 - 3] and [0.14 - 11.3], respectively. All GW150914-like systems form in low-metallicity dwarfs with M* < 5 10^6 Msun at 2.4 < z < 4.2. Despite these initial differences, by the time they reach coalescence the observed events are most likely hosted by star forming galaxies with M* > 10^{10} Msun. Due to tidal stripping and radiative feedback, a non negligible fraction of GW150914-like candidates end-up in galaxies with properties similar to dwarf spheroidals and ultra-faint satellites.

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