Periodic Reporting for period 1 - GRACE-BH (Gravitational waves from crowded environments: simulating intermediate-mass black hole formation and evolution with supercomputers.)
Periodo di rendicontazione: 2022-03-01 al 2024-02-29
Intermediate-mass black holes represent an elusive class of objects that are supposed to link stellar-mass black holes, formed from the death of massive stars, and supermassive black holes, which are routinely found in the centre of galaxies.
While stellar black holes have masses from a few to tens of times the mass of the Sun (solar mass), supermassive black holes can attain masses from millions to tens of billions of solar masses.
Intermediate-mass black holes, referred to as IMBHs in what follows, are believed to sit in the middle, with masses from a hundred to a few hundred thousand solar masses.
Only a few IMBH candidates, with masses around 50,000 solar masses, have been observed in the centre of dwarf galaxies, and only a dozen controversial observations found them in globular clusters.
In 2021, the LIGO-Virgo-Kagra collaboration discovered the first IMBH with a mass of 150 solar masses, formed from the merger of two smaller black holes.
The lack of an observational smoking gun in the broad 1,000-100,000 solar mass range, however, makes it hard to assess whether IMBHs constitute a class of black holes or, rather, they are representatives
of the populations of high-mass stellar black holes and low-mass supermassive black holes. Proving that the former hypothesis is true would have implications for the formation of supermassive black holes and the
evolution of dense stellar environments.
Understanding whether IMBHs link stellar and supermassive black holes requires finding a set of processes and environments capable of supporting the formation of IMBHs through the whole 100-100,000 solar mass range.
The GRACE-BH proposes to tackle this astrophysical challenge by addressing the following question:
What are the best conditions under which an intermediate-mass black hole forms in a massive stellar system?
Utilising unprecedented, state-of-the-art numerical simulations to model the evolution of dense star clusters, the GRACE-BH project explores whether, and how, IMBHs can form and possibly grow in such extreme environments.
In brief, the GRACE-BH main objectives are:
1) quantify the impact of mass-segregation on the formation of an IMBH
2) quantify the IMBH seed survival probability
3) quantify possible gravitational-wave signatures associated with the IMBH growth process
4) determine the most favourable environments to nurture IMBHs
By achieving these objectives, the GRACE-BH project provided clear evidence of the existence of particular star cluster configurations ideal for the seeding and growth of IMBHs.
While stellar black holes have masses from a few to tens of times the mass of the Sun (solar mass), supermassive black holes can attain masses from millions to tens of billions of solar masses.
Intermediate-mass black holes, referred to as IMBHs in what follows, are believed to sit in the middle, with masses from a hundred to a few hundred thousand solar masses.
Only a few IMBH candidates, with masses around 50,000 solar masses, have been observed in the centre of dwarf galaxies, and only a dozen controversial observations found them in globular clusters.
In 2021, the LIGO-Virgo-Kagra collaboration discovered the first IMBH with a mass of 150 solar masses, formed from the merger of two smaller black holes.
The lack of an observational smoking gun in the broad 1,000-100,000 solar mass range, however, makes it hard to assess whether IMBHs constitute a class of black holes or, rather, they are representatives
of the populations of high-mass stellar black holes and low-mass supermassive black holes. Proving that the former hypothesis is true would have implications for the formation of supermassive black holes and the
evolution of dense stellar environments.
Understanding whether IMBHs link stellar and supermassive black holes requires finding a set of processes and environments capable of supporting the formation of IMBHs through the whole 100-100,000 solar mass range.
The GRACE-BH proposes to tackle this astrophysical challenge by addressing the following question:
What are the best conditions under which an intermediate-mass black hole forms in a massive stellar system?
Utilising unprecedented, state-of-the-art numerical simulations to model the evolution of dense star clusters, the GRACE-BH project explores whether, and how, IMBHs can form and possibly grow in such extreme environments.
In brief, the GRACE-BH main objectives are:
1) quantify the impact of mass-segregation on the formation of an IMBH
2) quantify the IMBH seed survival probability
3) quantify possible gravitational-wave signatures associated with the IMBH growth process
4) determine the most favourable environments to nurture IMBHs
By achieving these objectives, the GRACE-BH project provided clear evidence of the existence of particular star cluster configurations ideal for the seeding and growth of IMBHs.
To achieve the goals set by the GRACE-BH project, I initially invested a significant amount of time to implement, upgrade, and test, the NBODY6++GPU code, a direct N-body code that permits the simulation of the evolution of star clusters simultaneously taking into account both stellar evolution and dynamics.
Using the upgraded code and exploiting the JUWELS Booster supercomputer, the fastest supercomputer in Europe and fifth in the world in 2022, we gathered an unprecedented sample of 19 N-body models of young massive star clusters, referred to as DRAGON-II simulations.
In DRAGON-II models, I identified three different processes that can lead to the formation of an IMBH: 1) stellar collisions, generally involving stars with a mass larger than 100 solar masses, 2) accretion of material from a stellar relic onto a black hole, 3) black hole - black hole mergers.
These processes are not mutually exclusive. For example, an IMBH seed can initially form via stellar collisions and the resulting IMBH can later grow by feasting on passing by stars.
For the first time, we found that star clusters with a density above 300,000 solar masses per cubic parsec form IMBHs through stellar collisions on very short timescales (a few million years), whilst sparser clusters form IMBHs via black hole mergers and stellar accretion episodes on hundreds of million years.
In clusters with masses above 1-10 million solar masses and sizes ~1 parsec, a cascade of repeated black hole mergers can onset and rapidly build up IMBHs as massive as 10,000 solar masses, regardless of the IMBH initial mass.
We found a substantial population of compact binary mergers and used them to infer the cosmic merger rate of black holes from star clusters. Exotic mergers involve black holes in the so-called upper mass gap, IMBHs, white dwarfs and neutron stars. We place constraints on the detectability of such sources with current and future gravitational wave detectors, possibly jointly with electromagnetic facilities. For the first time, we predict the detection rate of pair-instability supernovae from star clusters, an elusive type of cosmic explosion expected from the death of stars more massive than 100 solar masses.
To complement the limited sample of DRAGON-II models, I also used the semi-analytic code B-POP to study the growth of IMBHs via multiple stellar black hole mergers in a statistical way. I found two possible regimes leading to IMBH formation. The first in clusters with masses in the range 1-10 million solar masses per cubic parsec and pc-scale sizes, where densities are sufficiently large to trigger an avalanche of black hole mergers that rapidly builds-up an IMBH with final masses above 10,000 solar masses. The second in clusters where stellar processes (e.g. stellar collisions) can form an IMBH seed that further grows via repeated mergers with smaller black holes.
To communicate GRACE-BH results to the broad public, I was also involved in several outreach activities.
Using the upgraded code and exploiting the JUWELS Booster supercomputer, the fastest supercomputer in Europe and fifth in the world in 2022, we gathered an unprecedented sample of 19 N-body models of young massive star clusters, referred to as DRAGON-II simulations.
In DRAGON-II models, I identified three different processes that can lead to the formation of an IMBH: 1) stellar collisions, generally involving stars with a mass larger than 100 solar masses, 2) accretion of material from a stellar relic onto a black hole, 3) black hole - black hole mergers.
These processes are not mutually exclusive. For example, an IMBH seed can initially form via stellar collisions and the resulting IMBH can later grow by feasting on passing by stars.
For the first time, we found that star clusters with a density above 300,000 solar masses per cubic parsec form IMBHs through stellar collisions on very short timescales (a few million years), whilst sparser clusters form IMBHs via black hole mergers and stellar accretion episodes on hundreds of million years.
In clusters with masses above 1-10 million solar masses and sizes ~1 parsec, a cascade of repeated black hole mergers can onset and rapidly build up IMBHs as massive as 10,000 solar masses, regardless of the IMBH initial mass.
We found a substantial population of compact binary mergers and used them to infer the cosmic merger rate of black holes from star clusters. Exotic mergers involve black holes in the so-called upper mass gap, IMBHs, white dwarfs and neutron stars. We place constraints on the detectability of such sources with current and future gravitational wave detectors, possibly jointly with electromagnetic facilities. For the first time, we predict the detection rate of pair-instability supernovae from star clusters, an elusive type of cosmic explosion expected from the death of stars more massive than 100 solar masses.
To complement the limited sample of DRAGON-II models, I also used the semi-analytic code B-POP to study the growth of IMBHs via multiple stellar black hole mergers in a statistical way. I found two possible regimes leading to IMBH formation. The first in clusters with masses in the range 1-10 million solar masses per cubic parsec and pc-scale sizes, where densities are sufficiently large to trigger an avalanche of black hole mergers that rapidly builds-up an IMBH with final masses above 10,000 solar masses. The second in clusters where stellar processes (e.g. stellar collisions) can form an IMBH seed that further grows via repeated mergers with smaller black holes.
To communicate GRACE-BH results to the broad public, I was also involved in several outreach activities.
Deploying the GRACE-BH project resulted in a series of advancements in at least three fields of research:
1) Numerical modelling of star clusters.
The numerical simulations performed during the GRACE-BH project marked a cornerstone in the field of numerical simulations.
The DRAGON-II clusters have a combined mass, density, and binary fraction never explored before. DRAGON-II models represent a new point of reference for the community. In these regards, we recently started a line of research to reproduce the DRAGON-II simulations
with less accurate but less computationally demanding Monte Carlo simulations, to study the long-term evolution of DRAGON-II clusters
2) Compact binary coalescence and transient phenomena in the "Gravitational-Wave Era".
The GRACE-BH project explored the formation of compact binary mergers in a class of star clusters poorly studied before. We have demonstrated that a significant fraction of the mergers possibly detectable at low redshift can come from star clusters and develop in dynamical environments.
This will be used in future to build more accurate semi-analytic, predictive models to interpret gravitational wave observations
3) Intermediate-mass black holes.
The works performed within the GRACE-BH project are among the first showing how the environment crucially impacts the IMBH formation processes, probability, and survivability. This can have important implications for future gravitational-wave detectors in terms of number of detections per year.
In these regards, we started a new work where, using a catalogue of IMBHs generated through the B-POP code, we will show how future detectors like the ET, LISA, and the lunar gravitational wave antenna (LGWA) may jointly reconstruct the IMBH mass spectrum and help
unravelling the true nature of IMBHs.
1) Numerical modelling of star clusters.
The numerical simulations performed during the GRACE-BH project marked a cornerstone in the field of numerical simulations.
The DRAGON-II clusters have a combined mass, density, and binary fraction never explored before. DRAGON-II models represent a new point of reference for the community. In these regards, we recently started a line of research to reproduce the DRAGON-II simulations
with less accurate but less computationally demanding Monte Carlo simulations, to study the long-term evolution of DRAGON-II clusters
2) Compact binary coalescence and transient phenomena in the "Gravitational-Wave Era".
The GRACE-BH project explored the formation of compact binary mergers in a class of star clusters poorly studied before. We have demonstrated that a significant fraction of the mergers possibly detectable at low redshift can come from star clusters and develop in dynamical environments.
This will be used in future to build more accurate semi-analytic, predictive models to interpret gravitational wave observations
3) Intermediate-mass black holes.
The works performed within the GRACE-BH project are among the first showing how the environment crucially impacts the IMBH formation processes, probability, and survivability. This can have important implications for future gravitational-wave detectors in terms of number of detections per year.
In these regards, we started a new work where, using a catalogue of IMBHs generated through the B-POP code, we will show how future detectors like the ET, LISA, and the lunar gravitational wave antenna (LGWA) may jointly reconstruct the IMBH mass spectrum and help
unravelling the true nature of IMBHs.