9 research outputs found
The Dragon-II simulations -- II. Formation mechanisms, mass, and spin of intermediate-mass black holes in star clusters with up to 1 million stars
The processes that govern the formation of intermediate-mass black holes
(IMBHs) in dense stellar clusters are still unclear. Here, we discuss the role
of stellar mergers, star-BH interactions and accretion, as well as BH binary
(BBH) mergers in seeding and growing IMBHs in the \textsc{Dragon-II} simulation
database, a suite of 19 direct -body models representing dense clusters with
up to stars. \textsc{Dragon-II} IMBHs have typical masses of M and relatively large spins . We find a link between the IMBH formation mechanism and the cluster
structure. In clusters denser than M pc, the
collapse of massive star collision products represents the dominant IMBH
formation process, leading to the formation of heavy IMBHs ( M), possibly slowly rotating, that form over times Myr and
grow further via stellar accretion and mergers in just Myr. BBH mergers
are the dominant IMBH formation channel in less dense clusters, for which we
find that the looser the cluster, the longer the formation time ( Myr)
and the larger the IMBH mass, although remaining within M. Strong
dynamical scatterings and relativistic recoil efficiently eject all IMBHs in
\textsc{Dragon-II} clusters, suggesting that IMBHs in this type of cluster are
unlikely to grow beyond a few M.Comment: 15 pages, 6 figures, 2 tables, 1 appendix. Comments welcome.
Submitted to MNRA
The Dragon-II simulations -- III. Compact binary mergers in clusters with up to 1 million stars: mass, spin, eccentricity, merger rate and pair instability supernovae rate
Compact binary mergers forming in star clusters may exhibit distinctive
features that can be used to identify them among observed gravitational-wave
(GW) sources. Such features likely depend on the host cluster structure and the
physics of massive star evolution. Here, we dissect the population of compact
binary mergers in the \textsc{Dragon-II} simulation database, a suite of 19
direct -body models representing dense star clusters with up to stars
and of stars in primordial binaries. We find a substantial population
of black hole binary (BBH) mergers, some of them involving an intermediate-mass
BH (IMBH), and a handful mergers involving a stellar BH and either a neutron
star (NS) or a white dwarf (WD). Primordial binary mergers, of the
whole population, dominate ejected mergers. Dynamical mergers, instead,
dominate the population of in-cluster mergers and are systematically heavier
than primordial ones. Around of \textsc{Dragon-II} mergers are eccentric
in the LISA band and in the LIGO band. We infer a mean cosmic merger rate
of yr Gpc for BBHs, NS-BH, and WD-BH
binary mergers, respectively, and discuss the prospects for multimessenger
detection of WD-BH binaries with LISA. We model the rate of pair-instability
supernovae (PISNe) in star clusters and find that surveys with a limiting
magnitude can detect yr PISNe. Comparing
these estimates with future observations could help to pin down the impact of
massive star evolution on the mass spectrum of compact stellar objects in star
clusters.Comment: 22 pages, 14 figures, 3 tables. Comments welcome. Submitted to MNRA
The Dragon-II simulations -- I. Evolution of single and binary compact objects in star clusters with up to 1 million stars
We present the first results of the \textsc{Dragon-II} simulations, a suite
of 19 -body simulations of star clusters with up to stars, with up to
of them initially paired in binaries. In this work, we describe the main
evolution of the clusters and their compact objects (COs). All
\textsc{Dragon-II} clusters form in their centre a black hole (BH) subsystem
with a density times larger than the stellar density, with the cluster
core containing of the whole BH population. In all models, the BH
average mass steeply decreases as a consequence of BH burning, reaching values
M within relaxation times.
Generally, our clusters retain only BHs lighter than M over
relaxation times. Looser clusters retain a higher binary fraction, because in
such environments binaries are less likely disrupted by dynamical encounters.
We find that BH-main sequence star binaries have properties similar to recently
observed systems. Double CO binaries (DCOBs) ejected from the cluster exhibit
larger mass ratios and heavier primary masses than ejected binaries hosting a
single CO (SCOBs). Ejected SCOBs have BH masses M,
definitely lower than those in DCOBs ( M).Comment: 22 pages, 21 figures, 4 tables. Comments welcome. Submitted to MNRA
PHANGS-JWST First Results: A combined HST and JWST analysis of the nuclear star cluster in NGC 628
We combine archival HST and new JWST imaging data, covering the ultraviolet
to mid-infrared regime, to morphologically analyze the nuclear star cluster
(NSC) of NGC 628, a grand-design spiral galaxy. The cluster is located in a 200
pc x 400 pc cavity, lacking both dust and gas. We find roughly constant values
for the effective radius (r_eff ~ 5 pc) and ellipticity ({\epsilon} ~ 0.05),
while the S\'ersic index (n) and position angle (PA) drop from n ~ 3 to ~ 2 and
PA ~ 130{\deg} to 90{\deg}, respectively. In the mid-infrared, r_eff ~ 12pc,
{\epsilon} ~ 0.4, and n ~ 1-1.5, with the same PA ~ 90{\deg}. The NSC has a
stellar mass of log10 (M_nsc / M_Sun) = 7.06 +- 0.31, as derived through B-V,
confirmed when using multi-wavelength data, and in agreement with the
literature value. Fitting the spectral energy distribution, excluding the
mid-infrared data, yields a main stellar population's age of (8 +- 3) Gyr with
a metallicity of Z = 0.012 +- 0.006. There is no indication of any significant
star formation over the last few Gyr. Whether gas and dust were dynamically
kept out or evacuated from the central cavity remains unclear. The best-fit
suggests an excess of flux in the mid-infrared bands, with further indications
that the center of the mid-infrared structure is displaced with respect to the
optical center of the NSC. We discuss five potential scenarios, none of them
fully explaining both the observed photometry and structure.Comment: 26 pages, 10 figures, 6 tables. Accepted for publication by ApJ
The DRAGON-II simulations – III. Compact binary mergers in clusters with up to 1 million stars : mass, spin, eccentricity, merger rate, and pair instability supernovae rate
Compact binary mergers forming in star clusters may exhibit distinctive features that can be used to identify them among observed gravitational-wave sources. Such features likely depend on the host cluster structure and the physics of massive star evolution. Here, we dissect the population of compact binary mergers in the DRAGON-II simulation data base, a suite of 19 direct N-body models representing dense star clusters with up to 106 stars and < 33 per cent of stars in primordial binaries. We find a substantial population of black hole binary (BBH) mergers, some of them involving an intermediate-mass BH (IMBH), and a handful mergers involving a stellar BH and either a neutron star (NS) or a white dwarf (WD). Primordial binary mergers, ∼ 30 per cent of the whole population, dominate ejected mergers. Dynamical mergers, instead, dominate the population of in-cluster mergers and are systematically heavier than primordial ones. Around 20 per cent of DRAGON-II mergers are eccentric in the Laser Interferometer Space Antenna (LISA) band and 5 per cent in the LIGO band. We infer a mean cosmic merger rate of R ∼ 30(4.4)(1.2) yr−1 Gpc−3 for BBHs, NS–BH, and WD–BH binary mergers, respectively, and discuss the prospects for multimessenger detection of WD–BH binaries with LISA. We model the rate of pair-instability supernovae (PISNe) in star clusters and find that surveys with a limiting magnitude mbol = 25 can detect ∼1–15 yr−1 PISNe. Comparing these estimates with future observations could help to pin down the impact of massive star evolution on the mass spectrum of compact stellar objects in star clusters.Peer reviewe