Hierarchical mergers in young, globular and nuclear star clusters: black hole masses and merger rates. (arXiv:2007.15022v1 [astro-ph.HE])

Hierarchical mergers in young, globular and nuclear star clusters: black hole masses and merger rates. (arXiv:2007.15022v1 [astro-ph.HE])
<a href="http://arxiv.org/find/astro-ph/1/au:+Mapelli_M/0/1/0/all/0/1">Michela Mapelli</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Santoliquido_F/0/1/0/all/0/1">Filippo Santoliquido</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Bouffanais_Y/0/1/0/all/0/1">Yann Bouffanais</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Sedda_M/0/1/0/all/0/1">Manuel Arca Sedda</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Giacobbo_N/0/1/0/all/0/1">Nicola Giacobbo</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Artale_M/0/1/0/all/0/1">M. Celeste Artale</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Ballone_A/0/1/0/all/0/1">Alessandro Ballone</a>

Hierarchical mergers are one of the distinctive signatures of binary black
hole (BBH) formation through dynamical evolution. Here, we present a fast Monte
Carlo approach to simulate hierarchical mergers in nuclear star clusters
(NSCs), globular clusters (GCs) and young star clusters (YSCs). Hierarchical
mergers are orders of magnitude more common in NSCs than they are in both GCs
and YSCs, because of the different escape velocity. In our fiducial model, the
fraction of hierarchical mergers over all mergers is $sim{}0.15$,
$sim{}6times{}10^{-3}$ and $sim{}10^{-4}$ in NSCs, GCs and YSCs,
respectively. The mass distribution of hierarchical BBHs strongly depends on
the properties of first-generation BBHs, such as their progenitor’s
metallicity. In our fiducial model, we form black holes (BHs) with masses up to
$sim{}10^3$ M$_odot$ in NSCs and up to $sim{}10^2$ M$_odot$ in both GCs and
YSCs. When escape velocities in excess of 100 km s$^{-1}$ are considered, BHs
with mass $>10^3$ M$_odot$ are allowed to form in NSCs. Hierarchical mergers
lead to the formation of BHs in the pair instability mass gap and
intermediate-mass BHs (IMBHs), but only in metal-poor environments. In our
fiducial model, at metallicity $Zsim{}0.0002$, the fraction of BBH mergers
with primary BH in the pair instability mass gap is $sim{}7times{}10^{-3}$,
$3times{}10^{-4}$ and $5times{}10^{-6}$ in NSCs, GCs and YSCs, respectively.
In metal-poor NSCs, the fraction of BBH mergers with primary mass in the IMBH
regime is $sim{}5times{}10^{-4}$. The local BBH merger rate in our models
ranges from $sim{}10$ to $sim{} 60$ Gpc$^{-3}$ yr$^{-1}$; hierarchical BBHs
in NSCs account for $sim{}10^{-2}- 0.2$ Gpc$^{-3}$ yr$^{-1}$, with a strong
upper limit of $sim{}10$ Gpc$^{-3}$ yr$^{-1}$.

Hierarchical mergers are one of the distinctive signatures of binary black
hole (BBH) formation through dynamical evolution. Here, we present a fast Monte
Carlo approach to simulate hierarchical mergers in nuclear star clusters
(NSCs), globular clusters (GCs) and young star clusters (YSCs). Hierarchical
mergers are orders of magnitude more common in NSCs than they are in both GCs
and YSCs, because of the different escape velocity. In our fiducial model, the
fraction of hierarchical mergers over all mergers is $sim{}0.15$,
$sim{}6times{}10^{-3}$ and $sim{}10^{-4}$ in NSCs, GCs and YSCs,
respectively. The mass distribution of hierarchical BBHs strongly depends on
the properties of first-generation BBHs, such as their progenitor’s
metallicity. In our fiducial model, we form black holes (BHs) with masses up to
$sim{}10^3$ M$_odot$ in NSCs and up to $sim{}10^2$ M$_odot$ in both GCs and
YSCs. When escape velocities in excess of 100 km s$^{-1}$ are considered, BHs
with mass $>10^3$ M$_odot$ are allowed to form in NSCs. Hierarchical mergers
lead to the formation of BHs in the pair instability mass gap and
intermediate-mass BHs (IMBHs), but only in metal-poor environments. In our
fiducial model, at metallicity $Zsim{}0.0002$, the fraction of BBH mergers
with primary BH in the pair instability mass gap is $sim{}7times{}10^{-3}$,
$3times{}10^{-4}$ and $5times{}10^{-6}$ in NSCs, GCs and YSCs, respectively.
In metal-poor NSCs, the fraction of BBH mergers with primary mass in the IMBH
regime is $sim{}5times{}10^{-4}$. The local BBH merger rate in our models
ranges from $sim{}10$ to $sim{} 60$ Gpc$^{-3}$ yr$^{-1}$; hierarchical BBHs
in NSCs account for $sim{}10^{-2}- 0.2$ Gpc$^{-3}$ yr$^{-1}$, with a strong
upper limit of $sim{}10$ Gpc$^{-3}$ yr$^{-1}$.

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