Merger rate density of Population III binary black holes below, above, and in the pair-instability mass gap. (arXiv:2008.01890v1 [astro-ph.HE])
<a href="http://arxiv.org/find/astro-ph/1/au:+Tanikawa_A/0/1/0/all/0/1">Ataru Tanikawa</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Susa_H/0/1/0/all/0/1">Hajime Susa</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Yoshida_T/0/1/0/all/0/1">Takashi Yoshida</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Trani_A/0/1/0/all/0/1">Alessandro A. Trani</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Kinugawa_T/0/1/0/all/0/1">Tomoya Kinugawa</a>
We present the merger rate density of Population (Pop.) III binary black
holes (BHs) by means of a widely-used binary population synthesis code {tt
BSE} with extensions to very massive and extreme metal-poor stars. We consider
not only low-mass BHs (lBHs: $5-50M_odot$) but also high-mass BHs (hBHs:
$130-200M_odot$), where lBHs and hBHs are below and above the pair-instability
mass gap ($50-130 M_odot$), respectively. Pop.~III BH-BHs can be categorized
into three subpopulations: BH-BHs without hBHs (hBH0s: $m_{rm tot} lesssim
100M_odot$), with one hBH (hBH1s: $m_{rm tot} sim 140-260M_odot$), and with
two hBHs (hBH2s: $m_{rm tot} sim 270-400M_odot$), where $m_{rm tot}$ is the
total mass of a BH-BH. Their merger rate densities at the current universe are
$sim 0.1$ yr$^{-1}$ Gpc$^{-3}$ for hBH0s, and $sim 0.01$ yr$^{-1}$ Gpc$^{-3}$
for the sum of hBH1s and hBH2s, using pessimistic Pop.~III star formation
model. These rates are modestly insensitive to initial conditions and single
star models. The hBH1 and hBH2 mergers can dominate BH-BHs with hBHs discovered
in near future. They have low effective spins $lesssim 0.2$ in the current
universe. The number ratio of the hBH2s to the hBH1s is high, $gtrsim 0.1$. We
also find BHs in the mass gap (up to $sim 85 M_odot$) merge. These merger
rates can be reduced to nearly zero if Pop.~III binaries are always wide
($gtrsim 100R_odot$), and if Pop.~III stars always enter into chemically
homogeneous evolution. The presence of close Pop.~III binaries ($sim
10R_odot$) are crucial for avoiding the worst scenario.
We present the merger rate density of Population (Pop.) III binary black
holes (BHs) by means of a widely-used binary population synthesis code {tt
BSE} with extensions to very massive and extreme metal-poor stars. We consider
not only low-mass BHs (lBHs: $5-50M_odot$) but also high-mass BHs (hBHs:
$130-200M_odot$), where lBHs and hBHs are below and above the pair-instability
mass gap ($50-130 M_odot$), respectively. Pop.~III BH-BHs can be categorized
into three subpopulations: BH-BHs without hBHs (hBH0s: $m_{rm tot} lesssim
100M_odot$), with one hBH (hBH1s: $m_{rm tot} sim 140-260M_odot$), and with
two hBHs (hBH2s: $m_{rm tot} sim 270-400M_odot$), where $m_{rm tot}$ is the
total mass of a BH-BH. Their merger rate densities at the current universe are
$sim 0.1$ yr$^{-1}$ Gpc$^{-3}$ for hBH0s, and $sim 0.01$ yr$^{-1}$ Gpc$^{-3}$
for the sum of hBH1s and hBH2s, using pessimistic Pop.~III star formation
model. These rates are modestly insensitive to initial conditions and single
star models. The hBH1 and hBH2 mergers can dominate BH-BHs with hBHs discovered
in near future. They have low effective spins $lesssim 0.2$ in the current
universe. The number ratio of the hBH2s to the hBH1s is high, $gtrsim 0.1$. We
also find BHs in the mass gap (up to $sim 85 M_odot$) merge. These merger
rates can be reduced to nearly zero if Pop.~III binaries are always wide
($gtrsim 100R_odot$), and if Pop.~III stars always enter into chemically
homogeneous evolution. The presence of close Pop.~III binaries ($sim
10R_odot$) are crucial for avoiding the worst scenario.
http://arxiv.org/icons/sfx.gif
