The rate of stellar mass black hole scattering in galactic nuclei. (arXiv:1902.03242v1 [astro-ph.HE])
<a href="http://arxiv.org/find/astro-ph/1/au:+Rasskazov_A/0/1/0/all/0/1">Alexander Rasskazov</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Kocsis_B/0/1/0/all/0/1">Bence Kocsis</a>
We consider a black hole (BH) density cusp in a nuclear star cluster (NSC)
hosting a supermassive back hole (SMBH) at its center. Assuming the stars and
BHs inside the SMBH sphere of influence are mass-segregated, we calculate the
number of BHs that sink into this region under the influence of dynamical
friction. We find that the total number of BHs increases significantly in this
region due to this process for lower mass SMBHs by up to a factor of 5, but
there is no increase in the vicinity of the highest mass SMBHs. Due to the high
BH number density in the NSC, BH-BH binaries form during close approaches due
to GW emission. We update the previous estimate of O’Leary et al. for the rate
of such GW capture events by estimating the $langle n^2rangle/langle
nrangle^2$ parameter where $n$ is the number density. We find a BH merger rate
for this channel to be in the range $sim0.01-0.1 , mathrm{Gpc^{-3}yr^{-1}}$.
The total merger rate is dominated by the smallest galaxies hosting SMBHs, and
the number of heaviest BHs in the NSC. It is also exponentially sensitive to
the radial number density profile exponent, reaching $>100 ,
mathrm{Gpc^{-3}yr^{-1}}$ when the BH mass function is $m^{-2.3}$ or shallower
and the heaviest BH radial number density is close to $r^{-3}$. Even if the
rate is much lower than the range constrained by the current LIGO detections,
the GW captures around SMBHs can be distinguished by their high eccentricity in
the LIGO band.
We consider a black hole (BH) density cusp in a nuclear star cluster (NSC)
hosting a supermassive back hole (SMBH) at its center. Assuming the stars and
BHs inside the SMBH sphere of influence are mass-segregated, we calculate the
number of BHs that sink into this region under the influence of dynamical
friction. We find that the total number of BHs increases significantly in this
region due to this process for lower mass SMBHs by up to a factor of 5, but
there is no increase in the vicinity of the highest mass SMBHs. Due to the high
BH number density in the NSC, BH-BH binaries form during close approaches due
to GW emission. We update the previous estimate of O’Leary et al. for the rate
of such GW capture events by estimating the $langle n^2rangle/langle
nrangle^2$ parameter where $n$ is the number density. We find a BH merger rate
for this channel to be in the range $sim0.01-0.1 , mathrm{Gpc^{-3}yr^{-1}}$.
The total merger rate is dominated by the smallest galaxies hosting SMBHs, and
the number of heaviest BHs in the NSC. It is also exponentially sensitive to
the radial number density profile exponent, reaching $>100 ,
mathrm{Gpc^{-3}yr^{-1}}$ when the BH mass function is $m^{-2.3}$ or shallower
and the heaviest BH radial number density is close to $r^{-3}$. Even if the
rate is much lower than the range constrained by the current LIGO detections,
the GW captures around SMBHs can be distinguished by their high eccentricity in
the LIGO band.
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