= Tidal capture of galaxies
Tidal capture converts sufficient relative orbital energy into internal <tidal heating> to leave two initially unbound <galaxies> on a bound orbit. For identical spherical galaxies of mass $m_g$, a distant rapid flyby of <impact parameter> $p$ and relative speed $v$ gives the approximate capture condition $pv<[(32/3)G^2m_g^2\langle r^2\rangle]^{1/4}$. This requires the <impulse approximation> and weak <gravitational focusing>; it is not a general criterion for subsequent coalescence.
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