= Solution
At small separations the extended <dark-matter haloes> overlap, and <Chandrasekhar dynamical friction> transfers orbital <energy> to gravitational wakes while tidal interactions also redistribute orbital <energy>. A characteristic decay time scales as $t_{\rm df}\sim v r^2/(Gm\ln\Lambda)$, so sufficiently massive close pairs evolve rapidly towards a <galaxy merger> and spend relatively little time at small separation. \b[Short residence time depletes the close-pair population]; the quoted separation scale is characteristic of the population, not a universal exclusion radius.
Back to article page
