  The observed binary black hole (BBH) mergers indicate a large Galactic
progenitor population continuously evolving from large orbital separations and
low gravitational wave (GW) frequencies to the final merger phase. We
investigate the equilibrium distribution of binary black holes in the Galaxy.
Given the observed BBH merger rate, we contrast the expected number of systems
radiating in the low-frequency $0.1-10\,$mHz GW band under two assumptions: (1)
that all merging systems originate from near-circular orbits, as may be
indicative of isolated binary evolution, and (2) that all merging systems
originate at very high eccentricity, as predicted by models of
dynamically-formed BBHs and triple and quadruple systems undergoing Lidov-Kozai
eccentricity oscillations. We show that the equilibrium number of systems
expected at every frequency is higher in the eccentric case (2) than in the
circular case (1) by a factor of $\simeq 2-15$. This follows from the fact that
eccentric systems spend more time than circular systems radiating in the
low-frequency GW bands. The GW emission comes in pulses at periastron separated
by the orbital period, which may be days to years. For a LISA-like sensitivity
curve, we show that if eccentric systems contribute significantly to the
observed merger rate, then $\simeq 10$ eccentric systems should be seen in the
Galaxy.
