  We investigate the contribution of outer HI disks to the observable
population of merging black hole binaries. Like dwarf galaxies, the outer HI
disks of spirals have low star formation rates and lower metallicities than the
inner disks of spirals. Since low-metallicity star formation can produce more
detectable compact binaries than typical star formation, the environments in
the outskirts of spiral galaxies may be conducive to producing a rich
population of massive binary black holes. We consider here both detailed
controlled simulations of spirals and cosmological simulations, as well as the
current range of observed values for metallicity and star formation in outer
disks. We find that outer HI disks contribute at least as much as dwarf
galaxies do to the observed LIGO/Virgo detection rates. Identifying the host
galaxies of merging massive black holes should provide constraints on
cosmological parameters and insights into the formation channels of binary
mergers.
