The Supersonic Project: SIGOs, a Proposed Progenitor to Globular
  Clusters, and their Connections to Gravitational Wave Anisotropies
  Supersonically Induced Gas Objects (SIGOs), are structures with little to no
dark matter component predicted to exist in regions of the Universe with large
relative velocities between baryons and dark matter at the time of
recombination. They have been suggested to be the progenitors of present-day
globular clusters. Using simulations, SIGOs have been studied on small scales
(around 2 Mpc), where these relative velocities are coherent. However, it is
challenging to study SIGOs using simulations on large scales due to the varying
relative velocities at scales larger than a few Mpc. Here, we study SIGO
abundances semi-analytically: using perturbation theory, we predict the number
density of SIGOs analytically, and compare these results to small-box numerical
simulations. We use the agreement between the numerical and analytic
calculations to extrapolate the large-scale variation of SIGO abundances over
different stream velocities. As a result, we predict similar large-scale
variations of objects with high gas densities before reionization that could
possibly be observed by JWST. If indeed SIGOs are progenitors of globular
clusters, then we expect a similar variation of globular cluster abundances
over large scales. Significantly, we find that the expected number density of
SIGOs is consistent with observed globular cluster number densities. As a
proof-of-concept, and because globular clusters were proposed to be natural
formation sites for gravitational wave sources from binary black hole (BBH)
mergers, we show that SIGOs should imprint an anisotropy on the gravitational
wave signal on the sky, consistent with SIGOs' distribution.
