Dark matter constraints with stacked gamma rays scales with the number
  of galaxies
  Low-surface-brightness galaxies (LSBGs) are interesting targets for searches
of dark matter emission due to their low baryonic content. However, predicting
their expected dark matter emissivities is difficult because of observational
challenges in their distance measurements. Here we present a stacking method
that makes use of catalogs of LSBGs and maps of unresolved gamma-ray emission
measured by the Fermi Gamma-Ray Space Telescope. We show that, for relatively
large number of LSBGs, individual distance measurements to the LSBGs are not
necessary, instead the overall distance distribution of the population is
sufficient in order to impose dark matter constraints. Further, we demonstrate
that the effect of the covariance between two galaxies located closely -- at an
angular distance comparable to the size of the Fermi point spread function --
is negligibly small. As a case in point, we apply our pipeline to a sample of
800 faint LSBGs discovered by Hyper Suprime-Cam and find that, the 95 per cent
confidence level upper limits on the dark matter annihilation cross-section
scales with inverse of the number LSBGs. In light of this linear dependence
with the number of objects, we argue this methodology could provide extremely
powerful limits if it is applied to the more than 105 LSBGs readily available
with the Legacy Survey of Space and Time.
