Improved constraints from ultra-faint dwarf galaxies on primordial black
  holes as dark matter
  Soon after the recent first ever detection of gravitational waves from
merging black holes it has been suggested that their origin is primordial.
Appealingly, a sufficient number of primordial black holes (PBHs) could also
partially or entirely constitute the dark matter (DM) in the Universe. However,
recent studies on PBHs in ultra-faint dwarf galaxies (UFDGs) suggest that they
would dynamically heat up the stellar component due to two-body relaxation
processes. From the comparison with the observed stellar velocity dispersions
and the stellar half-light radii it was claimed that only PBHs with masses
$\lesssim10\,M_\odot$ can significantly contribute to the DM. In this work, we
improve the latter constraints by considering the largest observational sample
of UFDGs and by allowing the PBH masses to follow an extended (log-normal)
distribution. By means of collisional Fokker-Planck simulations, we explore a
wide parameter space of UFDGs containing PBHs. The analysis of the half-light
radii and velocity dispersions resulting from the simulations leads to three
general findings that exclude PBHs with masses
$\sim\mathcal{O}(1$-$100)\,M_\odot$ from constituting all of the DM: (i) We
identify a critical sub-sample of UFDGs that only allows for
$\sim\mathcal{O}(1)\,M_\odot$ PBH masses; (ii) for any PBH mass, there is an
UFDG in our sample that disfavours it; (iii) the spatial extensions of a
majority of simulated UFDGs containing PBHs are too large to match the
observed.
