Merger of Dark Matter Axion Clumps and Resonant Photon Emission
  A portion of light scalar dark matter, especially axions, may organize into
gravitationally bound clumps (stars) and be present in large number in the
galaxy today. It is therefore of utmost interest to determine if there are
novel observational signatures of this scenario. Work has shown that for
moderately large axion-photon couplings, such clumps can undergo parametric
resonance into photons, for clumps above a critical mass $M^{\star}_c$
determined precisely by some of us in Ref. [1]. In order to obtain a clump
above the critical mass in the galaxy today would require mergers. In this work
we perform full 3-dimensional simulations of pairs of axion clumps and
determine the conditions under which mergers take place through the emission of
scalar waves, including analyzing head-on and non-head-on collisions, phase
dependence, and relative velocities. Consistent with other work in the
literature, we find that the final mass from the merger
$M^{\star}_{\text{final}}\approx 0.7(M^{\star}_1+M^{\star}_2)$ is larger than
each of the original clump masses (for $M^{\star}_1\sim M^{\star}_2$). Hence,
it is possible for sub-critical mass clumps to merge and become super-critical
and therefore undergo parametric resonance into photons. We find that mergers
are expected to be kinematically allowed in the galaxy today for high
Peccei-Quinn scales, which is strongly suggested by unification ideas, although
the collision rate is small. While mergers can happen for axions with lower
Peccei-Quinn scales due to statistical fluctuations in relative velocities, as
they have a high collision rate. We estimate the collision and merger rates
within the Milky Way galaxy today. We find that a merger leads to a flux of
energy on earth that can be appreciable and we mention observational search
strategies.
