A first order dark $SU(2)_D$ phase transition with vector dark matter in
  the light of NANOGrav 12.5 yr data
  We study a dark $SU(2)_D$ gauge extension of the standard model (SM) with the
possibility of a strong first order phase transition (FOPT) taking place below
the electroweak scale in the light of NANOGrav 12.5 yr data. As pointed out
recently by the NANOGrav collaboration, gravitational waves (GW) from such a
FOPT with appropriate strength and nucleation temperature can explain their
12.5 yr data. We impose a classical conformal invariance on the scalar
potential of $SU(2)_D$ sector involving only a complex scalar doublet with
negligible couplings with the SM Higgs. While a FOPT at sub-GeV temperatures
can give rise to stochastic GW around nano-Hz frequencies being in agreement
with NANOGrav findings, the $SU(2)_D$ vector bosons which acquire masses as a
result of the FOPT in dark sector, can also serve as dark matter (DM) in the
universe. The relic abundance of such vector DM can be generated in a
non-thermal manner from the SM bath via scalar portal mixing. We also discuss
future sensitivity of gravitational wave experiments to the model parameter
space.
