Detection of the Intrinsic Spin Alignments in Isolated Spiral Pairs
  An observational evidence for the intrinsic galaxy alignments in isolated
spiral pairs is presented. From the catalog of the galaxy groups identified by
Tempel et al. in the flux limited galaxy sample of the Sloan Digital Sky Survey
Data Release 10, we select those groups consisting only of two spiral galaxies
as isolated spiral pairs and investigate if and how strongly the spin axes of
their two spiral members are aligned with each other. We detect a clear signal
of intrinsic spin alignment in the isolated spiral pairs, which leads to the
rejection of the null hypothesis at the $99.9999\%$ confidence level via the
Rayleigh test. It is also found that those isolated pairs comprising two
early-type spiral galaxies exhibit the strongest signal of intrinsic spin
alignment and that the strength of the alignment signal depends on the angular
separation distance as well as on the luminosity ratio of the member galaxies.
Using the dark matter halos consisting of only two subhalos resolved in the
EAGLE hydrodynamic simulations, we repeat the same analysis but fail to find
any alignment tendency between the spin angular momentum vectors of the stellar
components of the subhalos, which is in tension with the observational result.
A couple of possible sources of this apparent inconsistency between the
observational and the numerical results are discussed.
