Intrinsic alignments of the extended radio continuum emission of
  galaxies in the EAGLE simulations
  We present measurements of the intrinsic alignments (IAs) of the star-forming
gas of galaxies in the EAGLE simulations. Radio continuum imaging of this gas
enables cosmic shear measurements complementary to optical surveys. We measure
the orientation of star-forming gas with respect to the direction to, and
orientation of, neighbouring galaxies. Star-forming gas exhibits a
preferentially radial orientation-direction alignment that is a decreasing
function of galaxy pair separation, but remains significant to $\gtrsim 1$ Mpc
at $z=0$. The alignment is qualitatively similar to that exhibited by the
stars, but is weaker at fixed separation. Pairs of galaxies hosted by more
massive subhaloes exhibit stronger alignment at fixed separation, but the
strong alignment of close pairs is dominated by ${\sim}L^\star$ galaxies and
their satellites. At fixed comoving separation, the radial alignment is
stronger at higher redshift. The orientation-orientation alignment is
consistent with random at all separations, despite subhaloes exhibiting
preferential parallel minor axis alignment. The weaker IA of star-forming gas
than for stars stems from the former's tendency to be less well aligned with
the dark matter structure of galaxies than the latter, and implies that the
systematic uncertainty due to IA may be less severe in radio continuum weak
lensing surveys than in optical counterparts. Alignment models equating the
orientation of star-forming gas discs to that of stellar discs or the DM
structure of host subhaloes will therefore overestimate the impact of IAs on
radio continuum cosmic shear measurements.
