  By means of 3D hydrodynamical simulations, in a separate paper we have
discussed the properties of non-axisymmetric density wave trains in the
outermost regions of galaxy disks, based on the picture that self-excited
global spiral modes in the bright optical stellar disk are accompanied by
low-amplitude short trailing wave signals outside corotation; in the gas, such
wave trains can penetrate through the outer Lindblad resonance and propagate
outwards, forming prominent spiral patterns. In this paper we present the
synthetic 21~cm velocity maps expected from simulated models of the outer
gaseous disk, focusing on the case when the disk is dominated by a two-armed
spiral pattern, but considering also other more complex situations. We discuss
some aspects of the spiral pattern in the gaseous periphery of galaxy disks
noted in our simulations that might be interesting to compare with specific
observed cases.
