Redshift-space distortions of the Hi 21-cm intensity mapping signal due to the internal motions within galaxies

The Hi 21-cm intensity mapping signal experiences redshift-space distortions due to the motion of the galaxies which contain the Hi as well as the motions of the Hi gas within the galaxies. A detailed modelling is essential if this signal is to be used for precision cosmology. Considering dark-matter-only simulations where the Hi is assumed to reside in galaxies which are associated with haloes, in this work we introduce a technique to incorporate the Hi motions within the galaxies. This is achieved through a line profile which accounts for both the rotational and random (thermal and turbulent) motions of the Hi within galaxies. The functional form of the double-horned line profiles used here is motivated by observations of z= 0 spiral galaxies. Analyzing the simulated 21-cm power spectrum over the redshift range 1 <= z <= 6 we find that the Hi motions within galaxies make a significant contribution that is manifested as an enhancement in the Finger of God (FoG) effect which can be modelled reasonably well through a Lorentzian damping profile with a single free parameter sigma(p). The value of sigma(p) is significantly enhanced if motions within the galaxies are included. This is particularly important at z > 3 where sigma(p) is dominated by the internal motions and a measurement of the FoG effect here could provide a handle on the line profiles of high-redshift galaxies.