# Redshift space distortions of the {\rm HI} 21-cm intensity mapping signal due to the internal motions within galaxies

## TL;DR

This paper models the impact of internal galaxy motions on the HI 21-cm intensity mapping signal's redshift space distortions, highlighting their significance for precision cosmology and the Finger of God effect at high redshifts.

## Contribution

It introduces a new technique to incorporate internal HI motions into simulations using a line profile based on observed galaxy profiles, improving the modeling of redshift space distortions.

## Key findings

- Internal motions significantly enhance the Finger of God effect.
- The Lorentzian damping profile with a free parameter effectively models the FoG.
- Internal motions dominate at z>3, affecting line profile measurements.

## Abstract

The {\rm HI} 21-cm intensity mapping signal experiences redshift space distortions due to the motion of the galaxies which contain the {\rm HI} as well as the motions of the {\rm 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 {\rm HI} is assumed to reside in galaxies which are associated with haloes, in this work we introduce a technique to incorporate the {\rm 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 {\rm 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 $1 \le z \le 6$ we find that the {\rm HI} motions within galaxies makes 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.