# HI Kinematics and Mass Distribution of Messier 33

## TL;DR

This study presents a detailed HI survey of Messier 33, revealing complex gas kinematics, a warped disk, and a cuspy dark matter halo, challenging some predictions of the LambdaCDM model.

## Contribution

It provides new deep HI observations of M33, identifying a low brightness anomalous HI component and modeling its mass distribution with implications for dark matter halo profiles.

## Key findings

- Detection of a perturbed outer gas distribution and kinematic twist.
- Identification of a low brightness HI component with anomalous velocities.
- Mass modeling favors a cuspy dark matter halo with a high concentration.

## Abstract

A new deep HI survey of the galaxy Messier 33 is presented, based on observations obtained at the Dominion Radio Astrophysical Observatory. We observe a perturbed outer gas distribution and kinematics in M33, and confirm the disk warping as a significant kinematical twist of the major axis of the velocity field, though no strong tilt is measured, in agreement with previous work. Evidence for a new low brightness HI component with anomalous velocity is reported. It harbours a large velocity scatter, as its kinematics both exceeds and lags the rotation of the disk, and leaks in the forbidden velocity zone of apparent counter-rotation. The observations also reveal wide and multiple peak HI profiles which can be partly explained by crowded orbits in the framework of the warp model. Asymmetric motions are identified in the velocity field, as possible signatures of a lopsided potential and the warp. The mass distribution modeling of the hybrid Halpha-HI rotation curve favours a cuspy dark matter halo with a concentration in disagreement with the LambdaCDM dark halo mass-concentration relationship. The total mass enclosed in 23 kpc is 8 10^10 Msol, of which 11% are stars and gas. At the virial radius of the cuspy halo, the resulting total mass is 5 10^11 Msol, but with a baryonic mass fraction of 2% only. This strongly suggests a more realistic radius encompassing the total mass of M33 well smaller than the virial radius of the halo, maybe comparable to the size of the HI disk.