The degeneracy of M33 mass modelling and its physical implications

TL;DR

This study uses Bayesian analysis to explore the dark matter profile of M33, revealing that its halo likely remains unaltered by feedback and challenging previous NFW profile assumptions.

Contribution

It introduces a Bayesian method to analyze the dark matter density profile of M33 across a broad parameter space, providing new insights into its halo structure.

Findings

Models with inner log slope less than 0.9 are strongly excluded.

High stellar mass-to-light ratios are inconsistent with stellar population models.

M33's dark matter halo likely remains unmodified by feedback processes.

Abstract

The Local Group galaxy M33 exhibits a regular spiral structure and is close enough to permit high resolution analysis of its kinematics, making it an ideal candidate for rotation curve studies of its inner regions. Previous studies have claimed the galaxy has a dark matter halo with an NFW profile, based on statistical comparisons with a small number of other profiles. We apply a Bayesian method from our previous paper to place the dark matter density profile in the context of a continuous, and more general, parameter space. For a wide range of initial assumptions we find that models with inner log slope $\gamma_{\rm in}<0.9$ are strongly excluded by the kinematics of the galaxy unless the mass-to-light ratio of the stellar components in the $3.6\mu$m band satisfies $\Upsilon_{3.6}\geq2$. Such a high $\Upsilon_{3.6}$ is inconsistent with current modelling of the stellar population of M33. This suggests that M33 is a galaxy whose dark matter halo has not been significantly modified by feedback. We discuss possible explanations of this result, including ram pressure stripping during earlier interactions with M31.