The insight into the dark side. I. The pitfalls of the dark halo parameters estimation

TL;DR

This study investigates the reliability of dark matter halo parameter estimates from galaxy rotation curves, revealing significant uncertainties due to degeneracies and data limitations, but finds halo mass estimates within optical radii to be more dependable.

Contribution

It demonstrates the limitations of current mass-modelling methods for dark halo parameters and proposes that halo mass within optical radii can be reliably estimated without fixing mass-to-light ratios.

Findings

Dark halo parameters are highly uncertain due to degeneracies.

Lack of data in galaxy centers or outskirts affects parameter estimates.

Halo mass within optical radius can be accurately determined.

Abstract

We examined the reliability of estimates of pseudoisothermal, Burkert and NFW dark halo parameters for the methods based on the mass-modelling of the rotation curves.To do it we constructed the $\chi^2$ maps for the grid of the dark matter halo parameters for a sample of 14 disc galaxies with high quality rotation curves from THINGS. We considered two variants of models in which: a) the mass-to-light ratios of disc and bulge were taken as free parameters, b) the mass-to-light ratios were fixed in a narrow range according to the models of stellar populations. To reproduce the possible observational features of the real galaxies we made tests showing that the parameters of the three halo types change critically in the cases of a lack of kinematic data in the central or peripheral areas and for different spatial resolutions. We showed that due to the degeneracy between the central densities and the radial scales of the dark haloes there are considerable uncertainties of their concentrations estimates. Due to this reason it is also impossible to draw any firm conclusion about universality of the dark halo column density based on mass-modelling of even a high quality rotation curve. The problem is not solved by fixing the density of baryonic matter. In contrast, the estimates of dark halo mass within optical radius are much more reliable. We demonstrated that one can evaluate successfully the halo mass using the pure best-fitting method without any restrictions on the mass-to-light ratios.