Which Milky Way masses are consistent with the slightly declining 5-25 kpc rotation curve?

TL;DR

This paper investigates how different dark matter profiles, especially the Einasto profile, influence the estimated total mass of the Milky Way based on its rotation curve, highlighting potential biases of commonly used models.

Contribution

It demonstrates that the Einasto profile provides more consistent mass estimates for the Milky Way's rotation curve than NFW or gNFW profiles, which can bias results.

Findings

NFW and gNFW profiles produce inconsistent mass estimates.

Using Einasto profile suggests Milky Way mass as low as 2.6×10^11 M☉.

Mass estimates above 1.8×10^12 M☉ are inconsistent with the rotation curve.

Abstract

Discoveries of extended rotation curves have suggested the presence of dark matter in spiral galaxy haloes. It has led to many studies that estimated the galaxy total mass, mostly by using the Navarro Frenk and White (NFW) density profile. We aim at verifying how the choice of the dark-matter profile may affect the predicted values of extrapolated total masses. We have considered the recent Milky Way (MW) rotation curve, firstly because of its unprecedented accuracy, and secondly because the Galactic disk is amongst the least affected by past major mergers having fully reshaped the initial disk. We find that the use of NFW profile (or its generalized form, gNFW) for calculating the dark-matter contribution to the MW rotation curve generates apparently inconsistent results, e.g., an increase of the baryonic mass leads to increase of the dark matter mass. Furthermore we find that NFW and gNFW profile narrow the total mass range, leading to a possible methodological bias particularly against small MW masses. By using the Einasto profile that is more appropriate to represent cold dark matter haloes, we finally find that the Milky Way slightly decreasing rotation curve favors total mass that can be as small as 2.6 $\times 10^{11}$ $M_{\odot}$, disregarding any other dynamical tracers further out in the MW. It is inconsistent with values larger than 18 $\times 10^{11}$ $M_{\odot}$ for any kind of CDM dark-matter halo profiles, under the assumption that stars and gas do not influence the predicted dark matter distribution in the MW. This methodological paper encourages the use of the Einasto profile for characterizing rotation curves with the aim of evaluating their total masses.