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

Abstract

Context. Discoveries of extended rotation curves have suggested that spiral galaxy halos contain dark matter. This has led to many studies that estimated the total mass of the Galaxy, mostly using the Navarro, Frenk, and White (NFW) density profile. Aims. We determine the effect that the choice of the dark matter profile has on the predicted values of extrapolated total masses. Methods. We considered a recently reported Milky Way (MW) rotation curve, first because of its unprecedented accuracy, and second because the Galactic disk appears to be least affected by past major mergers that have fully reshaped the initial disk. Results. We find that the use of an NFW profile (or its generalized form, gNFW) to calculate the dark-matter contribution to the MW rotation curve generates apparently inconsistent results such as an increase in baryonic mass that leads to an increase in dark matter mass. Furthermore, we find that NFW and gNFW profiles narrow the total mass range, leading to a possible methodological bias particularly against low MW masses. Using the Einasto profile, which is better suited to represent cold dark matter halos, we finally found that the slightly decreasing rotation curve of the MW favors a total mass that can be as low as 2.6 × 1011 M⊙, disregarding any other dynamical tracers farther out in the MW. This is inconsistent with values higher than 18 × 1011 M⊙ for any type of cold dark matter halo profiles under the assumption that stars and gas do not affect the predicted dark matter distribution in the MW. Conclusions. This methodological paper encourages the use of the Einasto profile to characterize rotation curves with the aim of evaluating their total masses.