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

Y. B. Yang,Y. Jiao,J. L. Wang,F. Hammer

doi:10.1051/0004-6361/202141058

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.

Highlights

Gaia DR2 provided accurate stellar proper motions to calculate the circular velocity curve of the Milky Way (MW) up to 25 kpc (Eilers et al 2019; Mróz et al 2019)
The theoretical estimate of the circular velocity is derived at different disk radii (R) from the potential Φtot of the Galaxy through v2c (R)
– When the MW rotation curves (RCs) alone is used as a constraint, we find that the Einasto mass density profile leads to the largest range of MW total masses that can reproduce its RC, while both NFW and gNFW profiles lead to a narrow mass range, in particular, by excluding total mass (M200 = Mtot) values lower than ∼5 × 1011 M

Summary

Introduction

Gaia DR2 provided accurate stellar proper motions to calculate the circular velocity curve of the Milky Way (MW) up to 25 kpc (Eilers et al 2019; Mróz et al 2019). While Eilers et al (2019, see Hogg et al 2019) used spectrophotometric distances in their analysis, their finding was confirmed by Mróz et al (2019) using 773 Classical Cepheids with precise distances. Subsequent analyses of these rotation curves (RCs) have led to a total MW mass near or well below 1012 M (Eilers et al 2019; de Salas et al 2019; Grand et al 2019; Karukes et al 2020). Recent studies have shown that the three-parameter Einasto profile (Einasto 1965, see Retana-Montenegro et al 2012) provides a better description of the CDM halo density profile than the NFW profile (Navarro et al 2004, 2010; Gao et al 2008), and it is even than the three-parameter generalized gNFW (Klypin et al 2016)

Objectives

Methods

Discussion

Conclusion