The mass of our Galaxy from satellite proper motions in the Gaia era

Abstract

We use Gaia DR2 systemic proper motions of 45 satellite galaxies to constrain the mass of the Milky Way using the scale free mass estimator of Watkins et al. (2010). We first determine the anisotropy parameter β, and the tracer satellites' radial density index γ to be β=−0.67 +0.45 −0.62 and γ = 2.11 ± 0.23. When we exclude possible former satellites of the Large Magellanic Cloud, the anisotropy changes to β=−0.21 +0.37 −0.51 . We find that the index of the Milky Way's gravitational potential α, which is dependent on the mass itself, is the parameter with the largest impact on the mass determination. Via comparison with cosmological simulations of Milky Way-like galaxies, we carried out a detailed analysis of the estimation of the observational uncertainties and their impact on the mass estimator. We found that the mass estimator is biased when applied naively to the satellites of simulated Milky Way halos. Correcting for this bias, we obtain for our Galaxy a mass of 0.58 +0.15 −0.14 × 10 12 M ⊙ within 64 kpc, as computed from the inner half of our observational sample, and 1.43 +0.35 −0.32 ×10 12 M ⊙ within 273 kpc, from the full sample; this latter value extrapolates to a virial mass of M vir =1.51 +0.45 −0.40 ×10 12 M ⊙ corresponding to a virial radius of R vir =308±29 kpc. This value of the Milky Way mass lies in-between other mass estimates reported in the literature, from various different methods.

The orbital evolution of UFDs and GCs in an evolving Galactic potential

“…We stress, however, that more precise measurements of the Milky Way's mass exist (e.g. McMillan 2017; Callingham et al 2019;Cautun et al 2020;Fritz et al 2020;Li et al 2020). A comparison of mass estimates performed by Wang et al (2020) suggested that MMW ≈ 1.2×10 12 M⊙, though different methods yield systematically different results.…”

The mass of our Milky Way

“…Deviations from spherical symmetry and the existence of a companion halo are also found to contribute to their systematic biases [390,391]. For typical ap-000000-34 plications of the SJE and derived mass estimators, additional systematic biases arise when β is treated as a free but constant parameter [199,391] and when the radial density profile of tracer objects is not properly measured or modeled [138].…”

“…In two very recent papers [121,138], the Watkins et al [393] mass estimator was validated and calibrated against simulations. Fritz et al in 2020 [138] used dark matter only simulations to test and calibrate their measurements based on 45 satellite galaxies. After applying the mass estimator to simulations, significant biases were reported, which were mainly due to the deviation of satellite density profiles from a single power law.…”

“…After applying the mass estimator to simulations, significant biases were reported, which were mainly due to the deviation of satellite density profiles from a single power law. [138] also discussed systematic uncertainties arising from the LMC and LMC satellites. In another study, Erkal et al in 2020 [121] pointed out that since the infall of the massive LMC can induce a substantial reflex motion in the MW [e.g.…”

Exploring the Galaxy’s halo and very metal-weak thick disc with SkyMapper and Gaia DR2

“…As another possibility, we note that the total mass of the Galaxy may in fact be larger than that used in our modeling. If this is the case then, although on high-energy orbits, the stars would remain bound (e.g., Monari et al 2018;Fritz et al 2020).…”