The mass distribution and gravitational potential of the Milky Way (1608.00971v3)

Abstract: We present mass models of the Milky Way created to fit observational constraints and to be consistent with expectations from theoretical modelling. The method used to create these models is that demonstrated in McMillan (2011), and we improve on those models by adding gas discs to the potential, considering the effects of allowing the inner slope of the halo density profile to vary, and including new observations of maser sources in the Milky Way amongst the new constraints. We provide a best fitting model, as well as estimates of the properties of the Milky Way. Under the assumptions in our main model, we find that the Sun is $R_0 = (8.20\pm0.09)\,\mathrm{kpc}$ from the Galactic Centre, with the circular speed at the Sun being $v_0 = (232.8\pm3.0)\,\mathrm{km}\,\mathrm{s}^{-1}$; that the Galaxy has a total stellar mass of $(54.3\pm5.7)\times10^9\,{\rm M}\odot$, a total virial mass of $(1.30 \pm 0.30)\times10^{12}\,{\rm M}\odot$ and a local dark-matter density of $0.38\pm0.04\,\mathrm{GeV\,cm}^{-3}$, where the quoted uncertainties are statistical. These values are sensitive to our choice of priors and constraints. We investigate systematic uncertainties, which in some cases may be larger. For example, if we weaken our prior on $R_0$, we find it to be $(7.97\pm0.15)\,\mathrm{kpc}$ and that $v_0=(226.8\pm4.2)\,\mathrm{km}\,\mathrm{s}^{-1}$. We find that most of these properties, including the local dark-matter density, are remarkably insensitive to the assumed power-law density slope at the centre of the dark-matter halo. We find that it is unlikely that the local standard of rest differs significantly from that found under assumptions of axisymmetry. We have made code to compute the force from our potential, and to integrate orbits within it, publicly available.

• The paper refines Milky Way mass modeling by incorporating a gas disc, variable halo slopes, and maser constraints to yield precise measurements of stellar mass and local dark-matter density.
• The study employs astrometric maser data and updated priors to tightly constrain key Galactic parameters, including the Sun’s distance (8.20±0.09 kpc) and circular speed (232.8±3.0 km/s).
• It also provides a public computational tool for orbit integration, facilitating further research into Galactic dynamics and the interpretation of upcoming high-precision surveys.

Analysis of the Mass Distribution and Gravitational Potential of the Milky Way

The paper presented by McMillan offers a detailed mass model of the Milky Way, integrating both observational constraints and theoretical expectations. Building upon McMillan's previous work, this paper enhances prior models with the inclusion of a gas disc component, variable inner slope of the halo density profile, and recent maser observations, reflecting advancements in understanding the Milky Way's structure.

Key Findings and Numerical Precision

The mass model presented critically depends on observational data, including measurements from high-mass star-forming regions obtained through astrometric techniques like those from the Bar and Spiral Structure Legacy (BeSSeL) survey. Underpinning the analysis are parameters reflecting the Galaxy's attributes such as the Sun's distance to the Galactic Center at $R_0 = (8.20\pm0.09)\,\mathrm{kpc}$ and a circular speed at the Sun's location $v_0 = (232.8\pm3.0)\,\mathrm{km/s}$. The paper reports a total stellar mass of $(54.3\pm5.7)\times10^9\,M_\odot$, corroborated by a statistically significant local dark-matter density of $0.38\pm0.04\,\mathrm{GeV/cm}^3$.

These values portray sensitivity to the modeling assumptions, particularly the priors chosen for each parameter and the constraints applied. However, the robustness of properties like the local dark-matter density, even with variations in the inner halo density slope, demonstrates the model's resilience to certain systematic uncertainties.

Implications for Galactic Dynamics and Future Directions

The paper implies substantial practical applications in modeling the Milky Way's dynamics. By releasing a public code repository for computing forces and integrating orbits within the presented potential, the paper facilitates subsequent astrophysical research. Such availability ensures that the model can serve as a baseline for further explorations into Galactic dynamics and star orbit calculations, crucial for interpreting upcoming data sets with unprecedented volumes from ESA's missions, including Gaia.

Despite the comprehensive nature of this model, it is crucial to acknowledge the potential for systematic uncertainties, especially those related to Galaxy components' shapes and assumptions about spherical symmetry. Advances in observational technologies and methodologies will likely yield more precise parameters for such models. Hence, refining these assumptions and enhancing the resolution of constraint data will be instrumental in future investigations.

In summary, McMillan's updated mass model stands as both a significant reference and a computational tool for studying the Milky Way's potential and dynamics. By addressing the uncertainties, refining models' assumptions, and expanding datasets, the paper paves the way for evolving the theoretical frameworks governing our understanding of Galactic structure and evolution.