Mass models of the Milky Way

Abstract: We present a simple method for fitting parametrized mass models of the Milky Way to observational constraints. We take a Bayesian approach which allows us to take into account input from photometric and kinematic data, and expectations from theoretical modelling. This provides us with a best-fitting model, which is a suitable starting point for dynamical modelling. We also determine a probability density function on the properties of the model, which demonstrates that the mass distribution of the Galaxy remains very uncertain. For our choices of parametrization and constraints, we find disc scale lengths of 3.00 \pm 0.22 kpc and 3.29 \pm 0.56 kpc for the thin and thick discs respectively; a Solar radius of 8.29 \pm 0.16 kpc and a circular speed at the Sun of 239 \pm 5 km/s; a total stellar mass of 6.43 \pm 0.63 * 10^10 M_sun; a virial mass of 1.26 \pm 0.24 * 10^12 M_sun and a local dark matter density of 0.40 \pm 0.04 GeV/cm^3. We find some correlations between the best-fitting parameters of our models (for example, between the disk scale lengths and the Solar radius), which we discuss. The chosen disc scale-heights are shown to have little effect on the key properties of the model.

• The paper develops a Bayesian framework to integrate photometric and kinematic data for a comprehensive Milky Way mass model.
• The study determines key parameters, including disc scale lengths (~3.00 and ~3.29 kpc), Solar radius (8.29 kpc), and circular speed (239 km/s).
• The findings offer a refined baseline for Galactic dynamic modeling and challenge cosmological expectations with a high concentration dark matter halo.

An Analysis of "Mass Models of the Milky Way"

The paper "Mass models of the Milky Way" by P.~J.~McMillan presents a systematic approach to constructing parametrized models of the Milky Way's mass distribution, leveraging a Bayesian framework that integrates observational constraints from photometric and kinematic data with theoretical expectations. This study is motivated by the substantial uncertainties that persist regarding the Galactic mass distribution, a subject of significant interest in the context of galactic dynamics and cosmology.

At its core, the study delineates the process of developing a best-fitting mass model, suitable as an initial approximation for subsequent dynamical modeling. Essential to this process is the calculation of a probability density function (pdf) for the model's parameters, which underscores the inherent uncertainties in the Milky Way's mass distribution. Key parameter estimates from the study include disc scale lengths of approximately 3.00 kpc and 3.29 kpc for the thin and thick discs, respectively, a Solar radius of approximately 8.29 kpc, and a circular speed at the Sun of approximately 239 km/s. Furthermore, the work suggests a total stellar mass of about $6.43 \times 10^{10} M_\odot$, a virial mass of $1.26 \times 10^{12} M_\odot$, and a local dark matter density of 0.40 GeV/cm$^3$.

The methodological rigor of this study is evident in its comprehensive treatment of the Galactic components, including the bulge, disc, and the dark matter halo. Each component is modeled with careful consideration of observational data and theoretical understanding. For instance, the study builds upon the NFW profile for the dark matter halo, acknowledging limitations of such profiles when baryonic physics are considered. The significant correlations found between various model parameters highlight the complexity within the mass distribution, such as between disc scale lengths and the Solar radius. Interestingly, despite some parameters, like disc scale-heights, appearing less influential, they still add complexity to the model due to interactions with other constraints.

The implications of this research are multifaceted. Practically, the best-fitting model serves as a foundational tool for dynamical modeling of the Galaxy, relevant for understanding Galactic structure and evolution. Theoretically, the findings about the scale lengths and masses challenge typical expectations from cosmological simulations, suggesting a high concentration halo which deviates from average theoretical predictions.

The study also serves as a basis for future work, particularly in exploring models which break symmetry assumptions or incorporate updated data. One key area for further research is the impact of baryonic processes on dark matter distribution, a topic of ongoing debate and investigation in galaxy formation studies. The conclusions drawn in McMillan's study underscore the continuous need for refinement in Galactic models as new observational data becomes available, ensuring models remain aligned with both theoretical expectations and empirical evidence.

In summary, this paper contributes a robust framework for assessing Galactic mass models, providing both practical tools and theoretical insights that fuel further inquiry into the Milky Way’s complex structure. By systematically synthesizing observational data and theoretical perspectives, the study enhances our understanding of Galactic dynamics and presents avenues for refined models in the era of precise astronomical observations.