Trigonometric Parallaxes of High Mass Star Forming Regions: the Structure and Kinematics of the Milky Way (1401.5377v3)

Abstract: Over 100 trigonometric parallaxes and proper motions for masers associated with young, high-mass stars have been measured with the BeSSeL Survey, a VLBA key science project, the EVN, and the Japanese VERA project. These measurements provide strong evidence for the existence of spiral arms in the Milky Way, accurately locating many arm segments and yielding spiral pitch angles ranging from 7 to 20 degrees. The widths of spiral arms increase with distance from the Galactic center. Fitting axially symmetric models of the Milky Way with the 3-D position and velocity information and conservative priors for the solar and average source peculiar motions, we estimate the distance to the Galactic center, Ro, to be 8.34 +/- 0.16 kpc, a circular rotation speed at the Sun, To, to be 240 +/- 8 km/s, and a rotation curve that is nearly flat (a slope of -0.2 +/- 0.4 km/s/kpc) between Galactocentric radii of 5 and 16 kpc. Assuming a "universal" spiral galaxy form for the rotation curve, we estimate the thin disk scale length to be 2.44 +/- 0.16 kpc. The parameters Ro and To are not highly correlated and are relatively insensitive to different forms of the rotation curve. Adopting a theoretically motivated prior that high-mass star forming regions are in nearly circular Galactic orbits, we estimate a global solar motion component in the direction of Galactic rotation, Vsun = 14.6 +/- 5.0 km/s. While To and Vsun are significantly correlated, the sum of these parameters is well constrained, To + Vsun = 255.2 +/- 5.1 km/s, as is the angular speed of the Sun in its orbit about the Galactic center, (To + Vsun)/Ro = 30.57 +/- 0.43 km/s/kpc. These parameters improve the accuracy of estimates of the accelerations of the Sun and the Hulse-Taylor binary pulsar in their Galactic orbits, significantly reducing the uncertainty in tests of gravitational radiation predicted by general relativity.

• The paper presents high-precision VLBI measurements of maser parallaxes to map the Milky Way’s spiral arms and determine key rotation parameters.
• The analysis reveals spiral arm pitch angles ranging from 7° to 20° and a radial width scaling of approximately 42 pc/kpc, confirming detailed Galactic structure.
• Robust Bayesian modeling constrains fundamental parameters like the Galactic center distance (8.34 kpc) and circular rotation speed (240 km/s), enhancing our understanding of Galactic dynamics.

Insights into the Structure and Kinematics of the Milky Way: High Precision Parallax and Proper Motion Measurements

The paper by Reid et al. presents a comprehensive analysis of the structure and kinematics of the Milky Way, leveraging over 100 trigonometric parallaxes and proper motions for masers associated with young, high-mass star-forming regions (HMSFRs). Conducted as part of the Bar and Spiral Structure Legacy Survey (BeSSeL), the European VLBI Network, and the Japanese VERA project, this research provides a detailed mapping of spiral arm segments via Very Long Baseline Interferometry (VLBI) techniques. These measurements are pivotal in understanding the three-dimensional structure and dynamic properties of our Galaxy.

Core Findings

1. Spiral Arm Geometry and Dynamics: The data corroborates the existence of spiral arms, with pitch angles between approximately 7° and 20°. The arms are resolved at high precision, revealing a tendency for increasing arm width with radial distance from the Galactic center. This scaling factor was quantified at around 42 pc kpc$^{-1}$, signaling a significant interpolation in structured spiral arm modeling.
2. Galactic Rotation Parameters: Through a series of axially symmetric Galactic models, the authors estimate critical parameters, including the distance to the Galactic center (\Ro = $8.34 \pm 0.16$ kpc) and the circular rotation speed at the Sun’s position (\To = $240 \pm 8$ km s$^{-1}$), while confirming a near-flat rotation curve with a slope (\Tdot) of $-0.2 \pm 0.4$ km s$^{-1}$ kpc$^{-1}$.
3. Robust Estimation Techniques: Fit models employed Bayesian statistical methods, allowing for the examination of priors and parameter correlations critically, particularly concerning the solar motion ($U, V, W$) and the peculiar motions of HMSFRs. The parameter $\To + V = 255.2 \pm 5.1$ km s$^{-1}$ is tightly constrained, as is the angular speed of the Sun about the Galactic center, $(\To + V)/\Ro = 30.57 \pm 0.43$ km s$^{-1}$ kpc$^{-1}$.

Implications and Future Directions

• Astrophysical Modeling:

These precise measurements of the Milky Way's fundamental parameters enhance our understanding of the Galactic rotation curve and structure. Further refinement in defining the solar motion and its implications on Galactic dynamics is necessary, particularly given slight discrepancies between local and global rotational metrics.

• Testing General Relativity:

Improved knowledge of \Ro\ and \To\ enhances our ability to test predictions of General Relativity, such as in the orbital decay of binary pulsars like the Hulse-Taylor binary. These measures significantly reduce the uncertainties involved in deriving gravitational wave predictions.

• Galactic Evolution Theories:

By understanding spiral arm dynamics and their kinematic behaviors, we can better infer the formation history and future evolution of the Milky Way as well as its interaction with satellite galaxies and the Local Group overall.

In conclusion, the paper contributes significantly to the field of Galactic astronomy by providing unparalleled insight into the structure and kinematics of the Milky Way with implications ranging from fundamental physics to star formation processes across spiral arms. This paper exemplifies the vital role of VLBI in enhancing our comprehension of Galactic dynamics and stands as a benchmark for future studies aimed at unraveling the complexities of spiral galaxies.