Trigonometric Parallaxes of Massive Star Forming Regions: VI. Galactic Structure, Fundamental Parameters and Non-Circular Motions (0902.3913v2)

Abstract: We are using the VLBA and the Japanese VERA project to measure trigonometric parallaxes and proper motions of masers found in high-mass star-forming regions across the Milky Way. Early results from 18 sources locate several spiral arms. The Perseus spiral arm has a pitch angle of 16 +/- 3 degrees, which favors four rather than two spiral arms for the Galaxy. Combining positions, distances, proper motions, and radial velocities yields complete 3-dimensional kinematic information. We find that star forming regions on average are orbiting the Galaxy ~15 km/s slower than expected for circular orbits. By fitting the measurements to a model of the Galaxy, we estimate the distance to the Galactic center R_o = 8.4 +/- 0.6 kpc and a circular rotation speed Theta_o = 254 +/- 16 km/s. The ratio Theta_o/R_o can be determined to higher accuracy than either parameter individually, and we find it to be 30.3 +/- 0.9 km/s/kpc, in good agreement with the angular rotation rate determined from the proper motion of Sgr A*. The data favor a rotation curve for the Galaxy that is nearly flat or slightly rising with Galactocentric distance. Kinematic distances are generally too large, sometimes by factors greater than two; they can be brought into better agreement with the trigonometric parallaxes by increasing Theta_o/R_o from the IAU recommended value of 25.9 km/s/kpc to a value near 30 km/s/kpc. We offer a "revised" prescription for calculating kinematic distances and their uncertainties, as well as a new approach for defining Galactic coordinates. Finally, our estimates of Theta_o and To/R_o, when coupled with direct estimates of R_o, provide evidence that the rotation curve of the Milky Way is similar to that of the Andromeda galaxy, suggesting that the dark matter halos of these two dominant Local Group galaxy are comparably massive.

• The paper refines Galactic structure by mapping spiral arms with a pitch angle of 16° using VLBA and VERA parallax observations.
• The paper estimates fundamental parameters, reporting a Galactic center distance of 8.4 kpc and a rotation speed of 254 km/s, with a ratio of 30.3 km/s/kpc.
• The paper reveals non-circular motions of about 15 km/s slower than expected, indicating dynamic influences from spiral density waves.

Summary of "Trigonometric Parallaxes of Massive Star Forming Regions: VI. Galactic Structure, Fundamental Parameters and Non-Circular Motions"

This paper represents a landmark effort in refining our understanding of the Milky Way's structure and dynamics through precision measurements of trigonometric parallaxes and proper motions. Specifically, the research utilizes the Very Long Baseline Array (VLBA) and the Japanese VLBI Exploration of Radio Astronomy (VERA) project to track masers within high-mass star-forming regions (HMSFRs). The implications of this paper are twofold: advancement in determining fundamental Galactic parameters and insights into the non-circular motions of star-forming regions.

Methodology and Findings

The authors outline their methodology for observing and measuring the parallaxes and proper motions of masers associated with HMSFRs. These regions are pivotal in mapping the spiral arms of the Milky Way. The paper details the following major findings:

1. Galactic Structure and Spiral Arms:
   • The Perseus arm is mapped with a pitch angle of $16^\circ \pm 3^\circ$. This angle supports a model of the Milky Way possessing at least four spiral arms rather than two.
   • Parallax measurements provide a direct geometric method to overcome the inaccuracies of traditional photometric distances, which can be severely affected by issues like dust extinction and luminosity calibration.
2. Galactic Parameters:
   • The paper estimates the distance to the Galactic center ($R_0$) as $8.4 \pm 0.6$ kpc and calculates the circular rotation speed ($\Theta_0$) at $254 \pm 16$ km/s.
   • A more robust measure derived from these estimates is the ratio $\Theta_0/R_0 = 30.3 \pm 0.9$ km/s/kpc, aligning closely with findings from Sgr A*, adding confidence to the precision of these measurements.
3. Non-Circular Motions:
   • HMSFRs are identified to be orbiting the Galaxy approximately 15 km/s slower than expected from circular orbits. This discrepancy might highlight dynamic interactions with spiral density waves or potential Galactic eccentricities in mass distribution, requiring further investigation into the internal mechanics of star formation within spiral arms.

Broader Implications and Future Perspectives

The paper discusses practical implications, notably, providing a revised mechanism for calculating kinematic distances, which better aligns estimated kinematic measures to true parallax distances. This is significant because many previous studies have overestimated distances, hampering our understanding of Galactic structure.

The comparison of Milky Way kinematics with those of the Andromeda galaxy suggests that both galaxies have comparable dark matter halos, hinting at similar formation histories or structural compositions. Observational similarities to Andromeda also assist in constraining local models of dark matter distribution and Galactic formation scenarios.

Conclusion

The results presented in this paper contribute substantially to refining the models of Galactic rotation and structure, enabling advancements in Galactic astronomy and offering pathways for refined theoretical models. Future VLBA and VERA observations could refine these results, providing more sources, reducing uncertainties, and enhancing our understanding of both Galactic rotation curves and spiral arm characteristics. Additionally, as parallaxes and proper motions are refined, they can substantially contribute to verifying models of galaxy evolution and interaction within the Local Group, potentially leading to a more comprehensive cosmological understanding.