Untangling the Galaxy. II. Structure within 3 kpc (2004.07261v2)

Abstract: We present the results of the hierarchical clustering analysis of the Gaia DR2 data to search for clusters, co-moving groups, and other stellar structures. The current paper builds on the sample from the previous work, extending it in distance from 1 kpc to 3 kpc, increasing the number of identified structures up to 8292. To aid in the analysis of the population properties, we developed a neural network called Auriga to robustly estimate the age, extinction, and distance of a stellar group based on the input photometry and parallaxes of the individual members. We apply Auriga to derive the properties of not only the structures found in this paper, but also previously identified open clusters. Through this work, we examine the temporal structure of the spiral arms. Specifically, we find that the Sagittarius arm has moved by >500 pc in the last 100 Myr, and the Perseus arm has been experiencing a relative lull in star formation activity over the last 25 Myr. We confirm the findings from the previous paper on the transient nature of the spiral arms, with the timescale of transition of a few 100 Myr. Finally, we find a peculiar ~1 Gyr old stream of stars that appears to be heliocentric. It is unclear what is the origin of it.

Analysis of Galactic Structure within 3 kpc

The paper "Untangling the Galaxy. II. Structure within 3 kpc" presents an extensive analysis of the dynamic structure of the Milky Way using data from Gaia DR2 to identify clusters and stellar structures within a 3 kpc radius. The authors employ hierarchical clustering techniques and a neural network model named Auriga to estimate the age, extinction, and distance of these groups, leading to significant insights into Galactic dynamics.

The key contribution of this work is the identification of 8,292 stellar structures based on clustering analysis, which expands the spatial coverage of previously known open clusters. This catalog allows for examining properties of the Galaxy as a function of stellar age. The use of the Auriga neural network provides robust property estimates for both previously identified clusters and new structures, making it a critical advancement in studying Galactic dynamics.

Numerical Findings and Stellar Dynamics

1. Sagittarius Arm Movement: The paper finds that the Sagittarius arm has shifted by over 500 pc towards the Galactic Center over the past 100 million years, with a velocity of approximately 8-10 km/s. This movement suggests temporal and spatial evolution beyond typical stellar migration caused by Galactic rotation.
2. Star Formation Lull in the Perseus Arm: There is evidence of a decline in star formation activity in the Perseus arm during the last 25 million years. Unlike the Sagittarius arm, the Perseus arm exhibits stability in its positional evolution, providing insights into differential dynamics within spiral arms.
3. The Local Arm and Radcliffe Wave: Newly discovered structures are well-correlated with the Radcliffe wave—a vast molecular gas structure. The analysis suggests that the formation within this wave has occurred recently, with most populations younger than 12 million years.
4. Galactic Disk Ripple and Stream Phenomena: The analysis identifies ripples in the Galactic disk across different age groups, suggesting non-unique ripple formation events. A peculiar heliocentric stream is discerned, which lacks cohesive kinematic properties but remains distinct in spatial structure.
5. Transient Nature of Spiral Arms: Confirming prior results, the research underscores the transient nature of spiral arms with a transition timescale of a few hundred million years. This is characterized by complex interactions and evolving star formation patterns.

Implications and Speculations

This work substantially advances our understanding of Galactic structure and evolution. It implies transient spiral arm formation and dissolution, influenced by density waves and stellar dynamics. The recognition that star formation regions and stellar densities dynamically evolve provides crucial evidence for models predicting Galactic evolution.

The paper postulates that future developments in observational astronomy, including enhanced Gaia datasets and spectroscopic follow-up, will refine these findings further. Such advancements can offer refined insights into the velocity fields and population ages, validating the temporal dynamics of spiral arms and the structural integrity of identified streams.

Future Directions

With the evolving precision of long-term astrometric surveys, such as future Gaia releases, continued collection of radial velocity data will be instrumental in enhancing the reliability of cluster identification and membership assignment. This will provide a more nuanced understanding of Galactic dynamics and density wave movement.

In summary, this paper presents a thorough analysis of the transient nature of spiral arms, underscores the motion characteristics of the Sagittarius arm, and reveals dynamic features of star formation along the Local arm. The implications of these findings deepen our comprehension of Galactic structure, offering a foundation for theoretical models and future observational campaigns.