Untangling the Galaxy I: Local Structure and Star Formation History of the Milky Way (1907.07709v2)

Abstract: Gaia DR2 provides unprecedented precision in measurements of the distance and kinematics of stars in the solar neighborhood. Through applying unsupervised machine learning on DR2's 5-dimensional dataset (3d position + 2d velocity), we identify a number of clusters, associations, and co-moving groups within 1 kpc and $|b|<30^\circ$ (many of which have not been previously known). We estimate their ages with the precision of $\sim$0.15 dex. Many of these groups appear to be filamentary or string-like, oriented in parallel to the Galactic plane, and some span hundreds of pc in length. Most of these string lack a central cluster, indicating that their filamentary structure is primordial, rather than the result of tidal stripping or dynamical processing. The youngest strings ($<$100 Myr) are orthogonal to the Local Arm. The older ones appear to be remnants of several other arm-like structures that cannot be presently traced by dust and gas. The velocity dispersion measured from the ensemble of groups and strings increase with age, suggesting a timescale for dynamical heating of $\sim$300 Myr. This timescale is also consistent with the age at which the population of strings begins to decline, while the population in more compact groups continues to increase, suggesting that dynamical processes are disrupting the weakly bound string populations, leaving only individual clusters to be identified at the oldest ages. These data shed a new light on the local galactic structure and a large scale cloud collapse.

• The paper uses Gaia DR2 and machine learning to identify and analyze over 1,900 spatially coherent stellar structures, including filamentary strings, within 1 kpc of the Sun.
• Analysis reveals that about half these structures are filamentary strings, many primordial, with younger strings clustering near the Local Arm and showing distinct age stratification.
• Age and kinematic analysis indicates dynamical heating increases velocity dispersion with age, suggesting an evolutionary process from primordial strings to dispersed groups tied to galactic dynamics like spiral arms.

An Analysis of the Local Structure and Star Formation History of the Milky Way

The paper "Untangling the Galaxy I: Local Structure and Star Formation History of the Milky Way" by Marina Kounkel and Kevin Covey leverages the second data release of Gaia (Gaia DR2) to deliver insights into the structural composition and stellar dynamics within the Milky Way. This paper utilizes unsupervised machine learning techniques on Gaia DR2's extensive kinematic and positional data of stars to identify spatially coherent structures, hereafter referred to as clusters or strings, within 1 kpc of the solar vicinity. Through their analysis, Kounkel and Covey provide a comprehensive examination of the local galactic fabric, emphasizing its implications on understanding galactic formation and evolution.

Structural Groupings and Their Properties

The paper identifies 1,901 groups containing approximately 288,370 stars, of which about half are organized into filamentary strings. Many of these strings lack a central cluster, indicating they are primordial rather than the result of external forces such as tidal disruptions. The filamentary structures span several hundred parsecs and skew parallel to the Galactic plane. They are particularly evident in newly forming stars, weaving a coherent tapestry of galactic birth and lifecycle.

Age and Kinematic Characteristics

A significant facet of the work is the estimation of stellar ages with an uncertainty of approximately 0.15 dex using a combination of machine learning and traditional isochrone fitting methods. The association of age with physical and kinematic properties of the identified structures reveals a striking stratification. Younger structures, reflected as strings, cluster around the Local Arm of the Milky Way, while older stellar populations show alternate configurations. This evolutionary trend suggests a dynamic galactic environment where initially coherent molecular structures gradually diffuse into more discrete entities under the influences of dynamical heating and tidal forces.

The paper delineates that strings are not constrained by a characteristic length and are largely independent of age regarding their spatial extent, although older strings do exhibit larger velocity dispersions. The velocity dispersion increases with age, implying a dynamical heating rate on the order of ∼300 Myr. This trend points to a prolonged evolutionary process that transitions from primordial associations to the more dispersed and dynamically heated groups evident at older ages.

Large-Scale Galactic Structure

Furthermore, the research identifies distinct age-grouped streams termed Stream 1, Stream 2, Stream 3, and Stream 4. Each stream, characterized by parallel yet independent stellar strings, mirrors spiral arm structures now traced only residually through stars following previous epochs of star formation. The configuration of these strings, relative to the supposed spiral arms, offers a novel viewpoint on the temporal persistence and potential transience of galactic spiral features.

Implications and Future Directions

The implications of this paper extend to our understanding of the mechanisms governing star formation and Galactic dynamics. The filamentary nature of star clusters suggests a primordial shaping influenced by the molecular cloud morphology. The age-stratification of strings within streams signals an evolutionary trajectory heavily interlaced with spiral arm dynamics and galactic redistributive processes.

In synthesizing these insights, the paper not only contributes richly to Milky Way dynamics and structure but also posits new questions on the nature of galaxy formation, specifically the temporal evolution of spiral arms and large-scale star-forming structures. Future Gaia data releases could further refine these findings, facilitating deeper explorations into the patterns of large-scale molecular cloud collapse and subsequent star formation processes.

In summary, Kounkel and Covey's comprehensive analysis provides a significant contribution to understanding the Milky Way's complex structural framework and star formation history, highlighting the interplay between primordial formation influences and subsequent galactic dynamics.