The recurrent impact of the Sagittarius dwarf on the Milky Way star formation history (2003.12577v1)

Abstract: Satellites orbiting disc galaxies can induce phase space features such as spirality, vertical heating and phase-mixing in their discs. Such features have also been observed in our own Galaxy, but the complexity of the Milky Way disc has only recently been fully mapped thanks to Gaia DR2 data. This complex behaviour is mainly ascribed to repeated perturbations induced by the Sagittarius dwarf galaxy (Sgr) along its orbit, pointing to this satellite as the main dynamical architect of the Milky Way disc. Here, we model Gaia DR2 observed colour-magnitude diagrams to obtain the first detailed star formation history of the ~ 2-kpc bubble around the Sun. It reveals three conspicuous and narrow episodes of enhanced star formation that we can precisely date as having occurred 5.7, 1.9 and 1 Gyr ago. Interestingly, the timing of these episodes coincides with proposed Sgr pericentre passages according to i) orbit simulations, ii) phase space features in the Galactic disc, and iii) Sgr stellar content. These findings most likely suggest that Sgr has also been an important actor in the build-up of the Milky Way disc stellar mass, with its perturbations repeatedly triggering major episodes of star formation.

• The paper identifies three distinct star formation bursts, at ~5.7, 1.9, and 1 billion years ago, that correlate with pericentric passages of the Sagittarius dwarf galaxy.
• It employs high-precision Gaia DR2 photometry and astrometry combined with advanced CMD modeling to detail the temporal evolution of the Milky Way's star formation history.
• The research reveals that while the 5.7 Gyr burst impacts both thin and thick discs, later bursts predominantly affect the thin disc, challenging conventional galactic formation models.

The Recurrent Impact of the Sagittarius Dwarf Galaxy on the Star Formation History of the Milky Way Disc

The paper investigates the dynamic interplay between the Sagittarius dwarf galaxy (Sgr) and the Milky Way, particularly focusing on the effects of Sgr on the star formation history of the Milky Way's disc. Leveraging data from Gaia Data Release 2 (Gaia DR2), the authors analyze the star formation history (SFH) within a 2-kiloparsec bubble around the Sun, obtaining unprecedented detail about episodic enhancements in the Galactic star formation rate.

Key Findings

1. Star Formation Episodes: This paper identifies three distinct episodes of enhanced star formation within the analyzed region, occurring approximately 5.7, 1.9, and 1 billion years ago. Each episode is defined with high temporal precision and appears to coincide with pericentric passages of the Sgr galaxy, thereby suggesting a correlation between these events and gravitational perturbations introduced by Sgr interactions.
2. Significance of Data and Methodology: Utilizing Gaia DR2's precise photometry and astrometric data, combined with advanced color-magnitude diagram (CMD) modeling techniques, the authors successfully detailed the SFH over a broad spatial scale. The results showcase enhanced star formation throughout the Milky Way's history which could not be distinctly identified in prior studies using data from Hipparcos or methodologies with lower age resolution.
3. Chemical and Dynamical Diversity: The paper differentiates between components of the Milky Way disk by categorizing stars into kinematic groups, such as the thin and thick discs. This distinction reveals that while the 5.7 Gyr burst affects both disc components, subsequent bursts have a more pronounced impact on the thin disc.

Implications and Speculations

1. Dynamic Influences on Star Formation: The correspondence between the Sgr passages and star formation bursts supports the notion that minor mergers, such as those with satellite galaxies like Sgr, contribute significantly to modulation in star formation rates across galaxy discs—not merely through gravitational disturbances but potentially through induced gas accretion or shock wave propagation.
2. Constraints for Theoretical Models: These findings challenge existing theoretical simulations, which often predict that significant star formation enhancement occurs centrally during interactions. The demonstrated global enhancement across the disc necessitates theories that account for interactions at varied galactocentric radii and refines star formation feedback mechanisms.
3. Expansion of Cosmological Models: While simulations had previously highlighted the importance of massive mergers in galaxy formation and dynamics, this work emphasizes the need for detailed treatment of less massive satellite interactions, which appear critical not just for the dynamical history but also for building up the stellar mass in galaxy discs.

Future Directions

1. Further Observational Studies: To decisively attribute these star formation enhancements to specific interactions, further synchronous observational studies of dwarf galaxies and their disruption paths are suggested, possibly expanding beyond Sgr to other potential influencers like the Magellanic Clouds.
2. Simulation Refinement: Refining hydrodynamical simulations to incorporate complex multi-phase interstellar media and non-linear star formation triggers will help align theoretical predictions with observational evidence.
3. Exploration of Other Galactic Systems: The insights derived from the Milky Way should be tested against other spiral galaxies with known satellite interactions to assess if similar star formation histories can be mapped, thereby enhancing the understanding of galactic evolution patterns at a universal scale.

This paper illuminates an intricate aspect of the Milky Way's history and emphasizes the need for further integrative studies combining precise observational data with sophisticated cosmological models to decipher the causal relationships driving star formation events on galactic scales.