When the Milky Way turned off the lights: APOGEE provides evidence of star formation quenching in our Galaxy

Abstract: Quenching, the cessation of star formation, is one of the most significant events in the life cycle of galaxies. We show here the first evidence that the Milky Way experienced a generalised quenching of its star formation at the end of its thick disk formation $\sim$9 Gyr ago. Elemental abundances of stars studied as part of the APOGEE survey reveal indeed that in less than $\sim$2 Gyr the star formation rate in our Galaxy dropped by an order-of-magnitude. Because of the tight correlation between age and alpha abundance, this event reflects in the dearth of stars along the inner disk sequence in the [Fe/H]-[$\alpha$/Fe] plane. Before this phase, which lasted about 1.5 Gyr, the Milky Way was actively forming stars. Afterwards, the star formation resumed at a much lower level to form the thin disk. These events are very well matched by the latest observation of MW-type progenitors at high redshifts. In late type galaxies, quenching is believed to be related to a long and secular exhaustion of gas. In our Galaxy, it occurred on a much shorter time scale, while the chemical continuity before and after the quenching indicates that it was not due to the exhaustion of the gas. While quenching is generally associated with spheroids, our results show that it also occurs in galaxies like the Milky Way, possibly when they are undergoing a morphological transition from thick to thin disks. Given the demographics of late type galaxies in the local universe, in which classical bulges are rare, we suggest further that this may hold true generally in galaxies with mass lower than or approximately $M^*$, where quenching could be directly a consequence of thick disk formation. We emphasize that the quenching phase in the Milky Way could be contemporaneous with, and related to, the formation of the bar. We sketch a scenario on how a strong bar may inhibit star formation.

On Star Formation Quenching in the Milky Way

This paper investigates the phenomenon of star formation quenching in the Milky Way galaxy, utilizing data from the APOGEE survey. Star formation quenching, defined as a significant reduction or cessation in star formation activity, is a crucial event in the evolutionary timeline of galaxies. While typically associated with massive galaxies undergoing transformations, this work presents evidence that the Milky Way itself experienced such an event approximately 9 billion years ago.

The research utilizes elemental abundance data of stars in the solar vicinity, focusing on the relationship between age and $\alpha$-element abundances. By analyzing these elements, the study reconstructs the Galaxy’s star formation history (SFH). It identifies a significant drop in the star formation rate (SFR) by an order of magnitude in less than 2 billion years, marking the transition from a thick-disk dominated formation to the more quiescent thin-disk growth. This phase contrast is evident in the [Fe/H]-[$\alpha$/Fe] distribution, where a lack of stars is observed on the inner-disk sequence during this quenching period.

The paper also challenges several commonly held assumptions about galactic evolution and quenching mechanisms, providing a distinct perspective on the transition from thick to thin disk phases within the Milky Way. Notably, it excludes the possibility that gas exhaustion was the primary driver of this quenching phase, emphasizing the continuous chemical evolution between star-forming phases. By comparing the empirical data with modeling results, the authors argue that an internal dynamical process, potentially involving the formation of a strong bar within the disk, could have inhibited star formation by inducing heightened turbulence and preventing efficient gas cooling and collapse.

This study broadens the scope of quenching to encompass galaxies lacking significant bulge components, such as pure disk systems like the Milky Way, suggesting that mechanisms like bar formation may be common inhibitors especially in galaxies of masses approximating or below $M^*$. It further hypothesizes that these processes may contribute to the morphological transformation observed in spirals transitioning into lenticular forms, correlating with high redshift observations of Milky Way-like progenitors experiencing similar quenching phases.

The implications of this work extend to our understanding of galaxy formation and evolution, presenting a nuanced view that thick-disk formation and subsequent morphological transitions play a crucial role in the quenching phenomena. Future research in this domain could focus on detailing the physical processes during this quenching phase, refining the timeline, and establishing a broader comparative framework with other spiral galaxies. Additionally, continued observations and improved simulation models could further elucidate the association between internal galactic dynamics, such as bar formation, and the cessation of star formation activity in low to moderate mass galaxies.