(Re)mind the gap: a hiatus in star formation history unveiled by APOGEE DR17

TL;DR

This study confirms a gap in the star formation history of the Milky Way by analyzing APOGEE DR17 data, showing a sharp increase in [Fe/α] ratios during a transition phase, supporting models with a star formation hiatus.

Contribution

The paper demonstrates the existence of a star formation hiatus in the Milky Way using APOGEE DR17 data and interprets it with a detailed chemical evolution model based on the two-infall paradigm.

Findings

Clear evidence of a star formation hiatus from APOGEE data.

Sharp increase in [Fe/α] at nearly constant [α/H] during disc transition.

Model with a 3.5 Gyr star formation stop explains the observations.

Abstract

The analysis of several spectroscopic surveys indicates the presence of a bimodality between the disc stars in the abundance ratio space of [${\alpha}$/Fe] versus [Fe/H]. The two stellar groups are commonly referred to as the high-${\alpha}$ and low-${\alpha}$ sequences. Some models capable of reproducing such a bimodality, invoke the presence of a hiatus in the star formation history in our Galaxy, whereas other models explain the two sequences by means of stellar migration. Our aim is to show that the existence of the gap in the star formation rate between high-$\alpha$ and low-$\alpha$ is evident in the stars of APOGEE DR17, if one plots [Fe/$\alpha$] versus [$\alpha$/H], thus confirming previous suggestions by Gratton et al. (1996) and Fuhrmann (1998). Then we try to interpret the data by means of detailed chemical models. We compare the APOGEE DR17 red giant stars with the predictions of a detailed chemical evolution model based on the two-infall paradigm, taking also into account possible accretion of dwarf satellites. The APOGEE DR17 abundance ratios [Fe/$\alpha$] versus [$\alpha$/H] exhibit a sharp increase of [Fe/$\alpha$] at a nearly constant [$\alpha$/H] (where $\alpha$ elements considered are Mg, Si, O) during the transition between the two disc phases. This observation strongly supports the hypothesis that a hiatus in star formation occurred during this evolutionary phase. Notably, the most pronounced growth in the [Fe/$\alpha$] versus [$\alpha$/H] relation is observed for oxygen, as this element is exclusively synthesised in core-collapse supernovae. A chemical model predicting a stop in the star formation of a duration of roughly 3.5 Gyr, and where the high-$\alpha$ disc starts forming from pre-enriched gas by a previous encounter with a dwarf galaxy can well explain the observations.