Clues to the formation of Liller 1 from modeling its complex star formation history

TL;DR

This study reveals that Liller 1 experienced multiple star formation episodes over its lifetime, indicating a complex formation history that supports its origin as a relic of primordial structures contributing to the Galactic bulge.

Contribution

First detailed star formation history analysis of Liller 1, showing multiple episodes and challenging previous formation scenarios.

Findings

Liller 1 formed stars over its entire lifetime.

Three main star formation episodes identified: 12-13 Gyr ago, 6-9 Gyr ago, and 1-3 Gyr ago.

Old population makes up about 70% of stellar mass.

Abstract

Liller 1 and Terzan 5 are two massive systems in the Milky-Way bulge hosting populations characterized by significantly different ages ($\Delta t>7-8$ Gyr) and metallicities ($\Delta$[Fe/H]$\sim1$ dex). Their origin is still strongly debated in the literature and all formation scenarios proposed so far require some level of fine-tuning. The detailed star formation histories (SFHs) of these systems may represent an important piece of information to assess their origin. Here we present the first attempt to perform such an analysis for Liller 1. The first key result we find is that Liller 1 has been forming stars over its entire lifetime. More specifically, three broad SF episodes are clearly detected: 1) a dominant one, occurred some 12-13 Gyr ago with a tail extending for up to $\sim3$ Gyr, 2) an intermediate burst, between 6 and 9 Gyr ago, 3) and a recent one, occurred between 1 and 3 Gyr ago. The old population contributes to about $70\%$ of the total stellar mass and the remaining fraction is almost equally split between the intermediate and young populations. If we take these results at a face value, they would suggest that this system unlikely formed through the merger between an old globular cluster and a Giant Molecular Cloud, as recently proposed. On the contrary, our findings provide further support to the idea that Liller 1 is the surviving relic of a massive primordial structure that contributed to the Galactic bulge formation, similarly to the giant clumps observed in star-forming high-redshift galaxies.