Is Liller 1 a building block of the Galactic bulge? - Evidence from APOGEE

Abstract

Liller 1 is a stellar system orbiting within the inner 0.8 kpc of the Galactic centre, characterised by a wide spread in age and metallicity, indicating a high mass. Liller 1 has been proposed to be a major contributor to the stellar mass of the Galactic bulge, yet its origin is subject to debate. We employ Sloan Digital Sky Survey IV (SDSS-IV) data from the Apache Point Observatory Galactic Evolution Experiment (APOGEE) to test scenarios proposed to explain the nature of Liller 1. Using a random sampling technique, we contrast the chemical compositions of Liller 1 stellar members with those of the bulge, inner disk, outer disk and solar neighbourhood. The chemistry of Liller 1 deviates from that of the bulge population at the 2-3σ level for α-elements Mg, Si, and Ca. We conclude that the progenitor of Liller 1 was not a major contributor of stellar mass to the bulge. Furthermore, we find the abundance pattern of Liller 1 to deviate at the 2σ level from that of inner disk stars, ruling out the cluster rejuvenation scenario. Finally, we find that Liller 1 is chemically distinct from solar and outer disk populations, suggesting that the progenitor of Liller 1 is unlikely to be an in-situ massive clump formed at high redshift, from disk gravitational instabilities, that migrated inwards and coalesced with others into the bulge. Finally, we suggest that Liller 1 is a minor contributor to the stellar mass of the inner Galaxy, possibly of extragalactic origin.