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

TL;DR

This study tests whether Liller 1 is a major building block of the Galactic bulge by chemically tagging its members against bulge, inner disc, outer disc, and solar-neighborhood stars using APOGEE DR17 data. A robust, bootstrap-enabled, differential abundance metric reveals that Liller 1's $\alpha$-element patterns are depressed relative to the bulge at $2$–$3\sigma$, and its disc-analogues show significant chemical deviations, arguing against a disc-instability or bulge-building-clump origin. The analysis disfavors a rejuvenated globular cluster or globular-cluster merger origin and suggests Liller 1 is a minor contributor with possible extragalactic provenance; age and chemical-evolution considerations further support a low-mass progenitor scenario ($M_\star \approx 10^{8} M_\odot$). Overall, Liller 1 appears chemically distinct from bulge, disc, and solar neighbourhood populations, implying a limited role in the inner Galaxy’s mass assembly and motivating future chemodynamical tests. The work provides a framework for connecting stellar chemistry to Galaxy assembly through statistical comparisons across multiple Galactic components.

Abstract

Liller 1 is a stellar system orbiting within the inner 0.8kpc 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 disc, outer disk and solar neighbourhood. The chemistry of Liller 1 deviates from that of the bulge population at the 2-3$σ$ 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 2$σ$ 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 disc populations, suggesting that the progenitor of Liller 1 is unlikely to be an in-situ massive clump formed at high redshift, from disc 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.

Figures (15)

• Figure 1: Top panel: Right ascension $\alpha$ and declination $\delta$ (in degrees) of the Liller 1 stars colour coded by probability of cluster membership The cluster Jacoby radius $r_j$baumgardt2021 displayed as a reference. The smaller tidal radius from saracino2015 is shown with the dotted line. All but one of the stars in our sample of Liller 1 candidates are contained within the tidal radius. Bottom panel. Near-IR colour image from the VVV Survey made using the JHKs filters minniti_vista_2010 covering approximately $18^{\prime} \times 12^{\prime}$, oriented along Galactic coordinates, with longitude increasing to the left and latitude increasing upwards. Overlaid are the Liller 1 stars colour-coded by radial velocity.
• Figure 2: (J)-(J-K) colour magnitude diagram of Liller 1 stars in this paper in red with those from Alvarez2024Fanelli2024 plotted in grey. For a Liller 1 sample that extends further down the red giant branch we refer the reader to Valenti2010 and ferraro2021.
• Figure 3: Combined APOGEE spectra for star 2M17333292-3323168/67 in black (dotted line). The synthetic spectrum for the F24 parameters is shown in light blue, and the APOGEE parameters spectrum is shown in navy. The shaded regions around each spectrum indicates the error. (a), (b), (c): comparison between both spectra and the observed spectrum for the CO, OH, CN molecular bands respectively. The lines are indicated with the grey dashed lines. The grey shaded regions indicate further OH bands.
• Figure 4: Combined APOGEE spectra for star 2M17333292-3323168 indicated with the black dotted line. The blue shaded region indicates the parameter space covered by the spectra calculated with the stellar parameters and abundances adopted from Fanelli2024. To check the inference made from Figure \ref{['fig:spec']} is not a consequence of using the APOGEE line list and solar reference abundances from Grevesse2007, we model the spectrum using the VALD3 atomic linelist and Magg2022 solar reference abundances. The sources from which the molecular lines are from are shown in Table \ref{['tab:molecularlinelists']}. These encompass the molecular line list data for CO, CN and OH from VALD3, APOGEE and B. Plez. All in all, the use of different line list and solar reference abundances here did not significantly impact the spectral fit of the OH lines, the middle panel, as they are still overfit with respect to the observed spectrum.
• Figure 5: [N/Fe]-[C/Fe] of GCs from the VAC in the metallicity range ($\mathrm{[Fe/H]}$) spanned by Liller 1 stars in the sample. This includes GCS: NGC 6388, NGC 6582, NGC 6553, NGC 6441, Terzan 12, Palomar 10, Palomar 1. There is one Liller 1 star that exceeds the threshold $\mathrm{[N/Fe] = +0.5}$.