Multiple stellar populations found outside the tidal radius of NGC 1851 via Gaia DR3 XP spectra

¹ Istituto Nazionale di Astrofisica – Osservatorio Astronomico di Padova, Vicolo dell’Osservatorio 5, 35122 Padova, Italy
e-mail: giacomo.cordoni@unipd.it
² Istituto Nazionale di Astrofisica – Osservatorio Astrofisico di Arcetri, Largo Enrico Fermi, 5, Firenze, 50125 Firenze, Italy
³ Dipartimento di Fisica e Astronomia “Galileo Galilei”, Univ. di Padova, Vicolo dell’Osservatorio 3, 35122 Padova, Italy
⁴ Center for Galaxy Evolution Research and Department of Astronomy, Yonsei University, Seoul 03722, Korea

Received: 14 June 2023
Accepted: 8 August 2023

Abstract

Aims. Ancient galactic globular clusters (GCs) have long fascinated astronomers due to their intriguing multiple stellar populations (MPs), which are characterized by variations in light element abundances. Among these clusters, type II GCs stand out as they exhibit stars with large differences in heavy-element chemical abundances. These enigmatic clusters, comprising approximately 17% of analyzed GCs with MPs, have been hypothesized to be the remnants of accreted dwarf galaxies.

Methods. We focus on one of the most debated type II GCs, namely, NGC 1851, to investigate its MPs across a wide spatial range of up to 50 arcmin from the cluster center. By using Gaia Data Release 3 low-resolution XP spectra, we generated synthetic photometry to perform a comprehensive analysis of the spatial distribution and kinematics of the canonical and anomalous populations within this GC. By using appropriate color-magnitude diagrams from the synthetic photometry in the BVI bands and in the f415²⁵ band introduced in this work, we identified distinct stellar sequences associated with different heavy-element chemical compositions.

Results. Our results suggest that the canonical and the anomalous populations reside both inside and outside the tidal radius of NGC 1851, up to a distance that exceeds its tidal radius 3.5 times. However, about 80% of stars outside the tidal radius are consistent with characteristics that class them among the canonical population, emphasizing its dominance in the cluster’s outer regions. Remarkably, canonical stars exhibit a more circular on-sky morphology, while the anomalous population displays an elliptical shape. Furthermore, we delve into the kinematics of the multiple populations, examining velocity dispersions, rotation patterns, and potential substructures. Our results reveal a flat or increasing velocity dispersion profile in the outer regions. Additionally, we observe hints of a tangentially anisotropic motion in the outer regions, indicating a preference for stars to escape on radial orbits. Our work demonstrates the capability of synthetic photometry, based on Gaia spectra, to explore multiple populations across the entire cluster field.