Multiple Stellar Populations outside the tidal radius of NGC1851 through Gaia DR3 XP Spectra

TL;DR

This study uses Gaia DR3 XP spectra to analyze the spatial distribution and kinematics of multiple stellar populations in the globular cluster NGC1851, revealing their presence beyond the tidal radius and differences in morphology and motion.

Contribution

It provides the first comprehensive analysis of multiple populations in NGC1851 extending beyond its tidal radius using Gaia DR3 spectra and synthetic photometry.

Findings

Both canonical and anomalous populations are found outside the tidal radius.

80% of stars outside the tidal radius belong to the canonical population.

The canonical population has a circular shape, while the anomalous is elliptical.

Abstract

Ancient Galactic Globular Clusters (GCs) have long fascinated astronomers due to their intriguing multiple stellar populations 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. We focus on one of the most debated Type~II GCs, NGC1851, to investigate its MPs across a wide spatial range of up to 50 arcmin from the cluster center. By using Gaia DR3 low-resolution XP spectra, we generate 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 CMDs from the synthetic photometry in the BVI bands and in the $\rm f415^{25}$ band introduced in this work, we identify distinct stellar sequences associated with different heavy-element chemical composition. Our results suggest that the canonical and the anomalous populations reside both inside and outside the tidal radius of NGC1851, up to a distance that exceeds by 3.5 times its tidal radius. However, $\sim$80\% of stars outside the tidal radius are consistent with belonging to 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. Our results reveal a flat/increasing velocity dispersion profile in the outer regions and hints of a tangentially anisotropic motion in the outer regions, indicating a preference for stars to escape on radial orbits.