A Near-infrared RR Lyrae census along the southern Galactic plane: the Milky Way's stellar fossil brought to light

TL;DR

This study used near-infrared data and machine learning to identify and analyze nearly 1900 RR Lyrae stars along the southern Galactic plane, revealing insights into the Milky Way's ancient stellar populations and metallicity structure.

Contribution

It presents the first extensive near-infrared RR Lyrae census in the Galactic plane using novel data-driven methods and machine learning for characterization and metallicity estimation.

Findings

Identified 1892 high-confidence RR Lyrae stars in the Galactic plane.

Derived the metallicity distribution function showing structural similarities to other stellar populations.

Modelled the MDF with multiple components indicating metallicity gradients and different stellar populations.

Abstract

RR Lyrae stars (RRLs) are tracers of the Milky Way's fossil record, holding valuable information on its formation and early evolution. Owing to the high interstellar extinction endemic to the Galactic plane, distant RRLs lying at low Galactic latitudes have been elusive. We attained a census of 1892 high-confidence RRLs by exploiting the near-infrared photometric database of the VVV survey's disk footprint spanning $\sim$70$^\circ$ of Galactic longitude, using a machine-learned classifier. Novel data-driven methods were employed to accurately characterize their spatial distribution using sparsely sampled multi-band photometry. The RRL metallicity distribution function (MDF) was derived from their $K_s$-band light curve parameters using machine-learning methods. The MDF shows remarkable structural similarities to both the spectroscopic MDF of red clump giants and the MDF of bulge RRLs. We model the MDF with a multi-component density distribution and find that the number density of stars associated with the different model components systematically changes with both the Galactocentric radius and vertical distance from the Galactic plane, equivalent to weak metallicity gradients. Based on the consistency with results from the ARGOS survey, three MDF modes are attributed to the old disk populations, while the most metal-poor RRLs are probably halo interlopers. We propose that the dominant [Fe/H] component with a mean of $-1$ dex might correspond to the outskirts of an ancient Galactic spheroid or classical bulge component residing in the central Milky Way. The physical origins of the RRLs in this study need to be verified by kinematical information.