Discovery of RR Lyrae Stars in the Nuclear Bulge of the Milky Way

TL;DR

The paper reports the discovery of RR Lyrae stars near the Milky Way's center, providing evidence that ancient stars are present in the nuclear bulge, supporting the globular cluster merging formation scenario.

Contribution

This is the first direct detection of old RR Lyrae stars in the Galactic nuclear bulge, linking globular cluster mergers to bulge formation.

Findings

Presence of RR Lyrae stars indicates old stellar population.

Supports globular cluster merging as a formation mechanism.

Provides observational evidence of ancient stars in the nuclear bulge.

Abstract

Galactic nuclei, like the one of the Milky Way, are extreme places with high stellar densities and, in most cases, hosting a supermassive black hole. One of the scenarios proposed for the formation of the Galactic nucleus is by merging of primordial globular clusters (Capuzzo-Dolcetta 1993). An implication of this model is that this region should host stars characteristically found in old Milky Way globular clusters. RR Lyrae stars are primary distance indicators, well known representatives of old and metal-poor stellar populations, and therefore regularly found in globular clusters. Here we report the discovery of a dozen RR Lyrae ab-type stars in the vicinity of the Galactic center, i.e. in the so-called nuclear stellar bulge of the Milky Way. This discovery provides the first direct observational evidence that the Galactic nuclear stellar bulge contains ancient stars (>10 Gyr old). Based on this we conclude that merging globular clusters likely contributed to building-up the high stellar density in the nuclear stellar bulge of the Milky Way.