Discovery of Two New Globular Clusters in the Milky Way

TL;DR

This paper reports the discovery of two new globular clusters near the Galactic plane in the Milky Way, identified through infrared surveys, and characterizes their properties including distance, metallicity, and structure.

Contribution

The study presents the serendipitous discovery and detailed analysis of two previously unknown globular clusters using infrared survey data and isochrone fitting techniques.

Findings

Both clusters are located in the far-half region of the Milky Way.

Distances are approximately 29 kpc and 16 kpc, with metallicities around -2.2 and -2.1.

Clusters likely belong to the Milky Way halo.

Abstract

The spatial distribution of known globular clusters (GCs) in the Milky Way shows that the current census of GCs is incomplete in the direction of the Galactic plane. We present the discovery of two new GCs located close to the Galactic plane in the sky. These two GCs, RLGC 1 and RLGC 2, were discovered serendipitously during our new cluster survey (Ryu & Lee 2018) based on near-Infrared and mid-Infrared survey data. The two GCs show a grouping of resolved stars in their $K$ band images and a presence of faint diffuse light in their outer regions in the WISE $W1$ band images. They also show prominent red giant branches (RGBs) in their $K$ vs. $(J-K)$ color-magnitude diagrams (CMDs). We determine structural parameters of the two GCs using King profile fitting on their $K$ band radial number density profiles. The determined values are consistent with those of known GCs. Finally, we determine the distances, metallicities, and reddenings of the two GCs using the isochrone fitting on their CMDs. For the fitting, we assume that the ages of the two GCs are 12.6 Gyr and the brightest RGB stars of each cluster correspond to the tip of the RGB. Distances and metallicities of the two GCs are estimated to be $d=28.8\pm4.3$ kpc and $\textrm{[Fe/H]}=-2.2\pm0.2$ for RLGC 1 and $d=15.8\pm2.4$ kpc and $\textrm{[Fe/H]}=-2.1\pm0.3$ for RLGC 2. These results show that the two GCs are located at the far-half region of the Milky Way and they may belong to the halo of the Milky Way.