Multiple Stellar Populations of Globular Clusters from Homogeneous Ca-CN-CH-NH Photometry. VI. M3 (NGC 5272) is not a Prototypical Normal Globular Cluster

TL;DR

This study reveals that the globular cluster M3 contains two distinct stellar populations with different chemical compositions and structural properties, suggesting it is a merger remnant of two separate clusters, challenging the idea of M3 being a typical globular cluster.

Contribution

It provides evidence that M3 is composed of two independent stellar populations with distinct chemical and kinematic properties, indicating a merger origin.

Findings

M3 has two populations with different C and N abundances.

The populations show different spatial distributions and metallicity bimodality.

M3 likely originated from a merger of two globular clusters in a dwarf galaxy environment.

Abstract

We present Ca-CN-CH-NH photometry for the well-known globular cluster (GC) M3 (NGC 5272). We show new evidence for two M3 populations with distinctly different carbon and nitrogen abundances, seen in a sharp division between CN-weak and CN-strong red-giant branches (RGBs) in M3. The CN-strong population shows a C-N anticorrelation that is a natural consequence of the CN cycle, while the CN-weak population shows no or a weak C-N anticorrelation. Additionally, the CN-weak population exhibits an elongated spatial distribution that is likely linked to its fast rotation. Our derived metallicity reveals bimodal metallicity distributions in both populations, with $\langle$[Fe/H]$\rangle\approx-$1.60 and $-$1.45, which appear to be responsible for the discrete double RGB bumps in the CN-weak and the large $W^{1G}_{F275W-F814W}$ range. From this discovery, we propose that M3 consists of two GCs, namely the C1 (23\%, $\langle$[Fe/H]$\rangle\approx-1.60$) and C2 (77\%, $\langle$[Fe/H]$\rangle\approx-1.45$), each of which has its own C-N anticorrelation and structural and kinematical property, which is a strong indication of independent systems in M3. The fractions of the CN-weak population for both the C1 and C2 are high compared to Galactic GCs but they are in good agreement with GCs in Magellanic Clouds. It is believed that M3 is a merger remnant of the two GCs, most likely in a dwarf galaxy environment, and accreted to our Galaxy later in time. This is consistent with recent proposals of an ex-situ origin of M3.