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

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 at most a very weak C-N anticorrelation. Additionally, the CN-weak population exhibits an elongated spatial distribution that is likely linked to its fast rotation. Our derived metallicities reveal bimodal distributions in both populations, with ⟨[Fe/H]⟩ ≈ -1.60 and -1.45, which appear to be responsible for the discrete double RGB bumps in the CN-weak population and the large ${W}_{{\rm{F}}275{\rm{W}}-{\rm{F}}814{\rm{W}}}^{1G}$ <!-- --> range. From this discovery, we propose that M3 consists of two GCs, namely C1 (23%, ⟨[Fe/H]⟩ ≈ -1.60) and C2 (77%, ⟨[Fe/H]⟩ ≈ -1.45), each of which has its own C-N anticorrelation and structural and kinematical properties, which are strong indications of independent systems in M3. The fractions of the CN-weak population for both C1 and C2 are high compared to Galactic GCs but they are in good agreement with GCs in the Magellanic Clouds. We suggest that M3 is a merger remnant of 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 for M3. ^* Based on observations made with the Kitt Peak National Observatory (KPNO) 0.9 m telescope, which is operated by WIYN Inc. on behalf of a consortium of partner universities and organizations. † This work has made use of data from the European Space Agency (ESA) mission Gaia (https://www.cosmos.esa.int/gaia), processed by the Gaia Data Processing and Analysis Consortium (DPAC; https://www.cosmos.esa.int/web/gaia/dpac/consortium). Funding for the DPAC has been provided by national institutions, in particular the institutions participating in the Gaia Multilateral Agreement.