Globular Clusters as Chrono-Chemo-Kinematic Tracers of the Milky Way

Galactic Globular clusters (GCs) are among the oldest bound stellar systems in the Universe, hosting high-density populations of 10⁵–10⁷ stars, which make them invaluable fossils of galaxy assembly processes. High-resolution spectroscopy, coupled with high-precision photometry from the Hubble Space Telescope and the James Webb Space Telescope, and Gaia astrometry, has overturned the simple picture of GCs as single-age, single-chemical composition systems. Nearly every well-studied Galactic GC hosts multiple stellar populations: a first population (1P) which shares the light-element pattern of field halo stars at similar Fe abundance, and a second population (2P) with enhanced He, N, Na, and Al, and depleted C, O, and Mg. The abundance pattern of 2P stars bears the hallmarks of matter processed by hot proton-capture nuclear reactions, which has polluted the gas from which 2P objects have formed. A detailed model for the origin of these abundance anomalies, however, remains a matter of debate. Spectroscopic and photometric studies have provided evidence of a threshold in both mass and age, below which clusters do not host 2P stars. In addition, the so-called photometric chromosome maps and pseudo-color diagrams have revealed an unexpected complexity even within the “pristine” 1P population, suggesting the presence of [Fe/H] spreads of up to ~0.3 dex. The origin of such apparent spreads could be associated with primordial cloud inhomogeneity, internal enrichment, or unresolved binarity.

GCs account for a significant fraction of early star formation in galaxies. At the same time, both theory and observations suggest that the vast majority of GCs ever formed during the history of the Milky Way were destroyed. The 2P members of the long dissolved systems survive today as chemically anomalous field stars. Therefore, crucial information about the early histories of accretion and star formation of the Galaxy can be retrieved from studies of the GC remains that are found in the halo field. To distinguish major accretion events such as Heracles, Gaia Enceladus/Sausage from the early in situ halo (e.g., the Aurora population) will require a detailed mapping of the chromochemodynamical properties of the stellar remnants of these long-destroyed GCs in the halo field, in conjunction with those of their still existing counterparts. The data required to accomplish this goal involve the following: high-resolution spectroscopy for massive stellar samples, age determinations from deep color–magnitude diagrams and isochrone fitting, and Gaia-based kinematics and orbit integration. These data enable placing clusters on age–metallicity relations that disentangle the various progenitor systems, assigning clusters and field stars to specific accretion events or in situ formation.

These diagnostics extend beyond the Milky Way as well. Multiple populations have been identified in GCs belonging to the Magellanic Clouds and other dwarf galaxies in the Local Group, offering a comparative test of the mass and environmental thresholds for their formation. Multiple populations have also been identified in nuclear star clusters such as M54, and former nuclear clusters such as ω Centauri, suggesting a connection between GCs and dense stellar systems in the cores of dwarf galaxies. Finally, nitrogen-enhanced systems have been identified at high redshift, suggesting that proto-GC-like enrichment may be common in early, dense star-forming clumps, directly linking the local MP phenomenon with physical conditions in the early Universe.

This Research Topic calls for observational, theoretical, and data-driven contributions to advance an integrated view of GCs as both internally complex stellar laboratories and tracers of the evolutionary history of galaxies.

We welcome contributions on topics including, but not limited to:

● Origin and formation mechanisms of multiple stellar populations (light-element anticorrelations, polluter candidates, mass-budget problem)
● Chromosome maps and pseudo-color diagnostics, including new HST–JWST combinations reaching the M-dwarf regime
● Spectroscopic diagnostics at different metallicities, ages and masses, including He, C, N, O, Na, Mg, Al, Si, Li, and neutron-capture elements
● Possible [Fe/H] spread within first-generation populations and its physical origin
● Mass and environmental thresholds for multiple populations, including outer-halo and low-mass clusters
● Chemical tagging of dissolved/escaped 2P GC stars in the Galactic field (high-[N/O], high-[Na/Fe], high-[Al/Fe])
● Chemical properties of stellar populations in nuclear star clusters
● Machine-learning and large-survey approaches (e.g. Gaia BP/RP, APOGEE/Milky Way Mapper, 4MOST, WEAVE, MOONS) to identify GC-origin stars
● GCs as tracers of in-situ versus accreted halo Galactic components and individual accretion events
● Absolute and relative age-dating techniques (isochrone fitting, deep CMDs, white-dwarf cooling sequences, asteroseismology)
● Age–metallicity relations and reconstruction of GC progenitor systems
● Chronochemodynamical modelling linking ages, abundances, and orbital kinematics
● Reconciling spectroscopic and photometric multiple-population tagging
● Multiple populations in other environments: LMC, SMC and other dwarf galaxies
● Connections between GC chemistry and nitrogen-rich high-redshift galaxies
● Stellar mechanisms that could mimic abundance spreads and broaden the pseudo-color distribution in the CMD.

Article types and fees

This Research Topic accepts the following article types, unless otherwise specified in the Research Topic description:

• Brief Research Report
• Curriculum, Instruction, and Pedagogy
• Data Report
• Editorial
• FAIR² Data
• General Commentary
• Hypothesis and Theory
• Methods
• Mini Review
• ... View all formats

Articles that are accepted for publication by our external editors following rigorous peer review incur a publishing fee charged to Authors, institutions, or funders.

Article types

This Research Topic accepts the following article types, unless otherwise specified in the Research Topic description:

• Brief Research Report
• Curriculum, Instruction, and Pedagogy
• Data Report
• Editorial
• FAIR² Data
• General Commentary
• Hypothesis and Theory
• Methods
• Mini Review
• Opinion
• Original Research
• Perspective
• Review
• Study Protocol
• Technology and Code

Keywords: globular clusters, stellar populations, galaxy formation, galaxy kinematics and dynamics, spectroscopy, photometry

Important note: All contributions to this Research Topic must be within the scope of the section and journal to which they are submitted, as defined in their mission statements. Frontiers reserves the right to guide an out-of-scope manuscript to a more suitable section or journal at any stage of peer review.