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Nature and nurture: the dynamical ages of in situ and accreted globular clusters in the Milky Way

This study presents a homogeneous census of dynamical ages for 93 Milky Way globular clusters, revealing that accreted clusters are predominantly dynamically young and providing robust statistical evidence that enigmatic low-energy clusters, despite their ambiguous origins, likely experienced early accretion into the Galaxy.

Original authors: Eugenio Carretta (INAF - Osservatorio di Astrofisica e Scienza dello Spazio di Bologna)

Published 2026-04-30

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CC BY 4.0

Original authors: Eugenio Carretta (INAF - Osservatorio di Astrofisica e Scienza dello Spazio di Bologna)

Original paper licensed under CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

Imagine the Milky Way galaxy as a giant, bustling city. For a long time, astronomers knew this city was built from two main sources:

Locals: Stars and star clusters born right here in the city (called in situ).
Immigrants: Stars and clusters that were born in smaller, neighboring towns and were later absorbed into the Milky Way (called accreted).

Usually, we can tell these two groups apart by looking at their "chemical makeup" (metallicity). It's like telling a local from an immigrant by their accent or the food they eat. But this new paper by Eugenio Carretta asks a different question: How "old" do these star clusters feel on the inside?

The "Gym" Analogy: Chronological vs. Dynamical Age

To understand the paper's main discovery, we need to distinguish between two types of age:

Chronological Age: This is the actual number of years since the cluster was born. It's like a person's birth certificate.
Dynamical Age: This is how much the cluster has been "shaken up" by the galaxy's gravity. Think of this as a person's physical fitness.
- A 20-year-old who has never exercised is "dynamically young" (their muscles are soft, they haven't been stressed).
- A 20-year-old who has been lifting heavy weights every day is "dynamically old" (their body has changed structure due to the stress).

In the galaxy, the "weights" are the tidal forces (the gravitational pull) of the Milky Way. The closer a cluster gets to the center of the galaxy, the harder it gets "worked out." Over time, this stress changes the cluster's shape and how its stars move.

The Big Discovery: The "Immigrant" Paradox

Carretta looked at 93 star clusters and measured their "dynamical age" (how much they've been shaken up). Here is what he found:

1. The Immigrants are "Fresh"
Most of the clusters that came from other galaxies (the accreted ones) are dynamically young.

The Metaphor: Imagine a group of people who just moved to the city. Even if they are 50 years old, they haven't been "stressed" by the city's heavy traffic yet. They are still in their original, relaxed state.
The Data: About 61% of these immigrant clusters are "dynamically young." If we ignore a confusing middle group (the "Low Energy" clusters), that number jumps to 70%. They haven't been shaken up enough by the Milky Way's gravity to change their structure.

2. The Locals are "Worked Out"
The clusters born inside the Milky Way (in situ) are mostly dynamically old.

The Metaphor: These are the locals who have lived in the city center their whole lives. They have been buffeted by the city's gravity for billions of years. They are "fit" and their structure has changed significantly.
The Data: About 62% of the local clusters are "dynamically old."

The Mystery of the "Low Energy" Group

There is a specific group of clusters called Low Energy (LE) clusters. They are tricky because:

Their "accent" (chemistry) says they are immigrants (they belong to the satellite branch).
But their "fitness" (dynamical age) looks like locals. They are as "worked out" as the in-situ clusters.

The Paper's Explanation:
Why do these immigrant clusters look so "fit" if they just arrived?
Carretta suggests these clusters were immigrants who arrived a very long time ago.

The Analogy: Imagine a family that moved to the city 10,000 years ago. Even though they are technically "immigrants," they have lived in the city so long that they have been "worked out" by the city's gravity just like the locals.
The paper argues that the galaxy that these LE clusters came from merged with the Milky Way very early in the galaxy's history. This gave the Milky Way's gravity plenty of time to "train" them, making them look like locals even though they are immigrants.

The "Nature vs. Nurture" Conclusion

The title of the paper, "Nature and Nurture," perfectly sums up the findings:

Nature (Origin): Where the cluster was born determines its chemical "accent."
Nurture (Environment): How long the cluster has lived in the Milky Way's strong gravity determines its "dynamical age" (how much it has changed).

The Takeaway:
Most immigrant clusters are "dynamically young" because they arrived recently and haven't had time to be shaped by the Milky Way's gravity. The few that look "old" (like the Low Energy group) are the ones that arrived so long ago that the galaxy had time to reshape them. This helps astronomers understand not just what the Milky Way is made of, but when and how it grew.

1. Problem Statement

The age-metallicity relation (AMR) of Milky Way (MW) globular clusters (GCs) is bifurcated into two parallel branches: a metal-rich branch representing clusters formed in situ (within the Galaxy) and a metal-poor branch representing clusters accreted from satellite systems. While the origins of many GCs are now identifiable via chemo-dynamical parameters from the Gaia mission, the dynamical evolution of these populations remains less understood.

The central problem addressed is whether the "nature" (origin: in situ vs. accreted) and "nurture" (environment: the strength of the Galactic tidal field) can be distinguished by analyzing the dynamical ages of GCs. Specifically, the paper investigates if accreted clusters retain a "dynamically young" signature due to formation in weaker tidal fields, and how the enigmatic Low-Energy (LE) GCs (often associated with early merger events like Kraken/Koala/Heracles) fit into this framework, given their ambiguous placement on the AMR.

2. Methodology

The study utilizes a homogeneous census of 93 Milky Way globular clusters. The methodology involves the following steps:

Dynamical Age Definition: The authors define the dynamical age ( $N_h$ $N_{h}$ ) as the ratio between the chronological age ( $t_{age}$ $t_{a g e}$ ) and the half-mass relaxation time ( $t_{rh}$ $t_{r h}$ ):
$N_h = t_{age} / t_{rh}$
- $t_{age}$ values are sourced from the homogeneous set by Kruijssen et al. (2019).
- $t_{rh}$ values are taken from the Baumgardt et al. (2019) catalogue.
Classification Threshold: Clusters with $N_h < 8$ are classified as dynamically young. This threshold corresponds to the point where the chemically polluted stellar population (the dominant component) transitions from a radially anisotropic velocity distribution to an isotropic one, indicating significant dynamical evolution.
Progenitor Classification: GCs are categorized based on their progenitor systems, primarily following Massari et al. (2019, M19) and Gaia data:
- In Situ: Main Disc (MD) and Main Bulge (MB).
- Accreted: Gaia-Enceladus (GE), Helmi streams (H99), Sequoia (SEQ), Sagittarius (SGR), High-Energy (HE), and Low-Energy (LE).
- Note: NGC 6441 was reclassified from LE to MB based on chemical and AMR evidence.
Statistical Analysis: The study employs the two-sample Kolmogorov-Smirnov (K-S) test to compare the cumulative distributions of dynamical ages between different groups, testing the null hypothesis that samples are drawn from the same distribution.
Correlation Analysis: The paper correlates dynamical ages with the slope of the global mass function and half-mass relaxation times to assess the impact of the MW tidal field (mass loss and structural compactness).

3. Key Contributions

Homogeneous Census: Provides the first unified analysis of dynamical ages for a large sample of MW GCs, explicitly linking these ages to their specific progenitor systems identified by Gaia.
Quantification of "Nature vs. Nurture": Demonstrates that the dynamical youth of accreted clusters is a robust statistical feature, distinguishing them from in situ clusters.
Resolution of the LE GC Enigma: Offers a statistical explanation for the Low-Energy GCs. Despite being located on the satellite branch of the AMR, their dynamical age distribution resembles in situ clusters. The paper argues this is due to an early accretion event, allowing the MW tidal field sufficient time to drive their dynamical evolution.
Tidal Field Evidence: Links the dynamical state of GCs to the mass and accretion time of their progenitors, showing that clusters from smaller, later-accreted satellites remain dynamically young.

4. Key Results

Dynamical Youth of Accreted Clusters:
- 61% of all accreted GCs are dynamically young ( $N_h < 8$ ).
- When excluding the ambiguous LE GCs, this fraction rises to 70%.
- Specific groups like Sequoia, Sagittarius, Helmi streams, and High-Energy clusters are almost entirely dynamically young.
Dynamical Evolution of In Situ Clusters:
- The majority of in situ GCs (62%) are dynamically old.
- Bulge (MB) clusters show a wider range of dynamical ages compared to Disc (MD) clusters, with MB clusters being the most evolved.
The LE GC Anomaly:
- The distribution of dynamical ages for LE GCs is statistically indistinguishable from in situ (MB and MD) clusters (K-S p-value = 0.800).
- However, LE GCs are distinct from other accreted groups (e.g., K-S p-value = 0.000 vs. H99).
- Interpretation: LE GCs likely originated in a massive satellite that merged very early in the MW's assembly history. This early merger allowed the MW's strong tidal field to act on these clusters for a long duration, aging them dynamically despite their accreted origin.
Gaia-Enceladus (GE) as a Transition Group:
- GE clusters show a mixed distribution, containing both dynamically young and old members, suggesting a transition between the "early merger" (LE) and "late accretion" scenarios.
Tidal Field Correlation:
- A strong correlation exists between the mass function slope and relaxation times. In situ and LE GCs (subject to stronger tidal fields) show a wider range of slopes (indicating varying degrees of mass loss and evolution).
- Accreted GCs (subject to weaker fields in their progenitors) show a restricted range of slopes, confirming they have not undergone significant dynamical evolution even after accretion.
Accretion Time vs. Dynamical Age:
- There is a correlation where the fraction of dynamically young GCs increases as the estimated accretion time becomes more recent (later in MW history) and as the progenitor stellar mass decreases.

5. Significance

This paper provides statistically robust evidence that dynamical age is a powerful tracer of a globular cluster's origin and evolutionary history.

It validates the scenario that most accreted GCs are "fossils" of their formation in weak tidal fields, retaining a "dynamically young" signature even after merging with the Galaxy.
It resolves the paradox of the LE GCs by proposing an early accretion scenario, aligning their dynamical properties with in situ clusters while maintaining their chemical signature of accretion.
The study suggests that dynamical ages, when combined with chemo-dynamical data, offer a critical new observable for reconstructing the assembly history of the Milky Way, distinguishing between early massive mergers and late minor accretions.