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Globular Cluster NGC6569 Tidal Stripping Milky Way

Galactic Globular Cluster NGC 6569 Found Losing Stars Through Tidal Stripping

Candidate stars identified as tidal debris surrounding the globular cluster NGC 6569. Credit: arXiv (2025), DOI: 10.48550/arxiv.2512.19074.

Astronomers using the Anglo-Australian Telescope (AAT) have turned their attention to the nearby globular cluster NGC 6569, uncovering compelling evidence that the system is shedding stars as a result of tidal stripping. The findings were detailed in a study released on the arXiv preprint server on December 22.

NGC 6569: A Galactic Bulge Cluster From the Early Milky Way

NGC 6569 is a prominent globular cluster positioned in the dense bulge of the Milky Way, some 35,500 light-years away from Earth. Weighing in at roughly 230,000 solar masses and possessing a metallicity of -0.8 dex, it ranks as a relatively massive and moderately metal-enriched cluster. Astronomers believe the cluster is around 13 billion years old, making it a relic from the galaxy's earliest era.

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Observational Campaign Using the Anglo-Australian Telescope

A recent observational campaign led by Joanne Hughes of Seattle University utilized the Anglo-Australian Telescope to study NGC 6569 as part of the MWbest programme. The survey is designed to investigate how globular clusters gradually disintegrate within the Milky Way's inner bulge. Theoretical predictions suggest that clusters orbiting within 6,500 light-years of the galactic center cloud lose up to four-fifths of their mass over time.

Background reading on cosmic systems and long-term changes:

Tidal Tails Rare in the Galactic Bulge

Although tidal tails and extended stellar features are commonly found among halo globular clusters, they are notably scarce in the bulge. Observations indicate that 26% of halo clusters host tidal tails, while 42% show extra-tidal structures, a stark contrast to bulge populations.

Tidal Debris Detected in NGC 6569

Spectroscopic Evidence From 303 Stars

Using the Anglo-Australian Telescope (AAT), Hughes and her colleagues collected medium-resolution from 303 stars within NGC 6569. Detailed analysis of this data revealed clear spectroscopic signs of tidal debris linked to the cluster.

Key Observational Findings

  • The study reports the identification of 40 stars located between 7 and 30 arcminutes from the cluster's center, which the team considers genuine remnants of tidal stripping.
  • Of these, five stars appear to form part of an extended halo of tidal debris surrounding NGC 6569.

Science discoveries and observational research:

NGC 6569 Is Gradually Losing Its Stars

The researchers also compared the dynamically bound stars of NGC 6569 with the surrounding stellar field and discovered that roughly 35% of nearby stars share the cluster's proper motion. By combining chemo-dynamical constraints with detailed dynamical modelling, the team concludes that the cluster is experiencing steady but mild tidal stripping, shedding around 5.6% of its mass every billion years.

Cluster May Be Moving Through Its Own Stellar Stream

The findings further indicate that NGC 6569 may be travelling through a stream of its own tidal debris. However, the authors stress that additional investigations, particularly N-body simulations, are needed to confirm this scenario.

Stars Feeding the Galactic Bulge

Overall, the authors conclude that the structural features observed in NGC 6569 align closely with the ongoing loss identified through chemo-dynamical analysis. This coherence strongly suggests that the cluster is actively releasing stars into the surrounding bulge field.

The results confirm that NGC 6569 is undergoing tidal stripping, feeding the bulge's stellar population at an estimated rate of 1.0 to 1.6 solar masses per year. This level of loss equates to roughly 5.6% of the cluster's current mass over the course of a billion years.

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