Tidal stripping observed as a globular cluster sheds its stars

Using the Anglo-Australian Telescope (AAT), astronomers have observed a nearby galactic globular cluster known as NGC 6569. Results of the observational campaign, published December 22 on the arXiv preprint server, indicate that this galaxy is actively losing stars through tidal stripping.

Galactic bulge cluster

NGC 6569 is a globular cluster in the Milky Way’s bulge, at a distance of some 35,500 light years from Earth. With a mass of about 230,000 solar masses and metallicity of -0.8 dex, it is a relatively massive and moderately metal-rich cluster, and its age is estimated to be 13 billion years.

A team of astronomers led by Joanne Hughes of Seattle University has recently observed NGC 6569 with AAT as part of the Milky Way Bulge Extra-Tidal Star Survey (MWBest), which aims to understand globular cluster (GC) dissolution in the inner Milky Way. In general, theoretical models suggest that bulge GCs within 6,500 light years from the galactic center may lose up to 80% of their mass to the field.

Observations show that in the galactic halo about 26% of GCs exhibit tidal tails, while 42% of them show distinct extra-tidal features; however, these features are rarely detected around bulge GCs.

Tidal debris in NGC 6569

Using AAT, Hughes’ team obtained medium-resolution spectra of 303 stars in NGC 6569. By analyzing these spectra, they found spectroscopic evidence for tidal debris associated with this cluster.

According to the paper, the astronomers identified 40 stars about 7 to 30 arcminutes from the center of NGC 6569, which they interpret as bona fide tidal debris. Five of these stars appear to be part of a halo of tidal debris around this cluster.

Losing its stars

Moreover, the researchers compared the dynamically bound cluster stars of NGC 6569 with the surrounding field. They found that about 35% of the local stellar population shares its proper motion. Combining the chemo-dynamical constraints with a dynamical analysis, they estimate that the cluster is undergoing a continuous mild stripping, losing about 5.6% of its mass every 1 billion years.

The observations also suggest that NGC 6569 is moving through a tube of its own tidal debris. However, further studies, in particular N-body simulations, are required in order to verify this hypothesis.

All in all, the authors of the paper concluded that the structural signatures of NGC 6569 are fully consistent with the ongoing mass loss inferred from a chemo-dynamical analysis. This indicates that the cluster is actively shedding stars into the bulge field.

The findings provide evidence that NGC 6569 is actively losing stars through tidal stripping, contributing to the bulge field population at a present rate of 1.0–1.6 solar masses per year, which corresponds to about 5.6% of its present-day mass per billion years.

More information: arXiv