Observations investigate a compact stellar system around nearby galaxy
Tomasz Nowakowski
astronomy writer
Stephanie Baum
scientific editor
Robert Egan
associate editor
An international team of astronomers has performed photometric and spectroscopic observations of a compact stellar system around a nearby spiral galaxy known as NGC 7531. Results of the observational campaign, September 17 on the arXiv preprint server, indicate that the investigated stellar system is an ultra-compact dwarf galaxy.
The outer regions of galaxies may contain stellar substructures formed when satellite galaxies merge with their hosts and break apart into debris. Studies suggest that many of the disrupted satellite dwarf galaxies once harbored nuclear star clusters (NSCs) and tend to survive the tidal disruption of their host galaxy and morphologically evolve into—for instance—ultra-compact dwarf galaxies (UCDs).
NGC 7531 is an intermediate spiral galaxy located some 72.4 million light years away in the constellation Grus. The galaxy, which was discovered in 1836, has a size of about 95,000 light years. Previous observations of NGC 7531 have found that it hosts an enormous cloud of shell-like tidal stellar debris and a compact stellar system (CSS) embedded within it.
Now, a new study by a group of astronomers led by David MartÃnez-Delgado of the Aragon Center for Â鶹ÒùÔºics of the Cosmos in Spain sheds more light on the nature of this stellar system around NGC 7531. The new findings are based on the analysis of data mainly from the DESI Legacy Imaging Survey and from the Keck Observatory.
"Our goals are to determine the nature of the CSS, reconstruct its accretion history, and understand how the large, diffuse shell-like structure formed," the researchers explain.
The study found that the CSS around NGC 7531 has a stellar mass of about 3.7 million solar masses and a half-light radius of 45.6 light years. The age of the system was estimated to be around 3.7 billion years, while its metallicity was measured to be approximately 0.13 dex.
Furthermore, it turned out that the CSS experienced a burst of star formation some one billion years ago. It appears that the system was likely an NSC that experienced and may still be experiencing tidal stripping.
According to the authors of the paper, the findings strongly suggest that the investigated CSS is an ultra-compact dwarf galaxy. Therefore, they designated it NGC 7531-UCD1. They added that NGC 7531-UCD1 is a crucial addition to the list of objects confirming the NSC-UCD formation pathway.
"Our findings agree with theoretical predictions about the NSC to UCD formation pathway via tidal stripping, and further confirm the presence of these objects outside of our Milky Way," the researchers conclude.
The collected data also allowed the astronomers to reproduce the merger event, finding that two pericentric passages are needed to create the observed shell and the trailing stream. The timing of the first pericenter passage coincides with the star formation burst that took place one billion years ago.
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More information: David MartÃnez-Delgado et al, A Pearl in the Shell: an ultra-compact dwarf within the tidal debris surrounding spiral galaxy NGC 7531, arXiv (2025).
Journal information: arXiv
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