Observations investigate a compact stellar system around nearby galaxy
Image Amateur of NGC 7531 (left). Image of the study of the Desi inheritance imaging of NGC 7531 (right). The subtracted image of the sky of NGC 7531, as treated by the Gnuastro Noisechesel program, was used as a basis for photometry measures. The features are labeled: (a): Principal shell; (b): a weak exterior shell; (c): Counter Plume. Credit: arxiv (2025). DOI: 10.48550 / Arxiv. 2009.14038
An international team of astronomers has made photometric and spectroscopic observations of a compact stellar system around a neighboring spiral galaxy known as NGC 7531. Results of the observation campaign, published on September 17 on the arxiv Preprint Server, indicate that the studied stellar system is an ultra-compact dwarf galaxy.
The external regions of galaxies may contain stellar sub-structures formed when satellite galaxies merge with their hosts and separate into debris. Studies suggest that many dwarf galaxies disturbed have once hosted clusters of nuclear stars (NSC) and tend to survive the disturbance of the tides of their host galaxy and to evolve morphologically into nancial galaxies by Ultra Compact (UCD).
NGC 7531 is a galaxy in an intermediate spiral located at around 72.4 million light years in the GRUS constellation. The galaxy, discovered in 1836, has a size of around 95,000 light years. Anterior observations of NGC 7531 have discovered that it hosts a huge cloud of stellar tidal tide and a compact stellar system (CSS).
Now, a new study by a group of astronomers led by David MartÃnez-Delgado of the Aragon Center for Physics of the Cosmos in Spain throws more light on the nature of this stellar system around NGC 7531. The new results are based on the analysis of data mainly from the survey on the imagery of Desi and the Keck observatory.
“Our objectives are to determine the nature of the CSS, to rebuild its history of accretion and to understand how the great diffuse structure in the shape of a shell has formed,” explain the researchers.
The study revealed that the CSS around NGC 7531 has a stellar mass of around 3.7 million solar masses and a half-light radius of 45.6 light years. The age of the system was estimated at around 3.7 billion years, while its metallicity was measured at around 0.13 DEX.
In addition, it turned out that the CSS experienced an explosion of stars formation a billion years ago. It seems that the system was probably an NSC which knew and can still undergo a tide stripping.
According to the authors of the article, the results strongly suggest that the CSS studied is an ultra-compact dwarf galaxy. Therefore, they appointed it NGC 7531-UCD1. They added that NGC 7531-UCD1 is a crucial addition to the list of objects confirming the NSC-UCD training route.
“Our results agree with the theoretical predictions on the NSC training path to UCD via the stripping of the tides, and confirm more the presence of these objects outside our Milky Way”, conclude researchers.
The data collected also allowed astronomers to reproduce the merger event, noting that two percentric passages are necessary to create the observed shell and the leakage flow. The moment of the first passage of the pericentre coincides with the breakup of the formation of the stars which took place a billion years ago.
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More information:
David MartÃnez-Delgado et al, a pearl in the shell: an ultra-compact dwarf in the tidal debris surrounding the spiral galaxy NGC 7531, arxiv (2025). DOI: 10.48550 / Arxiv. 2009.14038
Newspaper information:
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