Using the Two-meter Twin Telescope (TTT3), Spanish astronomers have conducted deep optical imaging of an isolated dwarf galaxy known as NGC 6789. Results of the new observations, presented November 10 on the arXiv preprint server, shed more light on the star formation process in this galaxy.
Isolated but forming stars
Discovered in 1883, NGC 6789 is a blue compact dwarf (BCD) galaxy located some 12 million light years away in the Local Void—a region of space with far fewer galaxies than its surroundings. However, despite its extreme isolation, NGC 6789 shows recent central star formation activity.
Previous observations of NGC 6789 have found that approximately 4% of its total stellar mass—about 100 million solar masses—formed within the past 600 million years. It turned out that the central star-forming region of this galaxy is embedded within an apparently undisturbed, redder elliptical outer structure.
One question remains unanswered
Therefore, what still baffles scientists is the source of the gas sustaining recent star formation in NGC 6789, given that the galaxy is isolated and showcases an undisturbed shape.
To address this question, a team of astronomers led by Ignacio Trujillo of the University of La Laguna in Spain, has employed TTT3 to perform deeper optical observations of NGC 6789 than previously conducted.
“We present substantially deeper multiband imaging of NGC 6789 to explore its outer regions and search for faint features that may reveal evidence of past minor mergers or gas accretion events capable of supplying the fuel required to build its star-forming core,” the researchers explained.
Nothing there?
The new observations found that NGC 6789 maintains its elliptical shape down to the brightness of about 30 mag/arcsec2 (in g-band), or to a radial distance of 1.5 arcminutes, and that it exhibits a significant color gradient due to its central star-forming region. Therefore, no evidence of tidal features or merger remnants was found.
Furthermore, the collected data allowed the astronomers to place an upper limit on the amount of stellar mass that could potentially surround NGC 6789 as a result of the disruption of any small satellite. This value was calculated to be approximately 200,000 solar masses.
The researchers underlined that the number of new stars in the central region of NGC 6789 is around 4 million solar masses. Thus, they assume that the absence of any visible stellar streams around the galaxy indicates that its central starburst is either internally originated or produced by the recent infall of pristine gas.
“Its recent central star formation was likely produced either by in-situ residual gas or by the accretion of external pristine gas not associated with a minor merging activity,” the authors of the paper explained.
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More information:
Ignacio Trujillo et al, Deep imaging of the very isolated dwarf galaxy NGC6789, arXiv (2025). DOI: 10.48550/arxiv.2511.07041
Journal information:
arXiv
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Deep imaging suggests isolated galaxy forms stars without signs of past mergers (2025, November 18)
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