With very small stellar masses and tiny radii, ultracompact dwarf galaxies raise questions about the dividing line between the highest-mass star clusters and the lowest-mass galaxies. A recently published article leverages JWST’s high resolution to target one tiny galaxy and investigate its origins.
Location of UCD 736 in the Virgo Cluster with the other ultracompact dwarf galaxies with supermassive black hole detections labeled. Click to enlarge. [Taylor et al 2025]
Origins of Ultracompact Dwarf Galaxies
How tiny can a galaxy get? Discovered 20 years ago among the dense environments of the Virgo and Fornax clusters, ultracompact dwarf galaxies walk the line between the largest globular clusters and the smallest galaxies. Determining if these dwarfs are stellar in nature or actually of galactic origin is complex, but one sure way to get at this question is to determine if an ultracompact dwarf galaxy houses a supermassive central black hole.
While astronomers have identified hundreds of these tiny dense galaxies, supermassive black holes have been detected in only five. Hoping to extend this sample, Matthew Taylor (University of Calgary) and collaborators targeted UCD 736, an ultracompact dwarf galaxy located in the Virgo galaxy cluster. If a massive black hole is found in this galaxy, it will be the smallest and least luminous ultracompact dwarf to have such a detection. Exploring galaxies at such small sizes will allow researchers to determine what fraction of ultracompact dwarfs have galactic or star cluster origins, which can provide insights into massive black hole seeding mechanisms in the early universe.
Modeling Black Hole Mass
Summary of results for the three black hole modeling methods for UCD 736, indicating the quality of the models and how the observed velocity dispersion and spatial distribution of the galaxy fit with the models. Click to enlarge. [Taylor et al 2025]
However, the authors noted that the three models do not provide equivalent confidence levels in this estimate — while one model confidently rules out the possibility of no central black hole at 3σ significance, the other two models were less constraining, ruling out the possibility of there not being a black hole to only 1σ. Despite these differences, all three models agree on the central black hole mass within the first confidence interval, indicating a positive detection of a black hole in UCD 736.
Progenitor of UCD 736
From their observations and modeling, the authors presented the fifth positive detection of an ultracompact dwarf galaxy with a central supermassive black hole within the Virgo Cluster. Given the presence of this black hole, UCD 736 likely originated as a more massive galaxy that was tidally stripped as it interacted with other nearby cluster galaxies. Interestingly, though, other ultracompact dwarf galaxies with black hole detections in the Virgo Cluster are much closer to the massive galaxy that likely stripped them. UCD 736 sits about 160,000 light-years outside of the nearest giant galaxy Messier 59’s tidal radius, which does not rule out interactions but could indicate that Messier 59 is particularly good at stripping material from smaller galaxies.
Continued searches for black holes in ultracompact dwarfs will further probe the differences between the highest-mass globular clusters and the lowest-mass galaxies. Revealing more within this parameter space will allow astronomers to test black hole seeding and galaxy formation theories.
Citation
“A Supermassive Black Hole in a Diminutive Ultracompact Dwarf Galaxy Discovered with JWST/NIRSpec+IFU,” Matthew A. Taylor et al 2025 ApJ 991 L24. doi:10.3847/2041-8213/ae028e.