An international team of astronomers has captured the eroded remains of 106 dwarf galaxies for the first time. They represent the missing link in their evolution into ultra-compact dwarf galaxies, objects that resemble star clusters, particularly globular clusters, in terms of density but with much greater mass. These results confirm that many ultra-compact dwarf galaxies are probably the fossil remains of normal dwarf galaxies that have been stripped of their outermost stars.
Credit: NOIRLab/NSF/AURA/M. Zamani
Clusters of galaxies such as the Virgo Cluster can contain more than 1,000 galaxies of very different types: alongside giant elliptical galaxies and large spiral galaxies similar to our own Milky Way, there are myriads of smaller galaxies. These include dwarf galaxies, which can be elliptical or irregular, and some of which have a denser core of stars at their heart. But there are also Ultra-Compact Dwarf galaxies (known as UCDs), which have been known for around twenty years and are among the densest stellar groupings in the Universe. More compact than other galaxies of similar mass, but slightly larger than globular clusters – the objects they most closely resemble – UCDs have defied classification.
Credit: CTIO/NOIRLab/DOE/NSF/AURA, Image processing: T.A. Rector (University of Alaska Anchorage/NSF’s NOIRLab), M. Zamani (NSF’s NOIRLab), & D. de Martin (NSF’s NOIRLab)
The idea that UCDs are remnants of disrupted dwarf galaxies has been proposed since their discovery more than twenty years ago. However, previous searches have not revealed the large population of transition galaxies that one would expect to find. An international team of astronomers has therefore conducted a systematic search for these intermediate-stage objects around the Virgo cluster, a group of thousands of galaxies in the direction of the constellation Virgo.
The team involving astronomers from Strasbourg combined images of the Virgo cluster taken with the Canada-France-Hawaii telescope with follow-up spectroscopic measurements using the Gemini North telescope, located near the summit of Maunakea in Hawaii, to distinguish UCDs surrounded by an envelope of stars from normal galaxies located further out, beyond the Virgo cluster. These observations enabled the astronomers to eliminate all background galaxies from their samples.
In this vast study, we find numerous dwarf galaxies containing ultra-compact central star clusters. These galaxies represent the early stages of the transformation process and suggest that, once neighbouring massive galaxies have stripped these dwarf galaxies of their outer layers of stars and gas, what will remain is an object identical to the ultra-compact star clusters that have already been identified at an advanced stage.
Credit: NOIRLab/NSF/AURA/NASA/R. Gendler/K. Wang/M. Zamani
The researchers also found many objects surrounded by widespread and diffuse stellar envelopes, indicating that they are currently in transition as their stars and dark matter are removed. Within its large sample, the team identified objects at several other stages of the evolutionary process that, when sequenced, tell a compelling story of UCD morphology. In addition, almost all the candidates were in the vicinity of massive galaxies, suggesting that their local environment plays an important role in their formation.
Scientific contact : Pierre-Alain Duc – pierre-alain.duc@astro.unistra.fr
Article : An evolutionary continuum from nucleated dwarf galaxies to star clusters, (Wang et al., Nature DOI: 10.1038/s41586-023-06650-z).
More information: NOIRlab press release.