Astronomers using the Gemini North telescope have found the remnants of 106 galaxies as they transition into ultra-compact dwarf galaxies.

Strongly nucleated dwarf galaxies, UCDs with envelopes and UCDs in the core of the Virgo Cluster. Image credit: Wang et al., doi: 10.1038/s41586-023-06650-z.

Ultra-compact dwarf galaxies (UCDs) are among the densest stellar groupings in the Universe.

Being more compact than other galaxies with similar mass, but larger than star clusters — the objects they most closely resemble — these mystifying objects have defied classification.

The idea that UCDs are remnants of disrupted dwarf galaxies has been proposed since they were discovered over two decades ago.

However, previous searches have not revealed the large population of galaxies-in-transition that you would expect to find.

So Peking University astronomer Kaixiang Wang and colleagues conducted a systematic search for these objects around the Virgo Cluster, a group of about 1,300 — and possibly up to 2,000 — member galaxies in the direction of the constellation of Virgo.

Using the Gemini North telescope, they identified 106 of these missing-link galaxies that show every stage of the transformation process.

“Our results provide the most complete picture of the origin of this mysterious class of galaxy that was discovered nearly 25 years ago,” said NOIRLab astronomer Eric Peng.

“Here we show that 106 small galaxies in the Virgo cluster have sizes between normal dwarf galaxies and UCDs, revealing a continuum that fills the ‘size gap’ between star clusters and galaxies.”

The astronomers compiled their sample by first looking at images from the Next Generation Virgo Cluster Survey, taken with the Canada-France-Hawai’i Telescope.

And though they were able to identify hundreds of candidate UCD progenitors, they were unable to confirm their true nature.

The obstacle was that UCDs that are surrounded by envelopes of stars are indistinguishable from normal galaxies that are located farther away beyond the Virgo Cluster.

To distinguish the candidate UCD progenitors from the background galaxies, the researchers performed follow-up spectroscopic studies with Gemini North to obtain more concrete measurements of their distances.

These observations allowed the astronomers to eliminate all of the background galaxies from their samples until only the UCDs within the Virgo Cluster remained.

Scattered among this vast survey are many dwarf galaxies that contain ultra-compact central star clusters.

These galaxies represent the early stages of the transformation process and suggest that after neighboring massive galaxies strip these dwarfs of their outer layers of stars and gas, what remains will be an object identical to the late-stage UCDs that have already been identified.

The scientists also found many objects with very extended and diffuse stellar envelopes around them, indicating that they are currently in the throes of transitioning as their stars and dark matter is stripped away.

Within their extensive sample the team identified objects at several other stages of the evolutionary process that, when placed in sequence, tell a compelling story of the morphology of UCDs.

Furthermore, nearly all the candidates were near to massive galaxies, suggesting that their local environment plays an important role in their formation.

“Once we analyzed the Gemini observations and eliminated all the background contamination, we could see that these transition galaxies existed almost exclusively near the largest galaxies,” Wang said.

“We immediately knew that environmental transformation had to be important.”

Besides identifying the environment UCDs live in, these results also lend valuable insight into how many of these objects there are and what the full sequence of their evolutionary change looks like.

“It’s exciting that we can finally see this transformation in action,” Dr. Peng said.

“It tells us that many of these UCDs are visible fossil remnants of ancient dwarf galaxies in galaxy clusters, and our results suggest that there are likely many more low-mass remnants to be found.”

The findings were published in the journal Nature.

_____

K. Wang et al. 2023. An evolutionary continuum from nucleated dwarf galaxies to star clusters. Nature 623, 296-300; doi: 10.1038/s41586-023-06650-z