Mystery Solved. How We Get Ultra-Compact Dwarf Galaxies

I have been fascinated by galaxies for most of my adult life. I find it amazing that, just as we can ascertain the lifecycle of a tree by closely studying the trees in a forest, it is possible to study a sample of galaxies and understand galactic evolution.  A team of astronomers using the Gemini North Telescope have recently solved a long standing galactic mystery, namely how we get ultra-compact dwarf galaxies (UCDs to use their catchy acronym).

We are all familiar with spiral, barred-spiral and elliptical galaxies – ignoring the unclassifiable (if that’s even a word) irregular galaxy classification – but there are more. The UCD’s, which were discovered over twenty years ago, are one such lesser known classification and are among the densest stellar objects in the universe. They are more compact than most other galaxies, thought to be of the order of 200 light years across with about 100 million stars and have similar mass, but larger than star clusters.  They are almost like globular clusters but on steroids! 

The problem with them is that there is no evidence that supports why they exist! The dominant theory is that they are the nucleus of dwarf galaxies that remain after the outer layers have been stripped away.  However like a walk through our forest analogy where we could see examples of trees during all stages of their life, until now, there have been no observations of this process actually taking place. 

Previous searches have revealed no evidence of the transition phase so a team of astronomers have embarked on a systematic hunt for them among the Virgo Cluster of galaxies (a group of over a thousand galaxies, 5 million light years across). The international team of astronomers used the 8.1 metre Gemini North Telescope at Mauna Kea in Hawaii for their search. They first identified the galaxies they wanted to study by looking at images from the Next Generation Virgo Cluster Survey. The images were taken using the Canada-France-Hawaii Telescope and from this they identified hundreds of galaxies to observe.

The Gemini North Telescope operated by NSF’s NOIRLab. (Credit – NOIRLab/NSF/AURA/T. Slovinský)

The galaxies they were interested in were all thought to be potential progenitor UCDs, potential because UCDs that had stars surrounding them as they were being stripped were indistinguishable from more distant galaxies. Using the Gemini North Telescope, spectroscopic observations of the target galaxies revealed accurate distance measurements so they could eliminate distant objects leaving only those in the Virgo Cluster. Among the remaining sample are dwarf galaxies with ultra compact dense cores that are representative of the progenitor objects before they lose their outer layers of stars and gas that are subsequently stripped by nearby massive galaxies. 

Furthermore and to the delight of the team, in the same study they found almost 100 objects with very extended diffuse envelopes of stars which are believed to be dwarf galaxies in the throes of losing their outer layers. As if in a text book and as postulated by the theory, they were even close to massive galaxies who very likely played a great part in their formation. Until now, the missing piece of the jigsaw had eluded discovery but it seems now, the story of ultra-compact dwarf galaxies may be complete. 

Source : Gemini North Reveals the Formation of Dense Dwarf Galaxy ‘Fossils’

Mark Thompson

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