We’re going to talk about two different kind of “extreme” galaxies (I put “extreme” in quotes because it’s pretty hard to define what a “normal” galaxy is in the first place, but let’s go with it for now). The first is a kind of galaxy known as ultra-compact dwarfs, or UCDs. UCDs have way too many stars for their size and look like over-stuffed star clusters. They are thought to be the remnant cores of once-normal galaxies.
On the other end of the spectrum we have the ultra-diffuse galaxies, or UDGs. Quite unlike their ultra-compact cousins, the UDGs are extremely large but diffuse, taking up way too much space for their relatively low level of brightness.
And a pair of recently-published papers may have found a surprising connection between them.
A team of astronomers used the MegaCam, a wide-angle optical camera mounted on the Canada-France-Hawaii Telescope to map out the Virgo Galaxy cluster. This cluster is massive, containing over a thousand member galaxies, and is the closest cluster to our own Milky Way galaxy.
The survey – appropriately named the Next Generation Virgo Cluster Survey – mapped thousands of galaxies of all kinds in and around the Virgo cluster. Through that survey, the astronomers noticed that UDGs tended to sit towards the center of the cluster, a place rich with a violent history of mergers.
Since astronomers suspect that ultra-compact galaxies also have suffered major merger events in their pasts – causing them to lose most of their star-forming gas through tidal interactions – it’s suspicious that the ultra-diffuse galaxies may have also suffered a similar fate.
More work is needed, however, to fully untangle the complicated histories of these extreme galaxies.