Ultra Compact Dwarf Galaxies once crowded with stars

Astronomers think they’ve found a way to explain why Ultra Compact Dwarf Galaxies, oddball creations from the early universe, contain so much more mass than their luminosity would explain.

Pavel Kroupa, an astronomer at the University of Bonn in Germany, led a research team that’s proposing the unexplained density may actually be a relic of stars that were once packed together a million times more closely than in the solar neighbourhood. The new paper appears in the Monthly Notices of the Royal Astronomical Society.

UCDs were discovered in 1999. At about 60 light years across, they are less than 1/1000th the diameter of the Milky Way — but much more dense. Astronomers have proposed they formed billions of years ago from collisions between normal galaxies. Until now, exotic dark matter has been suggested to explain the ‘missing mass.’

The authors of the new study think that at one time, each UCD had an incredibly high density of stars, with perhaps 1 million in each cubic light year of space, compared with the 1 that we see in the region of space around the Sun. These stars would have been close enough to merge from time to time, creating many much more massive stars in their place. The more massive stars would consume hydrogen rapidly, before ending their lives in violent supernova explosions, leaving either superdense neutron stars or black holes as their remains. 

In today’s UCDs, the authors think, the previously unexplained mass comprises these dark remnants, largely invisible to Earth-based telescopes.

“Billions of years ago, UCDs must have been extraordinary,” study co-author Joerg Dabringhausen, also of the University of Bonn, said in a press release. “To have such a vast number of stars packed closely together is quite unlike anything we see today. An observer on a (hypothetical) planet inside a UCD would have seen a night sky as bright as day on Earth.”

PHOTO CAPTION: Background image taken by Michael Hilker of the University of Bonn using the 2.5-metre Du Pont telescope, part of the Las Campanas Observatory in Chile. The two boxes show close-ups of two UCD galaxies in the Hilker image. These images were made using the Hubble Space Telescope by a team led by Michael Drinkwater, at the University of Queensland.

Source: Royal Astronomical Society