Massive ultracompact galaxies have similar numbers of stars as our Milky Way but are confined to only a tenth the size or a thousandth the volume. They evolve in an accelerated way when compared to other galaxies in the Universe. By understanding their properties, astronomers can understand the eventual fate of all galaxies, including our own Milky Way.
Ultracompact Relics –80 Billion Stars With a 5,000-Light Year Radius
A 2018 study from the Instituto de Astrofísica e Ciências do Espaço (IA2) reported features of massive ultracompact galaxies, which have at least 80 billion stars densely packed within a radius of 5,000 light years. For comparison, our Milky Way has 100 – 200 billion stars spread across a disk that is about 140,000 light years wide. The researchers identified a set of 29 galaxies with these properties at distances of between 2 and 5 billion light-years from Earth.
Seven Found are Pristine Primordial Galaxies
Seven of these shy heavyweights are actually primordial galaxies that remained untouched by others since their formation more than 10 billion years ago. These so-called relic galaxies reveal how galaxies looked in the early ages of the universe, although they are located in our galactic neighborhood.
“When you study very small objects and you study them in the far away universe, it is very hard to tell anything about them,” says University of Valladolid (Spain) and the Institute of Astrophysics and Space Sciences (Portugal) astronomer, Fernando Buitrago . “As this sample of galaxies we studied is in the nearby universe and relatively close to us, though they are truly small, we have a better chance of probing them.”
Only 29 in the Local Universe
The researchers have found only 29 such galaxies in the most complete survey of the local universe. “They are so rare that we need roughly a volume with nearly 500 million light-years across to find a single one of them,” says Ignacio Ferreras, the second author of the study.
Ferreras determined the ages of the stars in the galaxies, separating the redder and older relic galaxies from the bluer and younger. How these relics are preserved untouched across cosmic time is something yet to be understood, says Buitrago.
Reside in Overly Populated Clusters of Galaxies
According to the paradigm of galaxy formation and evolution, these relic ultracompact galaxies could only be saved from merging with others and evolving by residing in overly populated clusters of galaxies. It may sound counterintuitive as galaxies in such crowded environments would more easily interact and loose their original properties, but Buitrago says, “In a place where there are many galaxies, there is also a lot of gravitational pull, and the velocities of the galaxies are very high. Thus, the galaxies pass by each other without enough time to interact significantly.”
“The surprise came when we realized that not all the galaxies in our sample live in such systems,” Buitrago adds. “We found them in a range of environments, and for those that live in under-dense neighborhoods, this is very hard to explain.”
Source: F. Buitrago et al. Galaxy and Mass Assembly (GAMA): Accurate number densities and environments of massive ultra-compact galaxies at 0.02 < z < 0.3, Astronomy & Astrophysics (2018). DOI: 10.1051/0004-6361/20183378.
Image at top of page: A team of astronomers led by Favio Faifer of the National University of La Plata, Argentina, discovered the first ultra-compact dwarf (UCD) galaxy in an X-ray bright galaxy group designated NGC 5044.