They are massive, they are very small, and they are extremely rare, but may hold the secrets of how galaxies form and evolve. A new study[1] lifts the tip of the veil over the timid life of the massive ultra-compact galaxies. It was published on the 16th of November in the journal Astronomy & Astrophysics and was carried out by an international team led by Fernando Buitrago, of Instituto de Astrofísica e Ciências do Espaço (IA[2]) and Faculdade de Ciências da Universidade de Lisboa (FCUL).

Massive ultra-compact galaxies have several times more stars than our Milky Way, more than the equivalent to 80 billion suns, and thus are very bright, but their stars are densely packed within a size much smaller than our galaxy’s. The researchers identified a new set of 29 galaxies with these properties, at distances between two and five billion light-years from Earth. 

Seven of these shy heavyweights are actually primordial galaxies that remained untouched by others since their formation, more than ten billion years ago. These so-called relic galaxies open windows onto how galaxies looked like and were in the early ages of the universe, although they are 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 Fernando Buitrago. “As this sample of galaxies we studied is in the nearby universe and relatively close to us, even being truly small, we have a better chance of probing them.”

One of the advances of the paper now published is to present the density of these massive ultra-compact galaxies in the universe, relics and non-relics altogether. The researchers have found only 29 in the most complete survey[3] of galaxies in 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 could those relics be preserved untouched across cosmic time is something yet to be understood, says Fernando Buitrago.

According to the paradigm of galaxy formation and evolution, these relic ultra-compact galaxies could only be saved from merging with others and evolving by residing in overly populated clusters of galaxies. It may sound counter-intuitive as one would expect that in such crowded environments they would more easily interact and lose their original properties, but Buitrago explains: “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 underdense neighborhoods, this is very hard to explain.”

In this study, the researchers tried to measure some of the properties of these objects, such as their sizes and ages, but they are requesting observing time with large ground-based telescopes to point directly at them. In order to understand their past history, they would like to study in greater detail the places where they live in, the other galaxies around them, and their relative positions in space.

“Massive galaxies evolve in an accelerated way when compared to other galaxies in the universe. By understanding the properties of the most massive galaxies, we could understand the eventual fate of all the other galaxies, including our own Milky Way,” Fernando Buitrago says.


1. The scientific paper “Galaxy and Mass Assembly (GAMA): Accurate number densities and environments of massive ultra-compact galaxies at 0.02].

2. The Instituto de Astrofisica e Ciencias do Espaco (Institute of Astrophysics and Space Sciences — IA) is the reference Portuguese research unit in space sciences and integrates researchers from the University of Lisbon and the University of Porto. The institute encompasses most of the field’s national scientific output and it was evaluated as Excellent in the last evaluation Fundacao para a Ciencia e Tecnologia (FCT) commissioned from the European Science Foundation (ESF). IA’s activity is funded by national and international funds, including Fundacao para a Ciencia e a Tecnologia (UID/FIS/04434/2013), POPH/FSE and FEDER through COMPETE 2020.

3. GAMA, short for Galaxy and Mass Assembly, is a galaxy survey of the nearby universe with spectroscopic data up to redshift 0.3, that is, up to a distance of roughly five billion light-years.