A bundle of ultra-compact massive galaxies found


Surprise discoveries afford new opportunities to study conditions in the early universe. Ben Lewis reports.


Images showing the multra-compact massive galaxies.

Images showing the multra-compact massive galaxies.

Buitrago et al, 2018

Astronomers have announced as many as 29 newly discovered massive ultra-compact galaxies within five billion light years of Earth.

The galaxies are so named because of their huge mass. They contain several times more stars than the Milky Way, packed into a much smaller volume. This makes them extremely bright, but also rare.

Using data from the Galaxy and Mass Assembly (GAMA) survey of the nearby universe, astronomers led by Fernando Buitrago from the Institute of Astrophysics and Space Sciences in Portugal have identified up to 29 of the galaxies between two and five billion light-years away.

The GAMA data was supplemented with observations made by the European Southern Observatory’s VLT Survey Telescope in Chile. The combined results have been published in the journal Astronomy & Astrophysics.

“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 co-author Ignacio Ferreras.

Of the 29, seven of the oldest are of particular interest. The group has remained largely untouched by others since its formation more than 10 billion years ago. Having existed without drastic external influences, its members reveal what galaxies looked like in the early ages of the universe.

Normally astronomers would need to look into the far distance to gain such insights, but these relics are in, galactically speaking, the local neighbourhood. That proximity provides a range of opportunities for Buitrago and his team.

“When you study very small objects and you study them in the far away universe, it is very hard to tell anything about them,” he explains.

“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.”

The discoveries defy some of the assumptions of galaxy formation and evolution. It was thought that massive ultra-compact galaxies could only exist in dense clusters of galaxies, where complex gravitational forces and high galactic velocities prevent them from interacting significantly or merging with others.

“The surprise came when we realised that not all the galaxies in our sample live in such systems,” says Buitrago.

“We found them in a range of environments, and for those that live in under-dense neighbourhoods, this is very hard to explain.”

With their relative proximity and time-capsule-like properties, the newly discovered batch will provide an unprecedented opportunity for astronomers to understand galaxies of all sizes, not just the massive ultra-compact type, according to Buitrago.

“Massive galaxies evolve in an accelerated way when compared to other galaxies in the universe,” he says. “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.”

  1. http://www.gama-survey.org/
  2. https://doi.org/10.1051/0004-6361/201833785
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