How to scout a cosmic monster


Enormous and unstoppable, a distant galaxy is making stars at a truly astronomical rate. Andrew Masterson reports.


An artist's impression of unstoppable monster galaxy, COSMOS-AzTEC-1.

An artist's impression of unstoppable monster galaxy, COSMOS-AzTEC-1.

National Astronomical Observatory of Japan

A gigantic galaxy 12.4 billion light years from Earth is forming stars at 1000 times the rate of the Milky Way and is, astronomers say, an unstoppable monster.

Called, with appropriate flair, COSMOS-AzTEC-1, the galaxy is of a type thought to be ancestors of the massive elliptical galaxies that dot the universe.

It was first recorded in 2007 by the James Clerk Maxwell Telescope in Hawaii.

Now, however, a team of scientists led by Ken-ichi Tadaki of the Japan Society for the Promotion of Science and the National Astronomical Observatory of Japan has used the Atacama Large Millimetre/submillimetre Array (ALMA) in Chile to construct a highly detailed “molecular map” of the monster.

The findings reveal a couple of very significant differences between COSMOS-AzTEC-1 and later, less active galaxies.

The latter typically contain a single large cloud of gas at the centre, out of which stars form. COSMOS-AzTEC-1 breaks that pattern.

“We found that there are two distinct large clouds several thousand light-years away from the centre,” explains Tadaki.

These two enormous clouds have other unusual properties that help to explain why the monster galaxy is producing stars like there’s no tomorrow.

In contrast to standard star-forming clouds, they are extremely unstable. This has dramatic effects on the equilibrium of the system.

With galaxies that form stars at only a moderate pace there is an alternating balance achieved between the inwards pull of gravity and the outwards force of gas pressure. As the gravity becomes stronger than the outward force of the gas, the cloud collapses and stars form.

Billions of years later, some of those stars come to an end, exploding as supernovas and blasting out gases, temporarily overcoming the force of gravity. Thus, over time, self-regulation emerges.

Monster galaxies such as COSMOS-AzTEC-1 behave very differently, Tadaki and his colleagues discovered. The outwards pressure of the gas is far weaker than gravity and equilibrium never develops. The result is runaway star formation.

Such productivity comes at a huge cost, however. The researchers calculate that the monster will completely consume itself in about 100 million years – 10 times the rate of standard star-forming galaxies.

The new data confirms the unstable nature of the galaxy, but does not yet explain why it is so.

One possible reason, suggests Tadaki, is that COSMOS-AzTEC-1 may be the result of a collision between two older galaxies. However, he adds, the hunt for evidence to support that contention still has a long way to run.

The research is published in the journal Nature.

  1. https://arxiv.org/abs/0801.2779
  2. https://www.nature.com/articles/s41586-018-0443-1
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