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Astronomers Unlock the Secrets of the World‘s Brightest Galaxies

Posted September 24, 2015

Scientists might have just found out what makes the world‘s brightest galaxies, called submillimetre galaxies (or SMGs), so incredibly radiant. These ancient astronomic entities, formed 12 to 13 billion years ago, and discovered in the early 1990s, are characterised by their unusually high rates of star formation (roughly 1,000 stars per year, compared to only 1 or 2 in the Milky Way), thought to stand behind their ultra-luminosity. What causes such rapid formation of stars, though, has been a mystery.

Submillimetre galaxies are not giant cosmic crashes after all, but a natural phase in the formation of massive galaxies. Image credit: ESO/WFI via, CC BY 4.0.

Submillimetre galaxies are not giant cosmic crashes after all, but a natural phase in the formation of massive galaxies. Image credit: ESO/WFI via, CC BY 4.0.

One proposed solution maintained that SMGs are sort of like cosmic car crashes – the fiery result of two disc galaxies colliding with each other, an even that would likely cause a spectacular, but short-lived burst of star formation. Another hypothesis held that SMGs are long-lived objects that slowly accrete mass.

Despite many attempts at modelling each of the two scenarios, the physics could never be fully worked out.

“People have hacked together different kinds of models, but they always violated some observed constraint,” said project leader Desika Narayanan, an Assistant Professor of Astronomy at the Haverford College and lead author on a new paper in Nature. “What we’ve done is develop the first model where we’ve been able to match the range of physical constraints that we know exist. So that is a pretty exciting result.”

The new model, which took Narayanan’s team over 18 month to develop, shows that SMGs, rather than leaking their copious reservoirs of gas into space, a process called galactic overflow, keep it inside due to the tight grip of gravity observed within these constellations, and recycle it to fuel the formation of new stars, which eventually become supernovas and explode, thereby emitting massive amounts of light.

This is now thought to be a natural phase in the evolution of massive galaxies, which keeps churning out 500 to a 1,000 solar masses per year for a billion years.

The accuracy of this simulation is owed to a newly-developed computer code which describes how light escapes the complex maze of dust and gas in an SMG.

“A lot of this work wouldn’t have been feasible without the cluster computing we have at the KINSC, and without the support of Joe Cammisa, who manages the cluster,” said Narayanan, who began the work when he joined Haverford’s faculty last January. “He really was integral in getting the cluster set up and the software together.”

Having solidified a method for simulation, the team will now proceed in modelling many additional galaxies to reach a statistically significant sample.

Sources: study abstract,,

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