Puffy Galaxies More Productive After Cosmic Noon

In its relative infancy, our Universe was producing new stars at a prodigious rate, fast enough that the majority were already formed about 10-billion years ago. But even though formation rates have been declining ever since, it seems that some galaxies are more likely than others to continue producing new stars.

Stars are born out of cool gas, mostly hydrogen and helium, that collapses in on itself due to gravity and ignites nuclear fusion. The process doesn’t work so well when the gas is hot because it is too energetic to collapse and coalesce together.

Playing a pivotal role in the regulation of star formation are the supermassive black holes at the centres of galaxies. The more active ones spew out enormous amounts of energy, expelling usable fuel into space and heating what remains.

But some galaxies seem to be able to continue forming new stars longer than others, though at a slower rate than at their peak. In massive galaxies with extended, stretched out disks, things don’t heat up as much as in galaxies with more compact disks, and the black holes are not quite so influential.

Though the hey-day for these puffy disk galaxies was billions of years ago, their star-making didn’t drop off quite as precipitously as it did for more compact galaxies. But why is it that galaxies are so much less productive today than they were all those years ago?

For the first few billion years after the Big Bang, the star formation rate was on the rise. There was plenty of pristine star-forming material around and the irrepressible force of gravity was tirelessly bringing galaxies together in mergers that replenished gas reservoirs.

The tipping point came about 10-billion years ago, a time that has been nicknamed ‘cosmic noon’. As dark energy forced the Universe to expand faster than gravity could pull things together, mergers became less prevalent, and the gas available to form new stars was becoming contaminated with heavier elements.

The rate at which stars formed plummeted. Previously active stellar nurseries were driven into a state of relative quiescence as fuel supplies dwindled and feedback mechanisms prevented further births. The whole process is referred to by astronomers as quenching.

This is not the end of star birth in the Universe; it will continue long into the future, albeit at a slower rate. We can expect new stars to be born for trillions upon trillions of years, but it does appear as though we are already in the ‘cosmic evening’.

The evidence for all of this comes from looking at the distant Universe with powerful telescopes and from state-of-the-art cosmological galaxy formation simulations.

Recent research by a team of astronomers led by Dr Anshu Gupta from the University of New South Wales looked at data from the MOSEL survey integrated with results of the TNG100 cosmological simulation to establish a connection between the onset of star formation quenching and the size of galaxies. This gave them plenty of data to cover most of the cosmic day.

MOSEL, the Multi-Object Spectroscopic Emission Line survey, is an ongoing study of star-forming galaxies around 12-billion light-years away that is attempting to identify the factors involved in the rise and fall of star formation in young galaxies. The TNG100 simulation, the results of which are publicly available data, is part of the IllustrisTNG project that looks to shed light on the physical processes that drive galaxy formation and evolution.

Dr Gupta explained how the team used the data in their research.

“We looked at the size of observed galaxies and compared it with the sizes of simulated galaxies. All MOSEL galaxies were relatively compact, but in the simulation, there was a large population of massive galaxies with really extended puffy stellar disks. We looked into the details of the possible origins of these puffy galaxies in the TNG100 simulations and how they will evolve over the next 10-billion years.”

What the team found was that extended puffy galaxies were able to continue making stars longer than galaxies with more compact disks. Rather than becoming quenched just after cosmic noon, they could continue right up until at least ‘cosmic afternoon-tea’.

“We found a close relationship between the distribution of existing stars and the length of time for which galaxies will continue to form new stars. The biggest galaxies in our Universe are usually not making new stars at the moment, but when these galaxies stopped making new stars remains a mystery.”

“This paper shows that we can now just look at the size of the stellar disk to predict how fast the new star making will stop.”

And where does our own galaxy, the Milky Way, fit into this?

The Milky Way is a massive galaxy that is still making stars at the rate of about one every year, but it was something of a cosmic late starter. At cosmic noon it was only about one-tenth of its current size, and did not attain massive status until after a series of mergers with smaller galaxies.

However, the Milky Way is not an extended puffy galaxy, so we can expect it to quench sooner rather than later. After about 13-billion years of birthing new stars, the violent, tumultuous business of lighting up the Universe will essentially be over.

Here's to a happy retirement.