Cosmic Snakes, Gravitational Lenses and Stellar Nurseries: Why Distant Star Clusters are so Dense

The key to why distant star clusters are more over-populated than those found in the Milky Way lies in the stellar nurseries of a serpentine galaxy 8 billion light-years away.

The galaxy cluster MACS1206.2–0847 with the Cosmic Snake visible on the right of the image (UNIGE)

A team of astronomers led by researchers from the University of Geneva (UNIGE) have discovered why clusters of stars in distant galaxies are so massive in comparison to those found locally.

The answer, the researchers in question believe, lies in the study of molecular clouds — stellar nurseries — comprised of cold, dense gas, which collapse under gravity to form the stars. Whilst molecular clouds in nearby galaxies can produce between 10³ and 10⁶ stars, distant galaxies have been observed to host giant star clusters with up to 100 times more stars than this. This leads astronomers to pose the question; how do distant molecular clouds produce so many more stars?

The international team of researchers gathered to discover if the properties of distant clouds are the same as those found close to the Milky Way. Reasoning, that if not, any disparity might explain why distant star clusters are so massive.

Choosing to test a model of star formation that puts disc fragmentation forward as the cause as its cause, the team conducted a study of molecular clouds in a galaxy nicknamed the Cosmic Snake — located 8 billion light-years from Earth.

“Molecular clouds are the cradle of star formation,” Miroslava Dessauges, a research associate in the astronomy department of UNIGE’s Faculty of Science, says. “The characterisation of the physical properties of these clouds in distant galaxies is necessary to understand star formation rates and determine if they are universal.”

Molecular clouds detected at the unprecedented resolution of 90 light-years in the Cosmic Snake, located more than 8 billion light-years away, a typical progenitor of our galaxy (left). Observed at resolutions 50,000 times better, each of these clouds resembles the very tormented gas of the Carina nebula located only 7500 light-years away, a veritable nursery of emerging stars (right). (UNIGE, NASA)

The team discovered that molecular clouds in distant galaxies had a mass, density and turbulence 10 to 100 times higher than similar clouds found in the Milky Way. “Such values had only been measured in clouds hosted in nearby interacting galaxies,” Dessauges, first author on the study published in the journal Nature Astronomy, adds.

The reason for these heightened properties lies in the fact that these distant galaxies have much more violent conditions than those found in closer galaxies. This means that in these hostile and extreme conditions, less dense molecular clouds like those found in our galaxy and nearby would be almost instantly destroyed, Dessauges explains. Thus, leaving only denser, more hardy clouds able to survive in these faraway regions.

“A molecular cloud typically found in a nearby galaxy would instantly collapse and be destroyed in the interstellar medium of distant galaxies.”

“Our findings show that the molecular cloud’s physical properties are clearly not universal in all galaxies and at all times,” Dessauges continues. “On the contrary, there are genuine variations amongst molecular clouds.

“Molecular clouds must adjust their properties to the ambient conditions of their host galaxy,” she continues.

The Interstellar Magnifying Glass and the desert of telescopes

The team was able to conduct their study thanks to unprecedented spatial resolution achieved in a distant galaxy achieved using gravitational lensing — the phenomena of mass distorting spacetime and thus the path of light travelling through it.

Dessauges says: “Gravitational lenses are a natural telescope that produces a magnifying glass effect when a massive object is aligned between the observer and a distant object.

“With this effect, some parts of distant galaxies are stretched on the sky and can be studied at an unrivalled resolution of 90 light-years.”

This effect was coupled with the use of the Atacama Large Millimetre/Submillimetre Array (ALMA) interferometer — comprised of a series of 50 radio antennas spread across the Chajnantor plateau in the Atacama desert, Chile. ALMA can construct the image of an entire galaxy instantly — a task beyond that of any single telescope. It can also be used to measure the level of carbon monoxide — a tracer of the molecular hydrogen gas which constitutes the molecular clouds.

Using this unprecedented resolution, the team was able to individually characterise the molecular clouds located in a distant galaxy which from our perspective winds its way through a cluster of galaxies known as MACS1206.2–0847. This “Cosmic Snake” — named for its serpentine appearance — lurks 8 billion light-years from Earth.

A closer view of the Cosmic Snake — a galaxy that winds its way through space 8 billion light-years from Earth (UNIGE)

Dessauges explains why the team selected the Cosmic Snake as the galaxy in which to conduct their observations: “The Cosmic Snake is one of those rare galaxies that are so strongly lensed — and therefore stretched on the sky — that it enables us to reach 30 parsecs [90 light years] spatial scales.

“It’s the first time that molecular gas content of a Milky Way progenitor has been studied at such high spatial resolution.”

Another advantage of using the Cosmic Snake is the fact that a parallel, highly detailed, study of the star clusters in this galaxy was conducted and published last year in the same journal, she says.

“The characteristic mass of molecular clouds in the Cosmic Snake appears to be in perfect agreement with the predictions of the scenario of fragmentation of turbulent galactic discs,” Lucio Mayer, a professor at the Centre for Physical and Cosmological Theory at the University of Zurich, adds.

The team also discovered that the Cosmic Snake galaxy is particularly efficient when it comes to star formation — likely because of the highly supersonic turbulence within the interiors of its molecular clouds. “In nearby galaxies, a molecular cloud donates what equivalent to about 5% of its mass to form stars,” remarks Daniel Schaerer, a professor in UNIGE’s Astronomy Department.

“In distant galaxies [like the Cosmic Snake] this rate climbs to about 30%.”

The only question that remains for researchers to consider is, are these denser, more turbulent, molecular clouds common enough to explain the number of massive star clusters in the observable Universe?

Hunting for more dense molecular clouds

The team of astronomers will now turn their attention to other distant galaxies in an attempt to confirm the observations made of the molecular clouds in the Cosmic Snake.

Discovering more about the Cosmic Snake’s molecular clouds will require pushing the ALMA interferometer (pictured) to its limits. ( © C. Padilla)

“Our findings clearly open up territory on the possibility of performing detailed studies of molecular clouds in a few other strongly lensed distant galaxies,” Dessauges explains. “And finding molecular clouds in other distant star-forming galaxies comparing their physical properties to the ones in the Cosmic Snake is needed to cross-check our results.”

But that doesn’t mean that Dessauges and her team are finished with the Cosmic Snake just yet.

“We want to reobserve the Cosmic Snake and obtain deeper and more resolved ALMA observations to determine more robustly the range of masses and densities of these distant molecular clouds,” the researcher says. “This is vital to put constraints on the formation mechanism of stars in molecular clouds.”

This means Dessauges concludes, pushing the resolution even further and taking full advantage of the unique performance of the ALMA Interferometer.