# Unraveling the Secrets of Distant Star Clusters
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Chapter 1: Stellar Nurseries in the Universe
The mystery behind the high density of star clusters in far-off galaxies compared to those in our Milky Way can be traced back to the stellar nurseries within a serpentine galaxy located 8 billion light-years from Earth.
The galaxy cluster MACS1206.2–0847, featuring the Cosmic Snake on the right side of the image (UNIGE).
A research team led by scientists from the University of Geneva (UNIGE) has made significant strides in understanding why distant star clusters are vastly more populous than those closer to home. Their findings suggest that the key lies in the characteristics of molecular clouds—stellar nurseries made up of cold, dense gas that collapse under gravitational forces to form stars. While molecular clouds in nearby galaxies typically generate between (10^3) and (10^5) stars, the clouds in distant galaxies can form star clusters with populations up to 100 times greater. This raises the intriguing question: How do these remote clouds manage to create so many stars?
The international team aimed to determine if the properties of molecular clouds in distant galaxies are similar to those in galaxies closer to the Milky Way. They hypothesized that any differences could shed light on the remarkable size of distant star clusters. To test this theory, they selected a galaxy nicknamed the Cosmic Snake, which is located 8 billion light-years away, and investigated its molecular clouds.
“Molecular clouds serve as the birthplace of stars,” explains Miroslava Dessauges, a research associate in UNIGE’s astronomy department. “Understanding the physical properties of these clouds in distant galaxies is essential for determining star formation rates and whether these rates are consistent across the universe.”
Section 1.1: Insights into Molecular Clouds
The research team found that molecular clouds in distant galaxies exhibited mass, density, and turbulence levels that were 10 to 100 times greater than those of similar clouds in the Milky Way. “These measurements had previously only been taken from clouds in nearby interacting galaxies,” Dessauges notes, the lead author of the study published in Nature Astronomy.
The researchers attribute these elevated characteristics to the more turbulent and violent conditions present in distant galaxies. Such extreme environments would likely obliterate less dense molecular clouds like those found in our galaxy. Dessauges elaborates, “Molecular clouds that would typically exist in a nearby galaxy would quickly collapse and be destroyed in the harsher interstellar conditions of distant galaxies.”
“Our findings indicate that the physical characteristics of molecular clouds are not universal across all galaxies and epochs,” Dessauges continues. “Instead, we observe significant variations among these clouds. They must adapt their properties to the specific conditions of their host galaxies.”
Molecular clouds in the Cosmic Snake observed with unprecedented resolution of 90 light-years (UNIGE, NASA).
Section 1.2: The Role of Gravitational Lensing
The team utilized the phenomenon of gravitational lensing to achieve remarkable spatial resolution in their study of this distant galaxy. Gravitational lenses act as natural telescopes, magnifying distant objects when a massive object aligns between the observer and the target.
Dessauges explains: “Gravitational lenses create a magnifying glass effect that allows us to study certain sections of distant galaxies at an unparalleled resolution of 90 light-years.”
This capability was further enhanced by the Atacama Large Millimeter/Submillimeter Array (ALMA) in Chile, which consists of 50 radio antennas spread over the Chajnantor plateau. ALMA can capture images of entire galaxies in a single observation, a feat unachievable by any single telescope. It also measures carbon monoxide levels, which serve as a tracer for the molecular hydrogen gas that constitutes these clouds.
Using this exceptional resolution, the team characterized individual molecular clouds in the Cosmic Snake galaxy, which winds through a cluster of galaxies known as MACS1206.2–0847.
Chapter 2: The Cosmic Snake and Its Star Formation Potential
The first video, titled "Globular Clusters: Like Stars Made Out of Stars," delves into the nature of these dense star clusters and their formation processes.
Dessauges explains the team's choice of the Cosmic Snake for their observations: “It’s one of the rare galaxies that are so intensely lensed that we can achieve 30 parsecs [90 light-years] spatial scales. This is the first time the molecular gas content of a Milky Way progenitor has been studied at such high spatial resolution.”
Another advantage of studying the Cosmic Snake is that a detailed analysis of its star clusters was conducted and published in the same journal the previous year. Lucio Mayer, a professor at the University of Zurich, mentions, “The characteristic mass of molecular clouds in the Cosmic Snake is consistent with the predictions of turbulent galactic disc fragmentation.”
The team also found that the Cosmic Snake is remarkably efficient in star formation, likely due to the highly supersonic turbulence within its molecular clouds. “In nearby galaxies, a molecular cloud typically contributes about 5% of its mass to star formation,” notes Daniel Schaerer, a professor in UNIGE’s Astronomy Department. “In distant galaxies like the Cosmic Snake, this rate increases to about 30%.”
As researchers continue their work, they ponder whether these denser and more turbulent molecular clouds are sufficiently common to account for the abundance of massive star clusters in the observable universe.
The second video, titled "What Causes Stars To Blink In Globular Clusters?" explores the fascinating phenomena associated with star behavior in these clusters.
Hunting for More Dense Molecular Clouds
The research team plans to investigate additional distant galaxies to confirm their observations regarding molecular clouds in the Cosmic Snake.
To further understand the Cosmic Snake's molecular clouds, the team aims to push the ALMA interferometer's capabilities to the limit (© C. Padilla).
“Our findings open up new avenues for studying molecular clouds in other strongly lensed distant galaxies,” Dessauges states. “We need to find molecular clouds in other star-forming galaxies and compare their properties to those in the Cosmic Snake to validate our results.”
However, Dessauges emphasizes that their work with the Cosmic Snake is far from over. “We plan to reobserve the Cosmic Snake and obtain deeper, more resolved ALMA observations to better define the masses and densities of these distant molecular clouds,” she concludes. “This is crucial for understanding the star formation mechanisms within molecular clouds.”
The researchers aim to enhance the resolution even further, maximizing the unique advantages offered by the ALMA Interferometer.
Special thanks to the University of Geneva Press Office and Miroslava Dessauges.