Fluffy Molecular Clouds Formed Stars in the Early Universe
Stars form in Giant Molecular Clouds (GMCs), vast clouds of mostly hydrogen that can span tens of light years. These stellar nurseries can form thousands of stars. Astronomers know this because they observe these regions in the Milky Way and the Magellanic Clouds and watch as stars take shape. But the Universe is more than … Continue reading "Fluffy Molecular Clouds Formed Stars in the Early Universe" The post Fluffy Molecular Clouds Formed Stars in the Early Universe appeared first on Universe Today.
Stars form in Giant Molecular Clouds (GMCs), vast clouds of mostly hydrogen that can span tens of light years. These stellar nurseries can form thousands of stars. Astronomers know this because they observe these regions in the Milky Way and the Magellanic Clouds and watch as stars take shape.
But the Universe is more than 13 billion years old and has been forming stars for almost that entire time. The early Universe was different in notable ways. Was star formation any different in the early Universe?
One of the main differences between the early Universe and the modern Universe is metallicity. Elements heavier than hydrogen and helium, called metals in astronomy, didn’t exist in the very early Universe. Only after massive stars formed and died did the Universe’s metallicity increase. Metallicity affects many different processes, including star formation. Metals help cool down clouds of gas and dust, allowing them to collapse and form stars.
Scientists know a lot about the star formation process, but there are many outstanding questions. One of them concerns star formation in the early, low-metallicity Universe. How different was the star formation process billions of years ago?
“We can’t go back in time to study star formation in the early universe, but we can observe parts of the universe with environments similar to the early universe.”Kazuki Tokuda, Kyushu University, Japan
New research in The Astrophysical Journal tackled the question. It’s titled “ALMA 0.1 pc View of Molecular Clouds Associated with High-mass Protostellar Systems in the Small Magellanic Cloud: Are Low-metallicity Clouds Filamentary or Not?” The lead author is Kazuki Tokuda, a Post-doctoral fellow in the Department of Earth and Planetary Sciences in the Faculty of Science at Kyushu University in Japan. Tokuda is also affiliated with the National Astronomical Observatory of Japan.
“Even today our understanding of star formation is still developing, comprehending how stars formed in the earlier universe is even more challenging,” said lead author Tokuda in a press release. “The early universe was quite different from today, mostly populated by hydrogen and helium. Heavier elements formed later in high-mass stars. We can’t go back in time to study star formation in the early universe, but we can observe parts of the universe with environments similar to the early universe.”
One of those places is the Small Magellanic Cloud (SMC), a dwarf galaxy near the Milky Way. The SMC’s metallicity is much lower than the Milky Way’s, containing only about one-fifth as many metals. This makes it analogous to the early Universe about 10 billion years ago.
In the Milky Way, star-forming molecular clouds tend to have a filamentary structure. Astronomers have wondered whether these same filamentary shapes are a universal feature found throughout cosmic time. “To test whether these structures are universal throughout cosmic star formation history, it is crucial to study low-metallicity environments within the Local Group,” the authors explain in their paper. Since the SMC is a close neighbour and also has a low metallicity, it’s a good place to look. However, searching the SMC for these filamentary features has been difficult due to the insufficient spatial resolution of many observatories.
The researchers used the Atacama Large Millimeter-submillimeter Array’s (ALMA) power to examine the SMC and see if it has the same star-forming filamentary structures. They focused on the molecular clouds associated with massive young stellar objects (YSOs) in the (SMC).
“In total, we collected and analyzed data from 17 molecular clouds. Each of these molecular clouds had growing baby stars 20 times the mass of our Sun,” said lead author Tokuda in a press release. “We found that about 60% of the molecular clouds we observed had a filamentary structure with a width of about 0.3 light-years, but the remaining 40% had a ‘fluffy’ shape. Furthermore, the temperature inside the filamentary molecular clouds was higher than that of the fluffy molecular clouds.”
In their paper, the authors describe it this way: “Our analysis shows that about 60% of the clouds have steep radial profiles from the spine of the elongated structures, while the remaining clouds have a smooth distribution and are characterized by lower brightness temperatures. We categorize the former as filaments and the latter as nonfilaments.”
The clouds were not uniform and displayed a diversity of shapes. The researchers classified them into four separate types: single filaments, hub filaments, spatially compact clouds, and diffuse clouds.
The temperature difference between the filamentary and fluffy shapes was probably due to their ages. The authors think all clouds started out as filamentary and had high temperatures due to cloud-to-cloud collisions. The clouds have weak turbulence when the temperatures are higher.
However, as the temperature drops, the movement of the incoming gas creates more turbulence. This smooths out the filamentary structure, creating the fluffy shapes.
According to the research, filamentary and fluffy clouds form stars differently. Clouds that hold onto their filamentary shapes are more likely to break apart along their length and form many lower-mass stars similar to our Sun, including planetary systems. When the filamentary structure changes to a fluffy structure, it becomes more difficult for such stars to form.
The implication is that the morphology of the clouds tells us about their evolutionary stages.
“Some of the filamentary clouds are associated with YSOs with outflows and exhibit higher temperatures, likely reflecting their formation conditions, suggesting that these clouds are younger than the nonfilamentary ones,” the authors write in their paper.
The study also emphasizes that the same temperature and structure changes have not been observed in higher metallicity environments like the Milky Way. “Such transitions in structure and temperature have not been reported in metal-rich regions, highlighting a key behaviour for characterizing the evolution of the interstellar medium and star formation in low-metallicity environments,” the authors explain.
With these results, Tokuda says the next step will be to compare them with observations of the Milky Way and other environments richer in heavy elements.
“This study indicates that the environment, such as an adequate supply of heavy elements, is crucial for maintaining a filamentary structure and may play an important role in the formation of planetary systems,” said Tokuda. “In the future, it will be important to compare our results with observations of molecular clouds in heavy-element-rich environments, including the Milky Way galaxy. Such studies should provide new insights into the formation and temporal evolution of molecular clouds and the universe.”
There are still more details to uncover about these filaments, what shapes them, and how they affect the stars they form. How does turbulence play its role? What role do magnetic fields play? Some filaments host YSOs with protostellar outflows. How does that radiative feedback affect the filaments?
Future research will address those questions.
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