Astronomers are a step closer to deciphering the origins of disk galaxies like our own Milky Way thanks to the James Webb Space Telescope.
Through the eyes of Webb they were able to study 111 single-disk and double-disk galaxies across a large span of cosmological time, peering as far back as 3.8 billion years after the Big Bang.
This is the first time a team of researchers has investigated thick- and thin-disk structures at such distances, bridging the gap between high-redshift observers probing the early universe’s composition and galactic archaeologists seeking to understand our own galaxy's history.
The new study has been published today in Monthly Notices of the Royal Astronomical Society.
Present-day disk galaxies often contain a thick, star-filled outer disk and an embedded thin disk of stars. For instance, our own Milky Way galaxy's thick disk is approximately 3,000 light-years in height, and its thin disk is roughly 1,000 light-years thick.
How the dual-disk structure comes to be over time and the factors that contribute to its formation have prompted various theories.
Now, by pulling archival data from multiple observational programs by Webb, a joint endeavour of the US, European and Canadian space agencies, astronomers are closer to understanding disk galaxies' origins, and thick- and thin-disk formation pathways.
"This unique measurement of the thickness of the disks at high redshift is a benchmark for theoretical study, and it was only possible with Webb," said Takafumi Tsukui, lead author of the paper and a researcher at the Australian National University in Canberra.
"Usually, the older, thick disk stars are faint, and the young, thin disk stars outshine the entire galaxy. But with Webb's resolution and unique near-infrared ability to see through dust and highlight faint old stars, we can identify the two-disk structure of galaxies and measure their thickness separately."
The results of their analysis indicate that galaxies form a thick disk first, followed by a thin disk. The timing of when this takes place is dependent on the galaxy's mass: high-mass, single-disk galaxies transitioned to two-disk structures around 8 billion years ago.
In contrast, low-mass, single-disk galaxies formed their embedded thin disks later on, about 4 billion years ago.
"This is the first time it has been possible to resolve thin stellar disks at higher redshift. What's really novel is uncovering when thin stellar disks start to emerge," said Emily Wisnioski, a co-author of the paper at the Australian National University in Canberra.
"To see thin stellar disks already in place 8 billion years ago, or even earlier, was surprising."
To explain this transition from a single, thick disk to a thick and thin disk, and the difference in timing for high- and low-mass galaxies, the team looked beyond their initial edge-on galaxy sample and examined data showing gas in motion from the Atacama Large Millimeter/submillimeter Array (ALMA) and ground-based surveys.
By taking into consideration the motion of the galaxies' gas disks, the team finds their results align with the "turbulent gas disk" scenario, one of three major hypotheses that has been proposed to explain the process of thick- and thin-disk formation.
In this scenario, a turbulent gas disk in the early universe sparks intense star formation, forming a thick stellar disk. As stars form, they stabilise the gas disk, which becomes less turbulent and, as a result, thinner.
Since massive galaxies can more efficiently convert gas into stars, they settle sooner than their low-mass counterparts, resulting in the earlier formation of thin disks. The team notes that thick- and thin-disk formation are not siloed events: The thick disk continues to grow as the galaxy develops, though it's slower than the thin disk's rate of growth.
Webb's sensitivity is enabling astronomers to observe smaller and fainter galaxies, analogous to our own, at early times and with unprecedented clarity for the first time. In this study, the team noted that the transition period from thick disk to a thick and thin disk roughly coincides with the formation of the Milky Way galaxy's thin disk.
With Webb, astronomers will be able to further investigate Milky Way-like progenitors – research that can help explain our galaxy's formation history.
In the future, the team intends to incorporate other data points into their edge-on galaxy sample.
"While this study structurally distinguishes thin and thick disks, there is still much more we would like to explore," said Tsukui. "We want to add the type of information people usually get for nearby galaxies, like stellar motion, age, and metallicity. By doing so, we can bridge the insights from galaxies near and far, and refine our understanding of disk formation."
Webb, the world's premier space science observatory, is solving mysteries in our solar system, looking beyond to distant worlds around other stars, and probing the mysterious structures and origins of our universe and our place in it.
ENDS
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Images and captions
Caption: Astronomers pulled from James Webb Space Telescope’s data to analyse a sample of 111 edge-on galaxies. The team’s analysis suggests that thick disk formation occurs first, and thin disk formation follows. When this process occurs depends on the galaxy’s mass.
Credit: NASA, ESA, CSA, T. Tsukui (Australian National University).
Further information
The paper 'The emergence of galactic thin and thick discs across cosmic history' by Takafumi Tsukui et al. has been published in Monthly Notices of the Royal Astronomical Society. DOI: 10.1093/mnras/staf604
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