This image from NASA’s James Webb Space Telescope shows a portion of the Leo P dwarf galaxy (stars at lower right represented in blue). Leo P is a star-forming galaxy located about 5 million light years away in the constellation Leo. A team of scientists collected data from about 15,000 stars in Leo P to deduce its star formation history. (Credit: Kristen McQuinn/NASA’s James Webb Space Telescope)
Webb Telescope reveals how galaxies can dodge death by taking a break from star formation
In a nutshell
- Scientists discovered that a tiny galaxy called Leo P took a 2.5-billion-year break from forming stars during its early life — yet somehow survived, unlike many similar-sized galaxies that died out during this period.
- Leo P’s isolation from larger galaxies appears to be key to its survival, suggesting that a galaxy’s environment may be just as important as its size in determining whether it lives or dies.
- This pattern of pausing and restarting star formation has now been seen in four isolated small galaxies, hinting that this might have been a common survival strategy for the universe’s smallest galaxies.
NEW BRUNSWICK, N.J. — Thirteen billion years ago, a small galaxy did something peculiar — it stopped making stars. Then, after an extended cosmic intermission lasting billions of years, it started again. This pattern, discovered in a galaxy called Leo P, is providing astronomers at Rutgers University with new insights into how the earliest and smallest galaxies managed to survive the universe’s turbulent youth.
Located in the constellation Leo about 1.62 million light-years away, Leo P is a cosmic lightweight. While our Milky Way contains roughly 100-400 billion stars, Leo P contains only about 290,000. The “P” in its name stands for “pristine,” reflecting its primitive nature. It contains only about 3% of the heavy elements found in our Sun, making it remarkably similar to the first galaxies that formed in the universe. The study, published in The Astrophysical Journal, represents one of the most detailed analyses of such a pristine galaxy ever conducted.
What makes Leo P particularly special is its isolation. It sits far enough from the Local Group (the cluster of galaxies that includes our Milky Way) to develop without being influenced by the gravitational fields of larger star systems. Leo P provides “a unique laboratory to explore the early evolution of a low-mass galaxy in detail,” writes lead study author Kristen McQuinn, from the Space Telescope Science Institute and Rutgers University, in a statement.
Using JWST’s powerful observing capabilities, the research team studied stars in Leo P that are approximately 13 billion years old. These ancient stars serve as “fossil records” of star formation from the universe’s early days, allowing astronomers to piece together the galaxy’s history without having to directly observe its past.
The observations revealed three distinct chapters in Leo P’s history. The galaxy began forming stars in the universe’s early years but then experienced something unexpected: a prolonged pause in star formation lasting about 2.5 billion years. This pause coincided with the Epoch of Reionization, a period between 150 million and one billion years after the Big Bang when the first stars and galaxies began flooding the universe with energetic light. After this extended pause, Leo P resumed making stars and continues to do so today.
The research team compared Leo P’s history with three other similar isolated small galaxies, Aquarius, Leo A, and WLM. Surprisingly, all four showed comparable patterns of early star formation, followed by a long pause and then renewed star activity. While this sample size is small, the consistency suggests this might be a common path for isolated small galaxies rather than a cosmic coincidence.
What’s particularly intriguing about Leo P is that during the universe’s reionization period, it had even fewer stars than many of the smallest known galaxies that stopped forming stars completely during this period. This suggests that a galaxy’s fate isn’t determined by size alone; its environment plays a crucial role in its survival and evolution.
“If the trend holds, it provides insights on the growth of low-mass structures that is not only a fundamental constraint for structure formation but a benchmark for cosmological simulations,” explains McQuinn. This knowledge will help astronomers better understand the timeline of cosmic events and the processes that led to the creation of stars.
The implications extend beyond just understanding galaxy evolution. During their extended pause in star formation, these small galaxies produced very little ultraviolet light, helping explain why astronomers observe fewer bright galaxies than expected during certain periods of cosmic history.
As telescopes like JWST continue to peer deeper into space and time, Leo P’s story may prove to be just the beginning. The discovery of this pattern in multiple isolated small galaxies suggests astronomers may need to revise their models of how the smallest galaxies evolved in the early universe, and how many might have survived through similar periods of dormancy.
Paper Summary
Methodology
The research team used NASA’s James Webb Space Telescope to observe Leo P for approximately 36 hours total, using three different filters on the telescope’s NIRCam instrument (F090W, F150W, and F277W). The telescope’s powerful capabilities allowed them to detect individual stars within the galaxy, including stars below what astronomers call the “oldest main-sequence turnoff point” – essentially letting them see the galaxy’s oldest surviving stars. Using sophisticated computer models, they compared the observed stars’ brightness and colors to theoretical predictions, allowing them to reconstruct the galaxy’s star formation history over time. The team also analyzed observations of three other isolated dwarf galaxies (Aquarius, Leo A, and WLM) for comparison.
Results
The study revealed that Leo P experienced three distinct phases of star formation:
- An early period of star formation that ended about 12.6 billion years ago, during which the galaxy formed about 15% of its total stars
- A prolonged pause lasting approximately 2.5 billion years
- Renewed star formation that continues to the present day
The team also discovered that Leo P is extremely metal-poor, containing only 3% of the heavy elements found in our Sun. This makes it similar to the primordial galaxies that existed in the early universe. The galaxy’s total current stellar mass is approximately 290,000 times the mass of our Sun.
Limitations
While the findings are significant, there are several important limitations to consider:
- The sample size of only four galaxies showing similar patterns is relatively small
- There are inherent uncertainties in dating very old stellar populations
- The theoretical models used to interpret the observations continue to be refined
- The team could only study galaxies that have survived to the present day, potentially missing other evolutionary patterns that might have existed in galaxies that didn’t survive
Discussion and Takeaways
- A galaxy’s environment, not just its mass, plays a crucial role in its evolution
- The pattern of paused and renewed star formation may be more common than previously thought
- Small galaxies’ reduced ultraviolet light output during their “pause” periods has implications for our understanding of the universe’s evolution
- The findings provide new benchmarks for cosmological simulations and models of galaxy formation
Funding and Disclosures
This research was supported by NASA through grant No. JWST-GO-1617 from the Space Telescope Science Institute under NASA contract NAS5-26555. The research team included scientists from Rutgers University (including Alyson Brooks, Roger Cohen, and Max Newman from the Department of Physics and Astronomy), the Space Telescope Science Institute, and several other institutions.
Publication Information
The study, titled “The Ancient Star Formation History of the Extremely Low-mass Galaxy Leo P: An Emerging Trend of a Post-reionization Pause in Star Formation,” was published in The Astrophysical Journal (Volume 976, Issue 60) on November 20, 2024. The research was led by Kristen B. W. McQuinn, with contributions from multiple international collaborators.
