For decades, astronomers have pictured “Lyman Alpha Emitters” (LAEs)—ancient, bright galaxies from the universe’s early days—as fresh-faced cosmic teenagers, experiencing their very first big growth spurt. This period, known as “Cosmic Noon,” roughly 2 to 3 billion years after the Big Bang, was a bustling time when galaxies were rapidly forming stars. The common wisdom was that when we saw these LAEs blazing brightly, it meant they were undergoing their initial major burst of star formation. But what if some of these galactic youngsters had already been through a few growth spurts, or even hit their peak, before we observed them? A new study in The Astrophysical Journal Letters turns this idea on its head, revealing a much richer, more complex history for these distant star systems.
Peeking into Galactic Baby Books
To figure out a galaxy’s “life story”—or more precisely, its “star formation history” (SFH)—scientists usually analyze the light it emits. Older methods of studying these histories were limited, often assuming a galaxy could only have one major period of intense star birth. This limitation meant they struggled to capture the full, complex story of a galaxy’s growth.
This new research takes a much more detailed approach. The team used a sophisticated technique called “Dense Basis” to analyze a galaxy’s “spectral energy distribution” (SED), which is its unique light signature across different wavelengths, from ultraviolet to infrared. By examining this signature, scientists can deduce crucial details like a galaxy’s age, how much cosmic dust it contains, and how fast it’s creating new stars. Unlike older methods, Dense Basis acts like a complete video diary, allowing scientists to see multiple periods of star formation, not just one.
The astronomers gathered their data from the “One-hundred-deg² DECam Imaging in Narrowbands” (ODIN) survey, which uses the Dark Energy Camera at the Cerro Tololo Inter-American Observatory in Chile. ODIN is designed to spot LAEs during Cosmic Noon by detecting their distinct “Lyman Alpha” light—ultraviolet light from hot, young stars that stretches into visible light as the universe expands. They then combined this with data from the UVCANDELS survey, which provided a wide range of light measurements essential for creating these detailed light fingerprints. The study focused on 74 LAEs, a significant sample size for such an in-depth, individual galaxy analysis.
Uncovering Diverse Galactic Pasts
The findings challenge our traditional understanding of early galaxy growth. The study found that while most LAEs (67%) were indeed experiencing their first major star-forming event, a substantial number had a more storied past:
- Nearly a third (28%) had actually experienced smaller bursts of star formation before, but their current burst was the most intense they had ever seen.
- A smaller group (5%) had already peaked, with their most intense star formation occurring even earlier in their history.
Combining these last two groups with the “first burst” galaxies, the researchers concluded that roughly 95% of LAEs are experiencing their largest burst of star formation yet—what they term a “formative burst.” This distinction is key: it means that while most LAEs are at a peak in their star formation, it’s not always their absolute beginning. This revelation pushes back against the idea that LAEs are exclusively young, pristine galaxies undergoing their initial major growth spurt.
The study also highlighted differences between LAEs and “Lyman Break Galaxies” (LBGs), another type of star-forming galaxy. LAEs, it turns out, formed a larger portion of their total stellar mass in the most recent 200 million years compared to LBGs. This indicates that LAEs are generally involved in more recent and intense star formation episodes than LBGs, even if it’s not their very first time. The overall picture painted here is that different paths can lead to a galaxy glowing brightly with strong Lyman Alpha light.
A New Chapter in Cosmic Evolution
This groundbreaking research underscores a vital point: galaxies, much like individuals, follow incredibly varied paths as they develop. The universe is not a place where every star-forming galaxy adheres to a single blueprint for growth. Instead, it’s a dynamic cosmic stage where galaxies experience multiple acts of intense star formation, gradually building up their mass and contributing to the universe’s grand story. This deeper insight into the star formation histories of LAEs is a crucial leap forward in deciphering the full saga of galaxy evolution, helping us better understand the diverse origins of all galaxies, including our own Milky Way.
Paper Summary
Methodology
This study analyzed 74 Lyman Alpha Emitters (LAEs) identified by the ODIN survey, which uses the Dark Energy Camera. These galaxies were cross-referenced with the UVCANDELS photometric catalog for light measurements. Researchers used the “Dense Basis” method to reconstruct each galaxy’s star formation history (SFH) by fitting its spectral energy distribution (SED). This advanced technique allowed for detailed, individual assessments of star formation episodes. The galaxies observed were from “Cosmic Noon,” approximately 2 to 3 billion years after the Big Bang.
Results
The study revealed that while 67% of LAEs showed their first major star formation burst, 28% had experienced earlier bursts with the current one being the strongest, and 5% had their highest star formation rate in the past. Overall, about 95% of LAEs were found to be in a “formative burst”—their largest star-forming event to date, though not necessarily their first. It was also found that LAEs formed about 1.3 times more of their total stellar mass in the last 200 million years compared to similar Lyman Break Galaxies (LBGs). These results indicate multiple evolutionary pathways can lead to strong Lyman Alpha emission in galaxies.
Limitations
The study’s primary strength, the Dense Basis method, addresses the limitations of older techniques that often assume a single star formation peak. However, the detailed analysis was conducted on a specific sample of 74 LAEs, which is a subsample of the larger ODIN survey. The study was also limited to LAEs with sufficient rest-UV-through-NIR photometry from the UVCANDELS catalog.
Funding and Disclosures
This research is an open-access publication by the American Astronomical Society. Lead author Nicole M. Firestone is a National Science Foundation Graduate Research Fellow. The ODIN survey, which provided the galaxy samples, is a NOIRLab program. Authors are affiliated with numerous academic and research institutions across the globe.
Publication Information
The paper is titled “ODIN: Star Formation Histories Reveal Formative Starbursts Experienced by Lya-emitting Galaxies at Cosmic Noon.” It was authored by Nicole M. Firestone and numerous collaborators. It was published in The Astrophysical Journal Letters, Volume 986, L8 (12pp), on June 10, 2025. The Digital Object Identifier (DOI) is https://doi.org/10.3847/2041-8213/adbf8c.
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