Editor’s Note: Astrobites is a graduate-student-run organization that digests astrophysical literature for undergraduate students. As part of the partnership between the AAS and astrobites, we occasionally repost astrobites content here at AAS Nova. We hope you enjoy this post from astrobites; the original can be viewed at astrobites.org.

Title: Hunting for the First Explosions at the High-Redshift Frontier
Authors: Junehyoung Jeon et al.
First Author’s Institution: The University of Texas at Austin
Status: Published in ApJ

Back in the 1920s, astronomers discovered that we live within just one of many, many galaxies in the big, wide universe. Since then, we’ve been racing to search for the most distant galaxy that can be observed — in other words, searching for the oldest starlight we can see, since the light from these distant sources has been travelling towards us for most of the age of the universe. (Remember: more distant = higher redshift = longer lookback time.)

This race to the redshift frontier has had a pretty eventful history (see a great overview video here), which became even more eventful with the launch of JWST. JWST rapidly smashed the previous redshift (z) record of z = 10.6 by discovering a galaxy at z = 13.2, and then it broke its own record twice more. The current title holder sits at z = 14.4, observed less than 300 million years after the Big Bang.

Several galaxy candidates (to date unconfirmed) have now even been proposed at z ~ 25–32 (e.g., Capotauro), only 100 million years after the Big Bang! If real, these sources would pose a serious challenge to our understanding of the formation of the first galaxies, as galaxies shouldn’t really be observable at such early times. In today’s article, the authors put forward an intriguing alternative: what if some of these ultra-high-redshift candidates aren’t galaxies at all, but transient explosions from the universe’s first stars?

The First Stars and Their Explosive Endings

The earliest generation of stars (Population III; see my previous bite on these here) formed from pristine hydrogen and helium gas. Without metals to cool the gas efficiently, theory predicts that these stars were extremely massive, often exceeding 100 solar masses. While such stars would be short-lived, their deaths could be spectacular.

Population III stars of sufficient mass are predicted to end their lives as hyper-energetic pair-instability supernovae (PISNe). This is a long-winded name for a rapid, intensely hot explosion that leaves no remnant behind — not even a trace of the pre-existing star. Whilst nothing would remain of the star, the light emitted in that explosion could be bright enough to masquerade as a high-redshift galaxy candidate in current JWST surveys, but only if three key conditions are met:

1. JWST must observe a sufficiently overdense region, where lots of Population III stars can form very early.
2. A PISN must occur while JWST is “watching.”
3. The explosion must be bright enough to rise above JWST’s detection limits.

Simulating a (Biased) Universe

To address the likelihood of these conditions having already been met by existing JWST observations, the authors turn to cosmological simulations. Rather than simulating an “average” patch of the universe, they focus on an extremely overdense region (Fig. 1). This creates a rare but important environment where structures collapse earlier than usual. These regions are exactly where large numbers of Population III stars are expected to form at the highest redshifts.

In their simulations, star formation begins as early as z ~ 30–40 (within the first hundred million years after the Big Bang), producing Population III stars and, by extension, potential PISNe very shortly afterwards. While such overdense regions are rare, the authors show that the total area already surveyed by JWST (including large surveys such as CEERS, JADES, PRIMER, and COSMOS-Web) is large enough that it is plausible JWST has already observed at least one such region.

Catching a Cosmic Explosion in the Act

So, we can tick off condition #1: it’s possible that a sufficiently overdense region has already been observed by JWST. Next up, how lucky do we have to be to catch an explosion in the act (condition #2), so to speak? For this condition, cosmic time dilation actually works in our favour. A PISN explosion at z > 20 that lasts only months in its own rest frame can last for decades in the observed frame. (This whole time-being-relative thing sounds wacky because it is — please join me, Neil deGrasse Tyson, and countless others in struggling to imagine this.)

But would any such explosion be bright enough (condition #3)? Using theoretical PISN spectra, the authors show that these explosions could reach observed magnitudes of ~28–29 at z ~ 30 — right at the depth of current JWST deep surveys (Fig. 2). In fact, the predicted brightness and colours are somewhat comparable to those of some proposed z ~ 30 candidates (Fig. 3), raising the possibility that these objects could be PISNe rather than galaxies.

So… Are We Seeing the First Stars Die?

It’s not time to throw a party just yet. The authors note there are several caveats and uncertainties. The nature of Population III stars is still highly uncertain, JWST does not continuously monitor the same patch of sky, and identifying a PISN at such high redshift would be extremely challenging. Thanks to time dilation, these explosions would fade very slowly, making them hard to distinguish from steady sources using photometry alone. Alternatively, there are other plausible explanations for these ultra-high-redshift candidates: lower-redshift interlopers (a rather infamous example is CEERS-93316), local brown dwarfs, or even nearby exoplanets.

Still, the idea is exciting. If JWST were to detect a genuine PISN at z > 20, it would represent a direct glimpse of the very first stars, pushing observational astronomy into truly uncharted territory. For now, the most distant explosions in the universe may already be hiding in JWST images; we just have to learn how to recognise them.

Original astrobite edited by Nathalie Korhonen Cuestas.

About the author, Lucie Rowland:

I’m a fourth (and final!) year PhD student at Leiden Observatory in the Netherlands, studying massive, star forming galaxies in the early universe with ALMA and JWST. It’s a really exciting time to be interested in astronomy, so I hope to make groundbreaking new research more accessible!

The post The Oldest Starlight appeared first on AAS Nova.

Editor’s Note: Astrobites is a graduate-student-run organization that digests astrophysical literature for undergraduate students. As part of the partnership between the AAS and astrobites, we occasionally repost astrobites content here at AAS Nova. We hope you enjoy this post from astrobites; the original can be viewed at astrobites.org.

Title: GLIMPSE: An Ultrafaint ≃10⁵ M_⊙ Pop III Galaxy Candidate and First Constraints on the Pop III UV Luminosity Function at z ≃ 6–7
Authors: Seiji Fujimoto et al.
First Author’s Institution: University of Toronto; The University of Texas at Austin
Status: Published in ApJ

You, me, your laptop, my $8 matcha, and just about everything else on Earth was forged in the fiery bellies of dying stars. Generations of stars had to live and die before the universe became enriched with any elements heavier than helium (what astronomers call “metals”). The first stars to undergo this cosmic cycle are known as Population III (Pop III) stars. Though their existence has been hypothesized since the 1960s, astronomers have failed to observe these distant metal-free stars or the faint, low-mass galaxies that host them.

The first Pop III stars likely formed around 100 million years after the Big Bang in pristine pockets of hydrogen gas. Although these are too distant for us to observe, we expect that as the universe started to become metal enriched, there were still existing pockets of gas introverted enough to survive unpolluted and form metal-free Pop III stars up to a redshift of z ~ 6–7 (when the universe was around 900 million years old)!

JWST is the perfect instrument to search for these systems. You can read other astrobites on the search for possible Pop III systems with JWST here and here. The authors of today’s article seek to develop the most efficient way of using JWST’s Near-Infrared Camera (NIRCam) to find the galaxies hosting Pop III stars. Using their selection method on existing NIRCam data, the authors identified one promising Pop III galaxy candidate.

I’m Not Like Other Galaxies

In order to find a Pop III galaxy, we need to take a look at galaxies’ spectral energy distributions (SEDs). These are graphs that show the energy emitted by a galaxy at different wavelengths of light. Pop III galaxies are expected to have SEDs that differ from your everyday, metal-enriched galaxy. NIRCam will be especially sensitive to three key spectral features that show up in the SEDs of Pop III galaxies: an absent [O III] line (light emitted by doubly ionized oxygen atoms), a strong H-alpha line (light emitted when a hydrogen atom transitions from its third to its second energy level), and a significant Balmer jump (light absorbed to ionize electrons in the second energy level of a hydrogen atom). To identify these key SED characteristics, the authors use SED fitting and color–color diagrams to execute an efficient Pop III search with NIRCam.

The first selection method involves SED fitting. Astronomers create template SEDs that represent different types of galaxies and then compare these templates to the observed SEDs to see which one matches best. In this work, the authors use metal-rich galaxy templates and Pop III templates to fit the galaxies observed with NIRCam. They then calculate the chi-squared χ² (a statistical measure of best fit) between the data and all the SED templates. A galaxy is selected as a Pop III candidate if the Pop III model provides a good fit (χ² < 10) to the photometry and is significantly better than any metal-rich model. It’s kind of like looking for Cinderella by making every woman in the kingdom try on the glass slipper.

A color–color diagram plots the difference in magnitude between two filters on each axis. NIRCam filters are specially chosen to emphasize the SED characteristics above. When these filters are chosen, Pop III galaxies occupy a distinct region of this diagram as compared to metal-rich galaxies. For example, subtracting the F356W filter from the F277W filter is sensitive to the presence of the [O III] line and the Balmer jump. Figure 1 demonstrates how this color selection separates Pop III galaxies from typical galaxies.

O Pop III, Pop III, Wherefore Art Thou?

The authors apply their fresh new selection criteria to publicly available NIRCam data from large surveys. And (drum roll please) the slipper fits! The Pop III galaxy candidate GLIMPSE-16043 is an ultra-faint galaxy at z = 6.5. It was imaged in the GLIMPSE survey, which uses the technique of gravitational lensing to observe faint and distant galaxies.

The GLIMPSE survey targeted a massive galaxy cluster, Abell S1063. The cluster bends the light from distant galaxies and, like a giant lens, magnifies faraway objects, providing some of the deepest JWST imaging to date. The Pop III candidate passes both tests: it resides in the Pop III region of the color–color diagram, and its SED is best fit by a Pop III model, not a metal-rich galaxy model (see Figure 2). Next, spectroscopic follow-up is needed to ensure that this galaxy is truly metal free and not just extremely metal poor.

The authors conclude that our best shot at identifying additional Pop III galaxy candidates is using NIRCam to image large numbers of gravitationally lensed clusters. Without magnification from gravitational lensing, it may be impossible to see these ultra-faint Pop III galaxies. Once candidates have been identified, they can be followed up with deep spectroscopy to confirm their redshift and their lack of metals. Who knows? With these new methods, we may soon get a glimpse of the universe’s very first stars.

Original astrobite edited by Chris Layden and Margaret Verrico.

About the author, Madison VanWyngarden:

I am a first-year PhD student in astronomy and NSF Graduate Research Fellow at the University of Arizona. I study galaxy formation and evolution in the distant universe and am particularly interested in dusty star-forming galaxies. In my free time, I love reading, hiking, and baking bread!

The post A Possible GLIMPSE of the Universe’s First Stars appeared first on AAS Nova.

Astronomers have long sought evidence of the universe’s first generation of stars, and as more distant galaxies come into view, it seems these stars may finally be within reach.

Excess Metals Popping Up

Since its 2021 launch, JWST has given astronomers eyes to peer into the distant past, discovering many galaxies whose light reveals the universe’s early stages of star and galaxy formation. Within the population of newly discovered galaxies are a few with bizarre chemical properties that, upon first pass, seem to be too enriched to exist so early in the universe. These hard-to-reconcile abundances may be a sign of the universe’s first stars. 

Known as Population III (Pop III) stars, the first stars in the universe were born out of giant clouds of pristine gas (hydrogen, helium, and a little lithium) and were able to form at masses hundreds to thousands of times the mass of our Sun. Though they burned bright, they did not burn for very long, ending their lives in violent supernovae and chucking their newly enriched guts back into their surroundings. While these stars are long dead, the chemical imprints they left on their host galaxies can persist  — and understanding how Pop III stars create and distribute metals could clue us in to the odd chemical signatures recently found with JWST.

Too Much Nitrogen in GS 3073

Researchers have identified a few galaxies exhibiting high nitrogen-to-oxygen (N/O) ratios that cannot be explained by stars similar to those in the universe today. A couple of these galaxies could be explained through multiple stellar populations, rapidly rotating stars, massive explosions, or the early stages of globular cluster formation. However, GS 3073, a galaxy with a redshift of z = 5.55 (about one billion years after the Big Bang), has an N/O excess so high that is has, so far, defied explanation.

Aiming to make sense of this bizarre phenomenon, Devesh Nandal (University of Virginia; Center for Astrophysics | Harvard & Smithsonian) and collaborators used stellar evolution models to see if Pop III stars could be the culprit. Modeling stars with masses 1,000–10,000 times the mass of our Sun, the authors traced the elemental yields of these supermassive stars as they go through the various stages of nuclear burning. The analysis takes into account mixing within the stars, mass loss throughout their lifetimes, and how the eventual supernova ejecta mixes within the interstellar medium.

From this modeling, the authors found that massive Pop III stars between 1,000 and 10,000 solar masses can produce the observed elemental abundances measured for GS 3073. Stars less massive do not produce high enough N/O ratios, and stars more massive have much lower oxygen-to-hydrogen ratios — strongly suggesting upper and lower mass limits to the possible supermassive stars that could have produced GS 3073’s chemical composition.

Finally Evidence of Pop III Stars?

This study of GS 3073 is the first of its kind to confirm the chemical imprints of Pop III stars on their host galaxy at this redshift. The unique nitrogen abundance can only be produced through the evolutionary phases of Pop III stars that burn quickly enough to produce and release an excess amount of nitrogen while other elements stay consistent. From their modeling, the authors suggest that galaxies with even higher nitrogen excess could exist, and further observations with JWST may just find them. 

The search for Pop III stars is booming — another recent study (Visbal et al 2025) examines the galaxy LAP1-B. While GS 3073 shows evidence of Pop III stars through chemical abundances, the study of LAP1-B finds that the galaxy matches theoretical predictions for the formation environments and mass distributions of Pop III stars. Both of these recent research works are laying the groundwork for the wealth of discovery possible with JWST, and the universe’s first stars are no longer out of reach.

Citation

“1000–10,000 M_⊙ Primordial Stars Created the Nitrogen Excess in GS 3073 at z = 5.55,” Devesh Nandal et al 2025 ApJL 994 L11. doi:10.3847/2041-8213/ae1a63

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