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Starts With A Bang

JWST finally makes sense of bright, early galaxies

With so many early galaxies of unexpectedly large brightnesses, JWST surprised us all. Here's how scientists made sense of what we see.
FIRE simulation JWST starburst star-forming
An artistic representation of a starburst galaxy, using data from the FIRE ( Feedback in Realistic Environments) simulation that includes strong bursts of star-formation. As recent research has shown, the bright, early galaxies seen by JWST might not be tracers solely of stellar mass, but of the brightness produced by giant and supergiant stars, as well as supernovae, found in actively starbursting galaxies.
Credit: Aaron M. Geller, Northwestern, CIERA + IT-RCDS
Key Takeaways
  • Ever since it opened its impressively sharp, high-resolution, infrared eyes, JWST has been seeing something unexpected: bright, early galaxies in far greater numbers than was expected.
  • Although two partial explanations existed, in JWST's optical overperformance due to cleanliness and an underestimate of early, massive galaxies due to simulation resolution, there were still too many bright, early galaxies.
  • At last, a third puzzle piece has been put into place: early galaxy brightnesses are not solely determined by mass, but also by brilliant bursts of star-formation. With all three pieces, the mystery may be finally solved.

From its very first glimpse of the distant Universe, JWST has shocked astronomers.

jwst smacs 0723 hubble
This almost-perfectly-aligned image composite shows the first JWST deep field’s view of the core of cluster SMACS 0723 and contrasts it with the older Hubble view. The JWST image of galaxy cluster SMACS 0723 is the first full-color, multiwavelength science image taken by the JWST. It was, for a time, the deepest image ever taken of the ultra-distant Universe, with 87 ultra-distant galaxy candidates identified within it. They await spectroscopic follow-up and confirmation to determine how distant they truly are. but even from this first image, JWST observations suggested that the number and density of bright, early galaxies may pose a problem for astronomers.
Credit: NASA, ESA, CSA, and STScI; NASA/ESA/Hubble (STScI); composite by E. Siegel

Its unprecedentedly deep views revealed a colossal surprise: bright galaxies.

JWST deep field vs hubble
This section of the latest JWST ultra-deep field, overlapping with Hubble’s eXtreme Deep Field and Ultra-Deep Field, reveals an enormous number of objects previously invisible to Hubble, even with only ~4% of the observing time. JWST is just that good, but what these galaxies mean for cosmology is still under review.
Credit: NASA, ESA, CSA, STScI, Christina Williams (NSF’s NOIRLab), Sandro Tacchella (Cambridge), Michael Maseda (UW-Madison); Processing: Joseph DePasquale (STScI); Animation: E. Siegel

Even at these earliest times, galaxies were too big, bright, and numerous to explain.

JWST deep field vs hubble
This portion of the newest JWST image that covered part of Hubble’s ultra-deep field reveals a number of distant galaxies, highlighted manually, that are present in the brief JWST views but not in the long-exposure Hubble views. Some of these may indeed be cosmic record-breakers.
Credit: NASA, ESA, CSA, STScI, Christina Williams (NSF’s NOIRLab), Sandro Tacchella (Cambridge), Michael Maseda (UW-Madison); Processing: Joseph DePasquale (STScI); Animation: E. Siegel

Our best cosmic predictions, based on ΛCDM cosmology, didn’t expect what JWST saw.

flight through universe CEERS JWST NASA
Taking us beyond the limits of any prior observatory, including all of the ground-based telescopes on Earth as well as Hubble, NASA’s JWST has shown us the most distant galaxies in the Universe ever discovered. If we assign 3D positions to the galaxies that have been sufficiently observed-and-measured, we can construct a visualized fly-through of the Universe, as the CEERS data from JWST enables us to do here. At greater distances, compact, star-forming galaxies are more common; at closer distances, more diffuse, quiescent galaxies are the norm.
Credits: Frank Summers (STScI), Greg Bacon (STScI), Joseph DePasquale (STScI), Leah Hustak (STScI), Joseph Olmsted (STScI), Alyssa Pagan (STScI); Science by: Steve Finkelstein (UT Austin), Rebecca Larson (RIT), Micaela Bagley (UT Austin)

Normally, galactic brightness traces stellar mass: the mass of the galaxy due to stars.

Southern Pinwheel M83
The Southern Pinwheel Galaxy, Messier 83, displays many features common to our Milky Way, including spiral arms and a central bar, as well as spurs and minor arms. The pink regions showcase transitions in hydrogen atoms driven by ultraviolet light. Since that light is primarily produced by hot, blue stars, it’s only the regions where new star-formation is actively occurring where those pink features appear. The overall brightness of the galaxy is directly related to its stellar mass: the amount of mass that has cumulatively formed stars within it, a typical property of modern galaxies.
Credit: CTIO/NOIRLab/DOE/NSF/AURA; Acknowledgment: M. Soraisam (University of Illinois); Image processing: Travis Rector (University of Alaska Anchorage), Mahdi Zamani & Davide de Martin

One potential culprit, the “first stars,” brighter and bluer than modern stars, have yet to be spotted.

jwst first stars
The very first stars and galaxies that form should be home to Population III stars: stars made out of only the elements that first formed during the hot Big Bang, which is 99.999999% hydrogen and helium exclusively. Such a population has never been seen or confirmed (although many have used insufficient, inconclusive measurements to indicate that they have), but some are hopeful that the James Webb Space Telescope will eventually reveal them. In the meantime, the most distant galaxies that we’ve seen are all very bright and intrinsically blue, but not quite pristine, still coming to us from several hundred million years after the start of the hot Big Bang and without compelling evidence for these “first stars” anywhere within them.
Credit: Pablo Carlos Budassi/Wikimedia Commons

One partial explanation comes from JWST’s optical overperformance.

Hubble spherical aberration jwst
This simulation of spherical aberration shows how a point source is seen by a perfectly spherical aperture if the object is overfocused (left), underfocused (right), or perfectly focused (center), along with being properly corrected for wavelength (middle row) versus being either slightly overcorrected (top row) or undercorrected (bottom row). The extreme lower-right image shows the original spherical aberration in Hubble’s original WFPC camera. Hubble’s primary mirror had problems with spherical aberration; JWST’s mirrors do not.
Credit: Mdf at English Wikipedia; NASA, ESA and the COSTAR Team

Due to its unprecedented cleanliness, JWST’s pristine optics return brighter, sharper views than anticipated.

James Webb Space Telescope
Shown during an inspection in the clean room in Greenbelt, Maryland in late 2021, NASA’s James Webb Space Telescope was photographed at the moment of completion. Only weeks later, it would successfully launch and deploy, leading to an unprecedented set of advances in astronomy. From mirrors to instruments, it was kept cleaner, from start to finish, than any observatory ever.
Credit: NASA/Desiree Stover

A second contribution arises from simulation resolution.

cold fuzzy dark matter simulations
This image shows a series of structure-formation simulations: at low resolution, medium resolution, and superior/high resolution, for both cold dark matter and fuzzy dark matter models. If we can measure the Universe precisely and accurately enough, we can distinguish between these types of models, contingent on matching dark matter density to a realistic galaxy distribution, and whether we simulate the cosmic web to great enough precision.
Credit: M. Sipp et al., MNRAS (submitted), 2023

We can increase to high-resolution and focus on initial, rare overdensities.

regions of various density renaissance dark matter simulations
Regions born with a typical, or “normal” overdensity, will grow to have rich structures in them, while underdense “void” regions will have less structure. However, early, small-scale structure is dominated by the most highly peaked regions in density (labeled “rarepeak” here), which grow the largest the fastest, and are only visible in detail to the highest resolution simulations.
Credit: J. McCaffrey et al., Open Journal of Astrophysics (submitted), 2023

These factors, combined, explain some, but not all, of JWST’s observed galaxies.

likelihood of early galaxies vs age for jwst
The three simulated regions highlighted earlier, using the Renaissance suite, lead to predictions for how massive galaxies should be in those three regions (orange, blue, and green lines). The 5 earliest galaxies revealed so far with JWST, with error bars shown, have about a probability of “1” of occurring within the observed regions. If they were truly rare, they’d be brighter and more massive, as shown by the ~10^-3 and ~10^-6 likelihood curves.
Credit: J. McCaffrey et al., Open Journal of Astrophysics (submitted), 2023

There are still too many bright galaxies seen too early on.

JWST deep field vs hubble
This region of space, viewed first iconically by Hubble and later by JWST, shows an animation that switches between the two. JWST reveals gaseous features, deeper galaxies, and other details that are not visible to Hubble. Although many of these galaxies are very distant, galaxies that are physically smaller, but more distant than 14.6 billion light-years away, can appear larger than their closer, smaller counterparts.
Credit: NASA, ESA, CSA, STScI, Christina Williams (NSF’s NOIRLab), Sandro Tacchella (Cambridge), Michael Maseda (UW-Madison); Processing: Joseph DePasquale (STScI); Animation: E. Siegel

But one factor may finally solve the puzzle: bright starbursts.

dwarf starburst galaxy henize 2-10
When a star-forming region becomes so large that it extends over an entire galaxy, that galaxy becomes a starburst galaxy. Here, Henize 2-10 is shown evolving toward that state, with young stars in many locations and active stellar nurseries in numerous locations galaxy-wide. If we were to count the number of stars within the galaxy and multiply that number by the Sun’s light-to-mass ratio, we’d underestimate the total flux by about a 3-to-1 ratio.
Credit: NASA, ESA, Zachary Schutte (XGI), Amy Reines (XGI); Processing: Alyssa Pagan (STScI)

Starbursts are brief star-forming episodes, dramatically enhancing a galaxy’s brightness.

Tarantula Nebula JWST stars
The central concentration of this young star cluster found in the heart of the Tarantula Nebula is known as R136, and contains many of the most massive stars known. Among them is R136a1, which comes in at about ~260 solar masses and shines brighter than more than 8 million suns, making it the heaviest known star. Although great numbers of cooler, redder stars are also present, the brightest, bluest ones dominate this image.
Credit: NASA, ESA, CSA, STScI, Webb ERO Production Team

Alongside normal stars, giants, supergiants, and supernovae temporarily inflate a galaxy’s luminosity.

starburst galaxies luminosity early jwst redshift
When burstiness is accounted for, rather than entirely “smoothed out” over long time intervals, brightness enhancements in a variety of galaxies can be seen at all redshifts where JWST has identified anomalously large number densities of bright galaxies. These three panels show those enhancements, relative to other simulations and photometric JWST data, at z = 8, 10, and 12, corresponding to times of 650, 480, and 380 million years after the hot Big Bang.
Credit: G. Sun et al., ApJ Letters, 2023

Especially common in low-mass galaxies, “burstiness” can explain what JWST sees.

starburst galaxies luminosity early jwst
Both the number density of galaxies as a function of redshift (left) and the rest-frame ultraviolet luminosity of galaxies (right) can be explained by a “bursty” scenario, where a young galaxy’s brightness is temporarily enhanced by the giant stars, supergiant stars, and stellar cataclysms that accompany a starburst galaxy.
Credit: G. Sun et al., ApJ Letters, 2023

At last, simulations can now reproduce JWST’s observed abundance of bright, early galaxies.

JADES deepest galaxies JWST
The viewing area of the JADES survey, along with the four most distant galaxies verified within this field-of-view. The three galaxies at z = 13.20, 12.63, and 11.58 are all more distant than the previous record-holder, GN-z11, which had been identified by Hubble and has now been spectroscopically confirmed by JWST to be at a redshift of z = 10.6. No doubt these records will themselves be broken, possibly with galaxy candidates that already exist within the same field-of-view.
Credit: NASA, ESA, CSA, M. Zamani (ESA/Webb), Leah Hustak (STScI); Science credits: Brant Robertson (UC Santa Cruz), S. Tacchella (Cambridge), E. Curtis-Lake (UOH), S. Carniani (Scuola Normale Superiore), JADES Collaboration

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