Might the universe have constructed its initial cities of stars sooner than science permits? That is the question now facing astronomers following NASA’s James Webb Space Telescope (JWST) uncovering a shocking array of 300 extremely bright objects from the beginning of time prospects for among the first galaxies ever to be observed.
The team, led by Haojing Yan of the University of Missouri, depended on JWST’s two main infrared workhorses: the Near-Infrared Camera (NIRCam) and the Mid-Infrared Instrument (MIRI). NIRCam’s detectors, attuned between 0.6 and 5 microns, are able to detect the dim, red-shifted light of galaxies more than 13 billion years old. MIRI, which reaches from 5 to 28 microns, explores even colder, dustier forms, adding depth to NIRCam’s reach. Together, they can see the feeble glow of very old starlight that has been redshifted deep into the infrared by the expansion of the universe a sign called redshift. The higher the redshift, the farther away the galaxy is from us on Earth, and the closer it is to the beginning of the universe, Yan said.
To remove these far-out candidates, researchers employed the dropout technique, a technique refined over decades but now amped up by JWST’s sensitivity. Bangzheng “Tom” Sun, author of the study, said, “It detects high-redshift galaxies by looking for objects that appear in redder wavelengths but vanish in bluer ones a sign that their light has traveled across vast distances and time.” The Lyman Break, caused by the presence of neutral hydrogen absorbing ultraviolet light, produces the spectral disappearance in the form of a sharp cutoff in a galaxy’s spectrum, and as redshift increases, the cutoff shifts towards redder wavelengths.
Detecting the dropouts is merely a starting point. With incomplete spectroscopy, the team resorted to spectral energy distribution fitting matching each object’s brightness in different filters to make educated guesses about redshift, age, and mass. Less conclusive than spectroscopy but still useful, this method can indicate whether light from a galaxy’s profile is plausible at extreme distances. Here, the many of the candidates are brighter than anticipated for galaxies that were already forming only a few hundred million years after the Big Bang, suggesting either very high rates of star formation or the presence of other light sources, such as active black holes.
The brightness of these objects is reminiscent of previous JWST surprises, when the masses of some of the first galaxies were so large that they challenged the standard ΛCDM cosmological model. Subsequent work provided the indication that a portion of this excess light arose from accreting black holes the so-called “little red dots” whose hot gas disks resemble the light of vast stellar populations. However even when such effects were allowed for, there are still about twice as many luminous early galaxies as theory would predict, which indicates that star formation in the early universe could have been much more efficient.
Spectroscopy will be the judge. By breaking a galaxy’s light up into its component wavelengths, devices such as JWST’s Near-Infrared Spectrograph can measure redshift to exquisite accuracy and disclose chemical signatures of stars, gas, and dust. “One of our objects is already confirmed by spectroscopy to be an early galaxy,” Sun said. But confirmation is only the first step; the team hopes to collect many more before announcing a challenge to prevailing models.
The stakes are high. Verification of even a small number of these 300 candidates at extreme redshifts would have the potential to make astrophysicists reexamine the assembly timeline of galaxies. Existing models, based on dark matter halo growth and controlled by supernova and black hole feedback, are not able to produce such luminous systems so early after the Big Bang. Others have suggested modifications to star formation physics, and still others play with radical possibilities, like pre-epoch dark energy bursts affecting structure growth.
In the meantime, the finding speaks to JWST’s revolutionary capability. Its infrared eyesight, combined with methods such as dropout selection, is not only completing the blank pages of cosmic history it is unveiling chapters no one even knew to write.
