How the Outskirts of Ancient Galaxies “Unburden” the Early Universe ✦
Analysis of color gradients in four massive (log(M*/M☉) > 11) quiescent galaxies at redshifts z > 3, previously considered problematic for models, shows that earlier estimates based on slit spectroscopy may have been inaccurate. Using MINERVA/JWST medium-band photometry, emission was resolved in a series of elliptical rings out to 0.7″ (~4 effective radii). Negative color gradients (bluer toward the periphery) were found in three galaxies; for the most extreme case (Δ(U–V)/ΔR = –0.126±0.030 mag kpc⁻¹), stellar mass is 0.1 dex lower than photometry within the NIRSpec slit. If gradients are entirely driven by age, tension with extreme-value statistics models weakens up to z~9.5, though the choice of stellar population models also matters. The results underscore the need for integral field spectroscopy: spatially resolved spectra will break the age-dust-metallicity degeneracy and reliably separate contributions from observed gradients and model assumptions.
Context
The Universe, according to the Big Bang theory proposed by Georges Lemaître and confirmed by observations of Edwin Hubble, is continuously expanding. In its first two billion years, there seemingly shouldn’t have been enough time to form such behemoths — galaxies with masses of hundreds of billions of suns, which had already ceased star formation. This contradiction challenges our basic understanding of galaxy evolution and the nature of dark matter, whose distribution governs the growth of structures.
Methods
Using the NIRCam camera on the James Webb Space Telescope and a unique set of 16 photometric filters (including 8 medium-band) from the MINERVA survey, astronomers obtained unprecedentedly detailed images of four candidates for “impossible” galaxies in the UDS field. For each galaxy, photometry was performed in concentric elliptical annuli 0.1 arcseconds wide, allowing measurement of color changes from center to outskirts. These data were then modeled with the spectrophotometric code PROSPECTOR, assuming as an extreme case that dust and metallicity contributions are constant with radius, and the entire color gradient is due solely to stellar age differences.
Results
Three out of four galaxies showed significant negative color gradients: central regions were noticeably redder than the outskirts. The record-holder was MINERVA-1084946, with a U−V color shift of −0.126 mag per kiloparsec. If interpreted as a pure age effect, the mass estimated from slit spectroscopy (which captures only the central region) is overestimated by 0.1 dex. Most importantly, when summing stellar masses over all annuli and comparing with theoretical limits based on extreme value statistics, the tension with the ΛCDM model is significantly reduced — up to z∼9.5. For MINERVA-1189865, however, a positive age gradient in the center was found, making it a possible post-starburst galaxy at z=4.62.
Implications
These results show that many “impossible” early galaxies may simply be an artifact of observation technique: when we look through a spectrograph slit, we only see the oldest and reddest core, while the young periphery goes unnoticed. So nature might not be breaking cosmological models — it’s just making us interpret data more carefully. It’s like archaeology: you wouldn’t judge the age of an entire city from a single potsherd found in its center.
Future development
As deep images accumulate from JWST and future telescopes, and with advances in integral field spectroscopy (IFU), astronomers will be able to build true “age maps” for distant galaxies. This will not only refine their assembly history but also shed light on the earliest epochs of star formation, possibly linked to cosmic dust, which plays a key role in cooling gas and fragmenting clouds.
Impact
This work directly impacts observational cosmology and galaxy formation theories, as well as the interpretation of data from any surveys using slit spectroscopy at high redshifts.
Next steps
To definitively separate the effects of age, dust, and metallicity, integral field spectroscopy of these galaxies is needed. Targeted JWST observations have already been planned, which will provide resolved maps of physical parameters.
Key open problems
The problem of overly early formation of massive galaxies is directly tied to unresolved questions about the nature of dark matter (cold, warm, or otherwise), the primordial perturbation spectrum, and the efficiency of star formation in mini-halos. Moreover, it's part of a broader mystery: why we see so many quiescent massive galaxies in the local Universe, and exactly when they “died”.
🎯 One of the galaxies, MINERVA-1092611, has an age comparable to the age of the Universe at that time — about 1.5 billion years, meaning it formed when the Universe was only ~300 million years old. If such a galaxy had planets, life could have existed on them even before the birth of the Solar System!
Key numbers
- Strongest color gradient (U-V): -0.126 mag/kpc
- Reduction in central stellar mass: 0.1 dex
- Galaxy redshifts: 3.19 – 4.62
- Age of the Universe at z=3.2: 1.97 Gyr
- Size of studied regions: 0.1–0.7 arcseconds