JWST's Massive Early Galaxies (2023): The Red Candidates Explained

Ivo Labbé and colleagues reported in 2023 six JWST candidate galaxies, seen 500 to 700 million years after the Big Bang, that seemed to hold over 10 billion Suns’ worth of stars each. Galaxies that massive so early would challenge models. Follow-up showed at least one is a growing black hole.

The question

In the standard picture, set out by White and Rees in 1978, galaxies grow gradually as gas cools inside dark matter halos that are themselves still assembling. In the first few hundred million years there should be few galaxies with large stellar masses. Before JWST, nobody could check: the light of the earliest galaxies is stretched into the infrared, beyond what Hubble could see well. JWST’s sharp infrared view made that test possible.

What the paper found

• The team found six candidate massive galaxies, each over 10 billion (10¹⁰) solar masses in stars, at redshifts 7.4 to 9.1, about 500 to 700 million years after the Big Bang.
• One candidate might have about 100 billion (10¹¹) solar masses, comparable to the Milky Way’s stellar mass today.
• If confirmed, the density of stellar mass locked in massive galaxies at that time would be far higher than models predicted.
• The authors were careful to call the objects candidates: their distances and masses came from colors, not spectra.

How they did it

The team searched early-release JWST imaging from the CEERS survey, taken at wavelengths from 1 to 5 micrometers, for galaxies that are intrinsically red, a sign of mature stars, dust or both. Redshift, the stretching of light by cosmic expansion, was estimated from how each object’s brightness changes across the filters, a technique called photometric redshift. Stellar masses came from fitting the colors with models of stellar populations. The paper first appeared as an arXiv preprint in July 2022 and was published in Nature in February 2023.

Why it mattered

The paper triggered a debate over whether JWST’s early galaxies “break” standard cosmology. The answer so far is mostly no. But the debate pushed astronomers to take spectra, and those spectra changed the story. It is one of the most discussed early results of the James Webb Space Telescope, whose science performance in commissioning made such deep searches possible.

What we know now

Follow-up spectroscopy showed that at least one candidate is not a giant galaxy at all but a dust-reddened active galactic nucleus, a feeding black hole, at redshift 5.62. It belongs to the population now called little red dots: abundant, puzzling red sources that may be growing black holes. Meanwhile, spectroscopically confirmed early galaxies such as JADES-GS-z14-0 at redshift 14.32 show that luminous galaxies did exist very early. See JWST discoveries and our galaxies guide for the wider picture.

Who wrote it

Ivo Labbé led the study with Pieter van Dokkum, Erica Nelson and colleagues, using some of the first public JWST data. The paper has about 410 citations (Semantic Scholar, October 2026).

Read the original

Labbé, I., van Dokkum, P., Nelson, E., et al. (2023). “A population of red candidate massive galaxies ~600 Myr after the Big Bang.” Nature 616, 266-269. doi:10.1038/s41586-023-05786-2

Free to read: the arXiv preprint.