Are Dusty Galaxies Getting in the Way of High-Redshift Studies?

JWST’s observations of galaxies in the distant universe have already shaken up our understanding of how the first galaxies evolved. But could our observations of faraway galaxies be misled by closer interlopers?

example of a high-redshift galaxy discovered with JWST

One of the many galaxies with redshift of z > 10, corresponding to the first 500 million years after the Big Bang, discovered with JWST observations. [Adapted from: Science: NASA, ESA, CSA, Tommaso Treu (UCLA); Image Processing: Zolt G. Levay (STScI)]

Distance Makes the Galaxy Grow Redder… but So Does Dust

During its first year, JWST observed many candidate high-redshift galaxies, corresponding to when the universe was just a few hundred million years old. As these candidates piled up, their numbers and masses started to stretch the bounds of what is likely under leading theories of galaxy formation and evolution. The tension between theory and observations has led some researchers to suggest that overhauling our theories is in order.

Before we give existing theories the boot, there’s another possibility to consider: some galaxies with reported redshifts of > 10 may actually be dusty star-forming galaxies at z < 7, skewing our statistics. Why might we confuse these two very different galaxy populations, and what can we do about it?

Lyman Breaks vs. Dusty Dropouts

Here’s how the mix-up can occur: researchers pick out extremely distant galaxies by searching for the Lyman break — a sharp drop-off in galactic emission at short wavelengths due to clouds of neutral hydrogen that absorb starlight beyond a certain wavelength. In practice, astronomers search for galaxies that are present in redder filters and “drop out” of bluer images.

Dusty star-forming galaxies may appear similar in our observations. When strong ultraviolet emission powered by ultra-hot young stars is soaked up by dust and re-emitted at longer wavelengths, the resulting color of the galaxy can mimic that of a more distant galaxy, including the drop-out behavior. As the authors point out, the confusion between nearby dusty galaxies and more distant galaxies isn’t unique to JWST; researchers analyzing Hubble Space Telescope data wrestled with the same issue, though the redshift ranges were different — in Hubble images, z ~ 6–8 galaxies vied with z ~ 2–3 galaxies for our attention.

images of CEERS-DSFG-1 in six JWST filters

Images of the galaxy CEERS-DSFG-1, which exhibits drop-out behavior in JWST images. The galaxy is clearly visible in the longer-wavelength filters and drops out at shorter wavelengths. [Zavala et al. 2023]

Inspecting High-Redshift Candidates

In a recent publication, Jorge Zavala (National Astronomical Observatory of Japan) and collaborators tackled this challenge by searching for thermal emission from dust in the galaxy CEERS-DSFG-1, which JWST observed as part of the Cosmic Evolution Early Release Science (CEERS) survey. CEERS-DSFG-1 shows drop-out behavior similar to other high-redshift galaxy candidates, but detecting dust emission could indicate that the galaxy is located at a lower redshift.

probability density function for the redshift of a candidate high-redshift galaxy.

Probability of CEERS-DSFG-1 having certain redshifts as derived from near-infrared (NIR) JWST data, longer-wavelength data (labeled FIR), and a combination of the two. The combined fit (green shaded area) places the strongest constraints on the redshift. This plot also shows the importance of using properly calibrated JWST data. Click to enlarge. [Zavala et al. 2023]

The team detected dust emission from the galaxy at a wavelength of 1.1 millimeters, and they tracked the galaxy at shorter wavelengths down to 2.0 microns (1 micron = 10-6 meter), at which point the emission abruptly dropped off. By using a range of models to fit the galaxy’s spectral energy distribution — the energy emitted by a source as a function of wavelength — the team showed that longer-wavelength data provide an important constraint on the galaxy’s redshift. Considering near-infrared JWST data and 1.1-millimeter data simultaneously places the galaxy at a redshift of z = 5.1 (>1 billion years after the Big Bang), while estimates based solely on the near-infrared JWST data leave open the possibility that the galaxy is located at z ~ 12–14 (300–400 million years after the Big Bang).

This study makes it clear that high-redshift galaxies detected by JWST need further investigation before they can be confirmed. Hopefully, follow-up long-wavelength observations of high-redshift candidates will confirm their redshift one way or another, allowing us to hone our models of the early universe further.


“Dusty Starbursts Masquerading as Ultra-high Redshift Galaxies in JWST CEERS Observations,” Jorge A. Zavala et al 2023 ApJL 943 L9. doi:10.3847/2041-8213/acacfe