Authors: Ryan L. Sanders, Alice E. Shapley, Michael W. Topping, Naveen A. Reddy, et al.
First Author’s Institution: Department of Physics and Astronomy, University of Kentucky, USA
Status: Preprint, Submitted to ApJ
Imagine a galaxy from 13 billion years ago, long before the Milky Way took its current form. Now picture trying to uncover the secrets of that ancient galaxy. How quickly did it create stars and evolve? Here’s the tricky part: its light has journeyed across an unimaginable distance to get here. On its journey, cosmic dust—tiny grains of carbon and silicates—has scattered and dimmed the light, making the task harder.
Dust doesn’t just block light—it changes it. Ultraviolet (UV) light gets scattered much more than light at longer wavelengths, which is why galaxies seem redder and dimmer than they truly are. This dual nature can be seen in the “Pillars of Creation“, in Figure 1. A new, near-infrared image from NASA’s James Webb Space Telescope, shown on the right panel below, helps us peer through more of the dust in this star-forming region. The thick, dusty brown pillars are no longer as opaque, and many more red stars that are still forming come into view.
Credits: NASA, ESA, CSA, STScI/ Joseph DePasquale (STScI), Anton M. Koekemoer (STScI), Alyssa Pagan (STScI)
To quantify how dust affects light at different wavelengths, astronomers use dust curves—tools that reveal how much light is blocked or scattered, helping them correct for dust-induced distortions. For instance, the Milky Way’s curve stands out with a distinct “bump” around 2175 Å, which comes from tiny carbon-based particles in the dust. But the Small Magellanic Cloud (SMC) tells a different story. Its curve is steeper in the UV and doesn’t have that bump at all. These differences aren’t just random—they give us clues about the size, makeup, and distribution of dust grains in each galaxy (See Figure 3 below for a visualization of these dust curves).
But here’s the catch: most of the dust curves we have were designed for galaxies much closer to us, like the Milky Way or the SMC, so they may not apply in the exact same way to distant galaxies. Whether those same curves work for the much younger, chaotic galaxies of the early universe is an open question. The dust in nearby galaxies is well-studied and relatively predictable, but do they behave the same way in galaxies billions of years in the past?
This question drove Ryan Sanders and his team to study GOODSN-17940, a starburst galaxy located at a redshift of 4.41, when the universe was just 1.36 billion years old. Observed as part of the AURORA Survey using JWST, this galaxy provided an unprecedented opportunity to investigate how dust behaved in the distant past. Using JWST’s NIRSpec instrument, the team constructed a unique dust curve by studying 11 hydrogen emission lines from Hα to H12 (the twelfth line in the rest-frame optical Balmer series), shown in Figure 2. These lines act like lighthouses, their brightness giving away how much light is being absorbed by dust at each wavelength. By comparing the observed brightnesses of these lines, the researchers created a detailed dust curve tailored to GOODSN-17940.
Image credit: Figure 1 in Sanders et al. (2024)
GOODSN-17940’s dust curve is shown in Figure 3. The y-axis represents the factor by which light is blocked/scattered by dust compared to this amount at 9550 Å. For example, a value of 2 means that light at that wavelength is attenuated two times more than light at 9550 Å.
This dust curve is surprising. In the near-infrared, it is much steeper than those of the Milky Way or SMC. This suggests its dust grains might be smaller or distributed differently. But in the UV, the curve flattens out, showing less absorption than classical models predict. And unlike the Milky Way’s dust, there’s no 2175 Å bump at all. These findings hint at a galaxy with truly unique dust properties, shaped by its extreme youth and chaotic star-forming environment. But here’s the twist—GOODSN-17940 isn’t just any galaxy. Its star formation rate is 40 times higher than typical galaxies at its redshift. While rare at redshifts ∼ 2–4, galaxies like this are more common in the epoch of reionization, suggesting that what we’ve learned from GOODSN-17940 could apply to reionization-era galaxies—the ones that played a key role in shaping the early universe.
Image credit: Figure 5 in Sanders et al. (2024)
So, why does this even matter? If you used the Milky Way’s dust curve to adjust for the dust in a galaxy like GOODSN-17940, you’d be way off. In fact, you’d underestimate its star formation rate (SFR) by up to 50%! This isn’t just a minor detail—it could explain why SFRs derived from Hα don’t always match those derived from UV luminosity. So such details can completely change how we interpret a galaxy. It’s the difference between a galaxy steadily forming stars and one caught in a dramatic starburst phase. That’s why astronomers focus on creating dust curves tailored to individual galaxies. This study isn’t just about one galaxy—it’s about rethinking how we interpret dust, star formation, and galaxy evolution in the early universe. Before JWST, isolating individual emission lines from distant galaxies was challenging, but it has become much more efficient now. As more galaxies like GOODSN-17940 are studied, we might find that many early galaxies had dust curves breaking the mold, forcing us to refine how we measure star formation and understand cosmic history.
Astrobite edited by Hilary Diane Andales, Delaney Dunne
Featured image credit: NASA, ESA, CSA, STScI, Webb Ero Production Team
