The James Webb Space Telescope (JWST) was designed to explore the mysteries of the Universe, not least what the first galaxies looked like. These galaxies formed during the epoch of reionization (also known as “Cosmic Dawn”), which lasted about 100 to 500 million years after the Big Bang. By observing these galaxies and comparing them to galaxies closer to our present one, astronomers hope to test the laws of physics at the largest scale and find out what role (if any) dark matter and dark energy played.
Unfortunately, early in its campaign, JWST discovered galaxies from this period that were so massive that they were inconsistent with our understanding of how the Universe formed. The most widely accepted theory of how all this is related is known as the Lambda Cold Dark Matter (LCDM) cosmological model, which best describes the structure and evolution of the Universe. According to the latest results from the Cosmic Dawn Center, these galaxies may be even more massive than previously thought, further challenging our understanding of the cosmos.
The research team was led by the Cosmic Dawn Center (DAWN) and included researchers from the Niels Bohr Institute (NBI) at the University of Copenhagen, the European Southern Observatory (ESO) and the Kavli Institute for Particle Astrophysics and Cosmology (KIPAC). Stanford University, and astronomers and astrophysicists from the Université de Genève, the University of Texas at Austin, the University of Colorado, and UC Santa Cruz. The article describing their findings appeared in the May 10 issue of the Astronomical Journal.
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The galaxy cluster SMACS 0723 with the five galaxies chosen for closer examination. Image credits: NASA/ESA/CSA/STScI/Giménez-Arteaga et al. (2023), Peter Laursen (Cosmic Dawn Center).
Among the first images shared by JWST was the stunning view of the SMACS 0723 galaxy cluster known as Webb’s First Deep Field (see above). This image was acquired with Webb’s Near-Infrared Spectrograph (NIRSpec) and provided a detailed look at how the galaxies in this cluster appeared 4.6 billion years ago. In addition, the image was filled with gravitational lensing, allowing astronomers to get a closer look at more distant objects, including the most distant galaxy ever seen (GL-z13, now known as GLz-12) and several dated Green Pea “-Galaxies to the Early Universe.
The only problem is that Webb noticed more galaxies than expected during this period, and some were more massive than had been thought possible. According to the LCDM model, there simply hasn’t been enough time since the Big Bang for so many galaxies to form or become so massive. This led to all sorts of claims, including the notion that the Big Bang model was wrong, a highly questionable claim by proponents of the steady-state hypothesis. While these findings did not upset our entire cosmological understanding, they nonetheless called for an explanation.
To clarify Webb’s earlier observations, a Ph.D. A student at the Cosmic Dawn Center (Clara Giménez Arteaga) and colleagues further analyzed the data. In their paper, they describe how they observed five galaxies in the SMACS 0723 deep field with redshifts (z) from 5 to 9 that look to us as if they had it about 12.7 to 13.2 billion years ago. Based on their analysis, the team believe what we’re seeing at work here is an effect that could mean these galaxies are even larger than they appear. As Arteaga explained in an NBI press release:
“We used the standard method to calculate stellar masses from the images James Webb took, but on a pixel-by-pixel basis, rather than looking at the entire galaxy. In principle, one might expect the results to be the same: add up the light from all pixels and get the total stellar mass, as opposed to calculating the mass of each pixel and adding up all the individual stellar masses. But they are not.”
The stellar mass of the five galaxies, showing the mass derived using the two different methods. Photo credit: Giménez-Arteaga et al. (2023), Peter Laursen (Cosmic Dawn Center).
Typically, astronomers calculate the stellar mass of galaxies by measuring the amount of light emitted and deriving the population required for that amount. However, when Arteaga and her team took a closer look at the sample of five galaxies, they found that viewing the galaxies as a unit made up of multiple stellar groupings (instead of a large mass) would change the picture drastically. Based on this alternative method, they found that the inferred stellar masses of these five galaxies would be up to 10 times larger.
The team then compared the mass of the five galaxies using the two methods and found that the values were always much higher, analyzing them pixel by pixel (instead of the inferred brightness approach). Arteaga and her team theorize that this is related to the composition of the galaxies, which is far from unique:
“Star populations are a mixture of small and faint stars on the one hand and bright, massive stars on the other. If we only look at the combined light, the bright stars tend to completely outshine the faint stars, leaving them unnoticed. Our analysis shows that bright, star-forming clumps can dominate the overall light, but most of the mass is found in smaller stars.”
Proper resolution is very important for accurately estimating stellar mass, one of the main properties that astronomers use to characterize galaxies. While this is relatively easy for galaxies relatively close to the Milky Way, it remains a challenge for more distant galaxies. The effect highlighted by Arteaga and her colleagues has been observed before, but only in galaxies occurring in later epochs of cosmic history. Thanks to Webb’s superior resolution, this is the first time it has been applied to the most distant galaxies.
An international team of astronomers has used data from the James Webb Space Telescope to report the discovery of the earliest galaxies confirmed to date. Image credit: NASA/ESA/CSA/STScI
Unfortunately, even Webb is limited when it comes to observing galaxies that existed about 13 billion years ago, when the universe was barely 1 billion years old. The next step will therefore be to look for signatures that correlate with the true mass of these galaxies, which do not require high-resolution imaging. As Arteaga summarized:
“Other studies from much later eras have also noted this discrepancy. If we can determine how common and severe the effect is in earlier epochs and quantify it, we will be closer to inferring robust stellar masses of distant galaxies, which is one of the greatest current challenges in studying galaxies in the early Universe.”
Further reading: NBI, The Astronomical Journal