Update on JWST Observations of Galaxy Cluster SMACS 0723

In the months since the release of the first JWST image of the galaxy cluster SMACS J0723.3–7327 (SMACS 0723), astronomers have studied the image from every possible angle. Today’s post takes a look at five research articles that have advanced our understanding of galaxies within the cluster and beyond.

How to Weigh a Cluster

SMACS 0723 is a galaxy cluster located four billion light-years away. It gets its name from the Southern MAssive Cluster Survey (SMACS), in which the Hubble Space Telescope observed 124 galaxy clusters that are bright at X-ray wavelengths. In addition to the galaxies bound to the cluster, observations of SMACS 0723 contain images of galaxies far more distant. This is because SMACS 0723 is so massive that it warps spacetime, causing photons from galaxies billions of light-years away to curve around the foreground galaxies and into our field of view. As a result, we see stretched-out, wiggly, or multiple images of distant galaxies.

two mass density distribution maps for galaxy cluster SMACS 0723

Top: Mass distribution derived by Mahler and collaborators. The magenta lines are mass contours and the white lines are X-ray surface brightness contours. Bottom: Mass distribution derived by Pascale and collaborators. The black lines show the surface mass density distribution. Note that there is a difference in scale and orientation between the two maps. [Adapted from Mahler et al. 2023; Pascale et al. 2022]

Researchers can use these wiggly galaxies to reconstruct a map of SMACS 0723’s mass distribution. Understanding how a cluster’s mass is distributed is important because it can tell us about the underlying distribution of dark matter within the cluster and how the cluster has evolved over time. As is often the case when exciting new data are released, several teams worked in parallel to map SMACS 0723’s mass distribution using different techniques. Two articles published in AAS journals detail efforts by teams led by Massimo Pascale (University of California, Berkeley) and Guillaume Mahler (Durham University) to use JWST’s superior capabilities to improve upon the existing mass map based on Hubble data.

In previous observations of the cluster, astronomers picked out five lensed background galaxies, but JWST’s impressive capabilities allowed the teams to pick out many more; at 6.5 meters in diameter, JWST is already a massive space telescope, but as Pascale and coauthors noted, pointing it toward a galaxy cluster that bends the light from more distant sources effectively creates a telescope with a diameter of 20–30 meters!

Pascale’s and Mahler’s teams used different computer algorithms to analyze the lensed galaxies newly identified in the JWST images, and both teams achieved a large increase in the precision of their mass maps compared to those based on Hubble data. Both teams’ maps show evidence of a past disturbance in the galaxy cluster, such as gravitational interactions or mergers between galaxies.

A Trio of Distant Galaxies

Karla Arellano-Córdova (The University of Texas at Austin) and collaborators took advantage of SMACS 0723’s lensing ability to study three galaxies with redshift, z, between 7.6 and 8.5, corresponding to when the universe was about 600–700 million years old. Using JWST’s Near-Infrared Spectrograph (NIRSpec), the team identified emission lines in the spectra of these distant galaxies. By measuring the strength of several prominent emission lines and the ratios of their strengths relative to one another, Arellano-Córdova’s team determined the properties of the gas in each galaxy, including the abundance of elements like carbon, oxygen, neon, and potentially iron. Notably, the team measured the neon-to-oxygen abundance ratio for a galaxy at z > 7 for the first time, and they obtained the most distant carbon-to-oxygen ratio measurement ever.

metallicity versus stellar mass for star-forming galaxies

Metallicity versus stellar mass for galaxies in the local universe (light blue circles) and at high redshift. Values from this work are shown with stars, while other researchers’ results are shown with triangles, diamonds, pentagons, and squares. [Arellano-Córdova et al. 2022]

By comparing the chemical abundances for the three high-redshift galaxies to those of nearby galaxies, the team gained a sense of how each element’s abundance has changed over time, which can inform us about how galactic stellar populations evolved early in the universe. While some of the abundances followed expected trends, others showed behavior that will require further inspection; a tentative detection of emission lines from iron atoms suggests that these galaxies might be rich in iron compared to oxygen, which the authors propose could be due to gaseous outflows. The authors emphasized that this is just the beginning of what JWST will accomplish in terms of investigating the chemical makeup of galaxies in the first billion years of the universe — we’ve cracked open a window into that time period, and future studies should open it wider still.

Investigating Point Sources in SMACS 0723

The final two articles focus on goings on within the cluster itself, using JWST’s exceptional precision to investigate individual star clusters four billion light-years away.

several panels illustrating the locations of the star clusters within the SMACS 0723 field

Location of the star clusters studied in this work within the larger SMACS 0723 field. Click to enlarge. [Faisst et al. 2022]

A team led by Andreas L. Faisst (California Institute of Technology — Infrared Processing and Analysis Center) searched the SMACS 0723 images for signs of star clusters. The team manually selected 178 promising sources from the images, finding that the human eye was better at picking out the faint clusters against the background galactic light than a search algorithm. Because the individual star clusters were so faint, the team combined the measurements for all 178 clusters to determine the average properties of the sample.

Overall, the clusters appear to be about 1.5 billion years old, although the team could not rule out the possibility that the clusters are up to 9 billion years old (roughly the age of the universe at SMACS 0723’s redshift). Given the clusters’ sizes (no more than 160 light-years across), masses (about 2.4 million solar masses, on average), and metallicity (about 20–30% the metal abundance of the Sun), the team concluded that the star clusters are most likely to be globular clusters: roughly spherical collections of hundreds of thousands of stars found on the outskirts of nearly all galaxies. However, the authors acknowledged another possibility: as dwarf galaxies surf the gravitational swells of the massive galaxy cluster where they were born, they may lose their outermost stars, resulting in dense star clusters similar to those seen in SMACS 0723.

Galaxy Cluster Globular Clusters

Myung Gyoon Lee (Seoul National University) and collaborators also went globular cluster–hunting in the SMACS 0723 images, using an algorithm to identify and characterize point sources throughout the galaxy cluster. By grouping point sources based on their color and brightness, the team identified those that are most likely to be globular clusters associated with the SMACS 0723 galaxy cluster.

illustration of the locations of the sources studied in this work

Illustration of the location of some of the sources studied in this work. Click to enlarge. [Lee et al. 2022]

Many of the sources appear to be linked to the brightest galaxy in the cluster, which lies near the cluster center. These star clusters are most concentrated near the bright central galaxy and become more scattered farther out, and similar concentrations are seen around other bright galaxies within the cluster.

In addition to tracing the distribution of galaxies in SMACS 0723, the candidate globular clusters also follow some of the intracluster structures, such as the distribution of diffuse intracluster light. This faint emission comes from stars that have been ejected from their home galaxy due to gravitational interactions between galaxies in the cluster. The team also compared the star cluster distribution to the dark matter distribution determined in other research articles, finding considerable similarities between the two distributions. These findings suggest that star clusters can be used to trace dark matter within galaxy clusters.


“Unscrambling the Lensed Galaxies in JWST Images behind SMACS 0723,” Massimo Pascale et al 2022 ApJL 938 L6. doi:10.3847/2041-8213/ac9316

“Precision Modeling of JWST’s First Cluster Lens SMACS J0723.3–7327,” Guillaume Mahler et al 2023 ApJ 945 49. doi:10.3847/1538-4357/acaea9

“A First Look at the Abundance Pattern—O/H, C/O, and Ne/O—in z > 7 Galaxies with JWST/NIRSpec,” Karla Z. Arellano-Córdova et al 2022 ApJL 940 L23. doi:10.3847/2041-8213/ac9ab2

“What Are Those Tiny Things? A First Study of Compact Star Clusters in the SMACS0723 Field with JWST,” Andreas L. Faisst et al 2022 ApJL 941 L11. doi:10.3847/2041-8213/aca1bf

“Detection of Intracluster Globular Clusters in the First JWST Images of the Gravitational Lens Cluster SMACS J0723.3–7327 at z = 0.39,” Myung Gyoon Lee et al 2022 ApJL 940 L19. doi:10.3847/2041-8213/ac990b