Update on JWST Observations of Galaxy Cluster SMACS 0723

The first image from JWST, a portion of which is shown here, offers a detailed view of the galaxy cluster SMACS 0723. Credit: NASA, ESA, CSA, and STScI

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.

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. Credit: Adapted from Mahler et al. 2023; Pascale et al. 2022

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.

 

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.

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. Credit: Arellano-Córdova et al. 2022

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.

 

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.

Location of the star clusters studied in this work within the larger SMACS 0723 field.
Credit: Faisst et al. 2022

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.

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.

Citation

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

 By Kerry Hensley

Source: American Astronomical Society - AAS Nova

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