Using Galaxy Clusters to Search for the Most Distant Objects in the Observable Universe

Dr. Felipe Andrade-Santos

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On November 25, 1915, Albert Einstein published his famous theory of General Relativity, which is a geometrical representation of gravity. In this work, Einstein described how spacetime curves in the presence of energy/matter. According to the theory of General Relativity, gravity is the manifestation of this curvature. Mathematically, the relationship between the spacetime curvature and the energy/matter content in space is given by Einstein’s field equation:
where the left side of this equation is the Einstein Tensor, which describes the geometry of spacetime. The right side of this equation is the Stress-Energy-Momentum Tensor, which describes the distribution of energy/matter in spacetime.

Physicists and Astronomers often say that matter tells spacetime how to curve, and curved spacetime tells matter how to move.

Figure 1 illustrates how the distribution of energy/matter will curve spacetime. The more massive the object, the more it will curve spacetime.

Figure 1: How different masses curve the fabric of spacetime. Image credit: ESA-C.Carreau

Interestingly, light also moves according to the curvature of spacetime. Because of that, large concentrations of mass distort the images of objects in the background. This effect is known as gravitational lensing. Figure 2 illustrates light rays moving through a curved spacetime.

Figure 2: Trajectory of light rays moving through curved spacetime. Image credit: NASA, ESA & L. Calçada.

Large concentrations of mass will produce gravitational lensing on the images of background objects, working as cosmic lenses. These cosmic lenses can magnify the sizes and brightnesses of background objects, serving as incredible tools to search for distant, faint objects.

Galaxy clusters are the most massive objects in the universe, with masses reaching a million times a billion times the mass of our Sun. They can produce incredible distortions and magnifications on the images of objects in the background, as shown in two extreme examples in Figure 3.

Figure 3: Left: Extreme gravitational lensing by a galaxy (The Horseshoe Einstein Ring from Hubble. Image credit: ESA/Hubble & NASA). Right: Extreme gravitational lensing by a cluster of galaxies (MACSJ0138.0-2155. Image credit: ESA/Hubble & NASA, A. Newman, M. Akhshik, K. Whitaker).

Gravitational lensing magnification by massive galaxy clusters has a long history of helping astronomers discover the most distant galaxies known. Twenty years ago the most distant known galaxy was a galaxy whose detected light was emitted 12.5 billion years ago. Its light was detected thanks to gravitational lensing by the galaxy cluster MS 1358+62. Currently the record belongs to the galaxy GN-z11, whose detected light was emitted 13.3 billion years ago (the universe is now 13.8 billion years old).

Astronomers also design specific programs to maximize the probability of finding those distant galaxies. By using powerful telescopes, astronomers can produce surveys of images of massive galaxy clusters to search for the most distant objects in the observable universe. The Reionization Lensing Clusters Survey1 (RELICS) is a joint Hubble Space Telescope (HST) and Spitzer program to discover the best and brightest very-distant galaxies. RELICS used more than 100 and 800 hours of HST and Spitzer observing time, respectively, to search for distant galaxies lensed by 41 massive galaxy clusters, which were carefully selected. The program was designed by many astronomers from leading astrophysics institutions, including Dan Coe from the Space Telescope Science Institute (leading scientist/coordinator of the Hubble observations) and Maruša Bradač from the University of California Davis (leading scientist/coordinator of the Spitzer observations). These massive galaxy clusters have the highest potential to magnify the light emitted by very distant galaxies. RELICS was primarily designed and optimized to search for brightly lensed very-distant galaxies in the epoch of reionization, when galaxies started to form and ionize the neutral hydrogen in the intergalactic medium (neutral hydrogen gas that resided among galaxies).

Typical magnifications for lensed galaxies are factors of a few but can also be as high as tens or hundreds, stretching galaxies into giant arcs. Even though the RELICS program was designed to search for the most distant galaxies, anything on the background of a galaxy cluster will be magnified. Individual stars can attain even higher magnifications given fortuitous alignment between the background star, the lensing cluster, and us.

Using the images of clusters of galaxies, astronomers can model these objects in terms of their mass distributions to predict how light traveling through them will be distorted to produce magnified images of background objects. Like a guitar pedalboard that processes the clean signal of an electric guitar to produce distorted sounds, a galaxy cluster will deflect the light rays traveling through it to produce distorted images. If musicians fully understand how a pedalboard works, they can do the exercise of reversing the process to infer what the clean signal input was that produced the distorted sound output. Similarly, astronomers can determine how large was the region in the background of a galaxy cluster that produced the distorted lensed image. Recently, using this approach, several individual stars whose detected light were emitted between 7 and 9 billion years ago have been discovered, magnified by factors of thousands, temporarily boosted by perfect cosmic alignments.

Their images were detected and the sizes of the regions in the background that produced those images were computed. The sizes of those regions in the background would fit at most a few stars.

In a more recent work published in Nature2, Brian Welch (from the Space Telescope Science Institute at the time of the publication, now at University of Maryland/NASA Goddard Space Flight Center), Dan Coe, and collaborators (among them Berklee’s professor Felipe Andrade-Santos), reported Hubble observations of an even more distant star whose light was emitted 12.9 billion years ago, just 900 million years after the Big Bang, tracing back to a moment in the history of our universe when the chemical composition was very different from what it is today, given that over billions of years stars have enriched the interstellar and intergalactic environments with newly forged chemical elements. This star (bright object pointed by the arrow in Figure 4), WHL0137-LS, nicknamed Earendel (old English for morning star), is magnified by a factor of thousands by the foreground galaxy cluster WHL0137–08.

Figure 4: Earendel (pointed by the arrow) as observed by the Hubble Space Telescope. Photo courtesy of NASA, ESA, Brian Welch, Dan Coe; Image processing by NASA, ESA, Alyssa Pagan (STScI).

Earendel was first identified on images taken with the Hubble Space Telescope (Figure 4). The RELICS team (led by Dan Coe) then obtained James Webb Space Telescope (JWST) images of Earendel. In these higher resolution images (Figure 5), Earendel remained a single point source (the higher resolution of JWST could have disentangled the original point source detected by HST (Figure 4) into an image containing multiple bright points) restricting the radius of the background region further to less than 0.07 light-years or about 4000 times the distance from Earth to the Sun. These new observations strengthened the conclusion that Earendel is best explained by an individual star or a multiple star system, and support the previous distance estimate (corresponding to the detected light being emitted 12.9 billion years ago).

Figure 5: Earendel as observed by the James Webb Space Telescope. Image credit: NASA, ESA, CSA, STScI, Cosmic Spring JWST; Welch et al., 2022 ApJ, 940,1.

Further investigation was required to determine whether Earendel was a single star or a stellar system. Any object emits electromagnetic radiation (light), due to the motion of its molecules and atoms (thermal radiation – not to be confused with reflected light). Figure 6 illustrates how objects with different temperatures emit different amounts of light (energy) at different frequencies (what astronomers call spectrum). The higher the temperature, the higher the frequency (and smaller the wavelength) where the peak of the emission happens. Also, the higher the temperature, the higher the emission at any frequency, resulting in more total energy being emitted. Astronomers use these properties to determine the temperatures of stars by measuring the amount of light that they emit at different frequencies. If more than one star is emitting the observed light, one can fit a multi-temperature emission model to the observed spectrum.

Figure 6: Black body radiation curves. Image credit: Light-Emitting Diodes (Cambridge University Press) by E. F. Schubert.

Using the JWST data from Earendel, Brian Welch, Dan Coe, and collaborators were able to estimate the number of stars in this stellar system3. Figure 7 shows two models that were fit to Earendel’s JWST data. On the left, a single temperature model is fit to the spectrum of Earendel. On the right, a double model is fit, with much better match, indicating that Earendel is more likely a binary stellar system, with the stars having surface temperatures of 10,000 K (17,500 degrees Fahrenheit) and 30,000 K (53,500 degrees Fahrenheit). For reference, our Sun has a surface temperature of about 5,800 K (10,000 degrees Fahrenheit).

Figure 7: Earendel’s spectrum as observed by the JWST. Left: a single model is used. Right: a double model is used. Image credit: Welch et al., 2022 ApJ, 940,1.

The discovery of Earendel, a stellar system that was formed in the first billion years of the history of the universe, opens up a new window into the exploration of the chemical composition and evolution of the early universe. This discovery marks the beginning of a new and exciting chapter that will be written in science, as astronomers discover more individual stars/stellar systems at extremely large distances.
Felipe Andrade-Santos is Associate Professor of Physics and Astronomy in Berklee’s Department of Liberal Arts and Sciences. He is also an Astrophysicist at the Center for Astrophysics | Harvard & Smithsonian, where he conducts research in the field of galaxy clusters. He has authored or co-authored more than 75 refereed scientific articles in Astrophysics, including in Nature and Nature Astronomy. He is passionate about Science, Music, and Bodybuilding.