• physics 17, 148
The unusual arrangement of seven galaxies behind a galaxy cluster allows researchers to study the distribution of dark matter within a galaxy cluster with high precision.
W. Hsu/UCLA; Hubble Space Telescope data
Sometimes the stars just align. Researchers discovered a perfect cosmic alignment involving a giant galaxy cluster acting as a “foreground” lens and seven other galaxies in the background of the lens. [1]. Located 5 billion light-years from Earth, this lens warps space-time in a way that distorts and magnifies images of background galaxies. This system, called a carousel lens because of its hall-of-mirrors appearance, provides an extremely powerful tool for investigating the distribution of dark matter within galaxy clusters.
As light from a distant source passes through space, its trajectory is bent by regions of space containing galaxies or galaxy clusters that act as gravitational lenses. If the material density in such a lens is large enough, the so-called strong lens action can produce multiple images, arcs, or rings of objects in the background of the lens. Occasionally, an object’s alignment almost perfectly matches that of an observer on Earth, forming a powerful gravitational lens. This is the case with the carousel lens, which has a huge foreground galaxy cluster within the same line of sight as seven background galaxies. Finding such a configuration is like finding eight aligned needles in a haystack, said David Schlegel, an astrophysicist at Lawrence Berkeley National Laboratory in California.
In 2019, researchers used supercomputers to scour wide-area observations from the Dark Energy Survey and identified the cluster as a potential powerful lens system. [2]. Two years later, Schlegel, working with UCLA graduate student William Hsu and other colleagues, independently discovered this feature in a data release from the Dark Energy Spectroscopy Instrument (DESI) Legacy Imaging Survey. Neural network analysis of morphology and color provides strong hints that the object forms a powerful lens system. The final confirmation came from images taken by the Hubble Space Telescope (HST). “When I saw this HST image, I was blown away,” Schlegel said. Multiple images of each background galaxy formed a concentric pattern around the foreground galaxy cluster. This is the result of the image being curved, stretched, and reflected by the distorted space-time of the lens (Figure 1).
Schlegel and his team then used spectroscopic data collected by the MUSE instrument on the Very Large Telescope in Chile to determine the redshift (or distance) of the lensed galaxy. These distances, ranging from 7.6 billion to 12 billion light-years from Earth, approach the limit of the observable universe (13.8 billion years). The researchers used relatively simple lens effect models fitted to the images, such as the unusual Einstein cross (a configuration in which a single object produces four separate images arranged in a cross around the lens). We identified several observable characteristics.
This pattern of multiple images near the center of the lens means that the mass distribution within the lens (mainly composed of dark matter) is symmetric. Schlegel says this simplicity makes the system suitable for high-precision studies not possible with more complex lens systems observed so far. “This is one of the best systems I know of for measuring the distribution of dark matter in galaxies,” Schlegel said. The lensed image of each galaxy serves as a measurement of the mass of dark matter within the foreground galaxy cluster. By using seven lenses at different distances, researchers can combine multiple measurements to significantly improve the accuracy with which they can constrain the mass distribution.
“The number of lens sources in this system and the quality of their imaging impose strong constraints on the mass model compared to other clusters of similar mass,” said Kenneth Wong, assistant professor at the Center for Early Universe Research at the University of Tokyo. says. . He noted that the lensing model presented by the researchers suggests that the mass profile near the center of the cluster is steeper than what would be expected from established dark matter models. He points out that this is a very surprising discovery. But this conclusion needs to be confirmed through more detailed modeling, he says.
The carousel system could help address dark energy and other cosmological questions related to the expansion of the universe. In particular, this lens system could help resolve discrepancies in measurements of the Hubble tension, and thus the current expansion rate. H0. If a supernova or other transient event occurs in one of the background galaxies, the time delay of the supernova light appearing in multiple images could be used as a constraint. H0. Thanks to precise knowledge of the matter distribution within the carousel lens, this measurement is likely to be much more accurate than similar measurements obtained with other gravitational lenses.
“I never thought we would be so lucky to find so many galaxies lined up like this,” Schlegel said. “This indicates that there are probably many other discoveries [in DESI’s billion-galaxy catalog] It’s for smart people who know what to look for. “These exceptional arrays will be particularly useful for studying the distribution of dark matter and luminescent matter in high-redshift galaxies, which are difficult to explore given their distances, he says.
–Rachel Berkowitz
Rachel Berkowitz is physics magazine Based in Vancouver, Canada.
References
- W. Shu Others.“Carousel Lens: A Well-Modeled Powerful Lens with Multiple Light Sources Spectroscopically Confirmed by VLT/MUSE.” Astrophy. J. 9733 (2024).
- C. Jacobs Others.“An expanded catalog of intergalactic strong gravitational lenses discovered in DES using convolutional neural networks.” Astrophy. J., supplement. Sir. 24317 (2019).
