Discovery of the Largest-class Monster Supercluster of Galaxies in the Universe 5.5 Billion Light-Years Away

Figure 1: This color composite image of a supercluster was captured by the Subaru Telescope's prime focus camera. The white contours in the center show the density distribution of galaxies, and the red regions indicate intense concentrations of dark matter. The numbered squares depict the locations of galaxy clusters associated with the supercluster. The surrounding panels are magnified views of these 19 clusters, showing the clusters of red galaxies that are common in galaxy clusters. The moon in the upper left depicts the apparent size of the full moon to provide a sense of scale. (Credit: NAOJ)

A team led by the National Astronomical Observatory of Japan (NAOJ) and Hiroshima University has discovered a massive supercluster of galaxies about 5.5 billion light-years away, based on the Big Data from the Subaru Telescope. Not only is there a strong concentration of galaxies and dark matter across an area of the sky roughly the size of 15 full moons, but there are at least 19 galaxy clusters associated with it, making it the largest supercluster ever identified in the Universe beyond 5 billion light years away.

Galaxies are comprised of gas and countless stars; and galaxy clusters, which are amalgamations of such galaxies, are known as the largest gravitationally-bound structures in the Universe. However, there is a still larger structure in the Universe called a supercluster, which develops after galaxy clusters further assemble. While superclusters extend over an area of about 100 megaparsecs (about 500 times the size of the Milky Way), the definition of a supercluster itself is still ambiguous; its true nature and what is going on inside it are still shrouded in mystery. In fact, the Milky Way is also inside the Laniakea supercluster, consisting of multiple galaxy clusters and superclusters (Note 1).

Hyper Suprime-Cam (HSC) on the Subaru Telescope has made a deep, wide-field survey, equivalent to 4,400 times the apparent size of the full moon, reaching over 10 billion light-years. The high-quality imaging data obtained from this program is currently the best resource for searching for unknown galaxy superclusters.

The research team examined the total stellar mass and dark matter distribution in the largest density excesses among the nearly 100 supercluster candidates (Note 2), which were discovered by the same team in the past (Note 3). As a result, the team detected a supercluster structure consisting of at least 19 clusters of galaxies centered on three dark matter-dense regions (Figure 1).

Comparison with cosmological simulations suggests that this supercluster has a dark matter mass about 10 times the mass of the Virgo supercluster in the local Universe. On top of that, two giant structures equivalent to superclusters have been identified immediately outside of the cluster, which means that the discovered supercluster may be a precursor to supermassive structures such as the Laniakea supercluster, the largest in the nearby Universe.

The lead author, Dr. Rhythm Shimakawa, Project Assistant Professor at NAOJ, says, "Indeed, the probability of finding such a supercluster about 5.5 billion light-years away, was 50-50 based on the data from the Subaru Telescope's strategic program. We plan to further investigate the three-dimensional structure and the morphology of the galaxies by using such instruments as Subaru Telescope’s PFS (wide field spectrograph) and the Euclid space telescope in the near future."

These results appeared as Shimakawa et al. "King Ghidorah Supercluster: Mapping the light and dark matter in a new supercluster at z = 0.55 using the Subaru Hyper Suprime-Cam" in Monthly Notices of the Royal Astronomical Society Letters on November 26, 2022.

Notes:

(Note 1) It is known that our Milky Way is located inside the Virgo supercluster, the core of which is composed of the Virgo cluster. The definition of a supercluster itself is still ambiguous, and thus in some cases, the term "supercluster" is also used to refer to a giant structure that envelops smaller superclusters.

(Note2) The distribution of dark matter was obtained using the weak gravitational lensing effect. The gravitational lensing effect is a phenomenon in which light emitted from distant galaxies appears distorted or brightened due to the bending of the light path when it passes through a strong gravitational field such as a galaxy cluster in the foreground. Weak gravitational lensing refers to relatively weak cases of this phenomenon. The supercluster in this study is the largest structure over 5 billion light-years away ever identified by weak gravitational lensing analysis.

(Note 3) "Subaru Hyper Suprime-Cam excavates colossal over- and underdense structures over 360 deg2 out to z = 1", Shimakawa et al, 2021, MNRAS

Source: Subaru Telescope

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