Webb observes galaxies interacting to form huge cluster about 13 billion light-years from Earth

A protocluster is defined as galaxies that are gradually approaching each other to form an enormous cluster.
Mrigakshi Dixit
The galaxy protocluster.

Astronomers have been able to peer back into the early years of the universe since the arrival of the James Webb Space Telescope. 

Now, this powerful space observatory has been able to observe the early stages of seven galaxies interacting to form a protocluster. A protocluster is defined, as where galaxies are gradually approaching each other to form an enormous cluster.

​​This is said to be one of the farthest-located clusters of galaxies that astronomers have observed and confirmed to date. 

Teamwork of Hubble and Webb

Initially, these seven galaxies were spotted using NASA's Hubble Space Telescope and appeared to be aligned in the same direction. Hubble, on the other hand, was unable to determine the distance between each galaxy. 

The team then used the Webb telescope to confirm that these galaxies are indeed located at the same distance. Webb determined the distance between protocluster galaxies that existed during the initial years of the universe, just 650 million years after the Big Bang.

According to the press statement, these galaxies would have already merged by now, resulting in the formation of one of the largest and oldest known galaxy clusters in the universe. The cluster is likely to be more than 100 times more massive as compared to the Milky Way galaxy.

Scientists highlight that these early stage development of galaxy clusters are rare to spot. Additionally, these seven galaxies may have been among the first formed galaxies in the universe's early years. 

“This is a very special, unique site of accelerated galaxy evolution, and Webb gave us the unprecedented ability to measure the velocities of these seven galaxies and confidently confirm that they are bound together in a protocluster,” said Takahiro Morishita of IPAC-California Institute of Technology, the lead author of the study, in an official statement

Measuring the redshift

The Webb used the telescope's Near-Infrared Spectrograph (NIRSpec) instrument to calculate the precise distance. Using this instrument, astronomers were able to measure the redshift, which determines the distance of galaxies from the Earth. Simply put, the higher the redshift, the farther the galaxy is from the Earth. The wavelengths emitted by the galaxy are used to calculate the redshift.

The findings revealed that each of these seven galaxies has the same redshift (7.88). The galaxy cluster is located in the constellation Sculptor, nearly 13 billion light-years from Earth.  

"We knew for a while from the Hubble data that there was an interesting over-density of galaxies. It came as a surprise when we first saw the spectra from JWST—all seven galaxies were aligned at the exact same redshift," said Takahiro Morishita, lead author of this study, in a statement.

Such discoveries will be accelerated by the upcoming Nancy Grace Roman Space Telescope. This, in turn, could shed light on the evolution of the universe, the formation of galaxies, and even some of the most perplexing mysteries surrounding black holes.

The results have been published in the Astrophysical Journal Letters, and the study is led by Caltech's IPAC astronomy center.

Study abstract:

We present the spectroscopic confirmation of a protocluster at z = 7.88 behind the galaxy cluster Abell 2744 (hereafter A2744-z7p9OD). Using JWST NIRSpec, we find seven galaxies within a projected radius of 60 kpc. Although the galaxies reside in an overdensity around ≳20× greater than a random volume, they do not show strong Lyα emission. We place 2σ upper limits on the rest-frame equivalent width <16–28 Å. Based on the tight upper limits to the Lyα emission, we constrain the volume-averaged neutral fraction of hydrogen in the intergalactic medium to be xHI > 0.45 (68% C i). Using an empirical MUV–Mhalo relation for individual galaxies, we estimate that the total halo mass of the system is ≳4 × 1011M⊙. Likewise, the line-of-sight velocity dispersion is estimated to be 1100 ± 200 km s−1. Using an empirical relation, we estimate the present-day halo mass of A2744-z7p9OD to be ∼2 × 1015M⊙, comparable to the Coma cluster. A2744-z7p9OD is the highest redshift spectroscopically confirmed protocluster to date, demonstrating the power of JWST to investigate the connection between dark-matter halo assembly and galaxy formation at very early times with medium-deep observations at <20 hr total exposure time. Follow-up spectroscopy of the remaining photometric candidates of the overdensity will further refine the features of this system and help characterize the role of such overdensities in cosmic reionization.

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