Saraswati Supercluster: One of the largest known supercluster

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Saraswati Supercluster: One of the largest known supercluster 

What is a Supercluster?

A group of galaxies is usually known as a galaxy cluster. When these clusters group together, they form a supercluster. The superclusters are one of the largest structures in the universe. A supercluster contains a large number of galaxies, usually in thousands and millions. Our home galaxy, the Milky Way is a part of a small cluster called the Local Group, which contains more than 54 galaxies. The Local Group is part of the Virgo Supercluster, which is a part of the Laniakea Supercluster. the Laniakea is about 500 million light-years wide. Astronomers estimated that there are more than 10 million superclusters in the observable universe. The empty space between two superclusters is termed void.

Introducing the Saraswati:

The Saraswati Supercluster was discovered by a group of Indian researchers from Pune and Jamshedpur. The group includes Joydeep Bagchi, Pratik Dabhade, Shishir Sankhyayan, Joe Jacob and Somak Raychaudhury. The Saraswati is the largest supercluster discovered till today. This massive galaxy supercluster is located about 4 billion light-years away in the constellation Pisces. It is also one of the most massive structures in the universe with an average mass of 2×10^16 solar masses and a diameter of 652 million light-years. 

The supercluster consists of more than 43 galaxy clusters. This supercluster reveals the role of dark energy during the early, young stages of supercluster formation. The mass of Saraswati is comparable to the Shapley supercluster. Shapley is the largest concentration of galaxies in the universe. Saraswati is the Indian goddess of wisdom, knowledge and music. In Sanskrit, the word Saraswati (or Sarasvati) means 'ever-flowing stream with many pools'. Hence 'the Saraswati Supercluster' resembles the large scale structure of clusters and galaxies moving and merging together.

Searching a supercluster:

Superclusters are large scale concentrations of smaller galaxy clusters. A large data of spectroscopic redshift covering a vast area of the sky, targeting the galaxies are needed to identify a supercluster. There are generally two methods to search for a supercluster.

(1) Identifying overdense structures using the Friends of Friends (FoF) algorithm 
(2) Using smoothed density field approach

Here, the astronomers used the FoF algorithm. FoF is a tool used to identify groups of particles. The algorithm uses the concept of linking length. If any particle is found within a certain distance of the selected particle, then they are linked. The particle is directly linked to all other particles within a certain distance (friends) and indirectly to all other particles linked with its friends (friends of friends). This way the algorithm identifies a group of particles. Once the group is detected, its total mass, density or centre of mass can be calculated.

The same concept is applied to find the Saraswati supercluster. They decided on a certain linking length (l) for the selected galaxy distribution. If the distance between two galaxies is less than or equal to l, then they are linked to the same cluster, otherwise not. The FoF algorithm was mostly applied to the clusters rather than galaxies.

Here the team considered the value of l to be 19 Mpc (or 6×10^7 light-years). This is because, between 19 Mpc, a maximum of 46 clusters with at least 4 galaxies in each cluster can be obtained. Hence astronomers considered 19 Mpc as a standard linking length. Thus by linking galaxies and clusters, the team finally identified the largest ever supercluster, the Saraswati.

Properties of Saraswati Supercluster:

Astronomers identified Saraswati as a region with a high density of galaxies. The centre has a very massive cluster called the Abell cluster. The Abell is an extremely massive and hot galaxy cluster. Astronomers detected very high emission of X-rays from the cluster. The second most massive cluster is ZwCl 2341.1+0000. Further studies of ZwCl showed that the galaxies in ZwCl were merged to form a cluster during the early stages of the formation of the Saraswati Supercluster. The main body of Saraswati contains more than 43 clusters. 23out of 43 clusters are extremely massive, having a mass of more than 10^14 solar masses. This implies an unusual concentration of high mass clusters inside a supercluster. 

Saraswati Supercluster: One of the largest known supercluster
3D model of Saraswati Supercluster
Credit: The Astrophysical Journal

Any supercluster of the scale of Saraswati cannot be in equilibrium. The next thing the astronomers wanted to find was whether the Saraswati supercluster is gravitationally bounded or not. 3-D modelling suggests that Saraswati is more planar than spherical. This can be done by using the spherical collapse model. To know more about the spherical collapse model I've shared the link below:

Using the ratio of turnaround radius and total radius, astronomers proved that the Saraswati is not gravitationally bounded. However, there are many groups that are gravitationally bounded. Four massive clusters, inducing the Abell, are considered to be bounded. These four bounded clusters are responsible for 20% of the total mass of Saraswati. 

Saraswati Supercluster: One of the largest known supercluster
Optical image of Abell and ZwCl - Most massive cluster in Saraswati
Credit: The Astrophysical Journal

Very few superclusters are known to present date having the mass equivalent to the Saraswati. Furthur to verify Einstein's general relativity for the formation of massive structures, the study of superclusters like Saraswati is extremely necessary. Numerical simulations or mathematical calculations are also necessary for the precise modelling of such large structures. Dark energy also plays a major role here. The gravitational potential of superclusters depends mostly on dark energy. Also, dark energy plays its part in the evolution and expansion of the universe.

Completely understanding the formation and structure of such massive superclusters is still a challenge for astronomers. Astronomers never expected to search a supercluster as large as the Saraswati. This discovery has made the scientific community think differently for large structures. 


   


 

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