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Discovery of Saraswati Supercluster and Giant Radio Galaxies  Aug 03, 2017

Shishir Sankhyayan Shishir Sankhyayan, a PhD student in the Physics Programme at IISER Pune has been involved in two major astronomical discoveries that have received world-wide attention-- the supercluster Saraswati, and 25 rare Giant Radio Galaxies. In this feature article, Harshini Tekur gives us a detailed breakdown of what these discoveries mean and how they add on to our understanding of the universe.
In the universe of Douglas Adams's science fiction masterpiece, The Hitchhiker's Guide to the Galaxy, there exists a device called the Total Perspective Vortex, which he describes as follows:
When you are put into the Vortex you are given just one momentary glimpse of the entire unimaginable infinity of creation, and somewhere in it there's a tiny little speck, a microscopic dot on a microscopic dot, which says, "You are here.”
The sense of perspective this device provides can apparently turn a person's brain to mush. In the author's own words, the Total Perspective Vortex illustrated that "In an infinite universe, the one thing sentient life cannot afford to have is a sense of proportion."
Life, the Universe and Everything
Yet humankind has persistently looked towards the skies, and sought to understand its place in the universe. We have come a long way since Galileo first pointed a telescope at the sky, and now, armed with larger telescopes that are capable of detecting different kinds of radiation, we may perhaps comprehend the enormity of the universe we live in. The observable part of it, that is.
On a cosmic scale, we live on a little planet orbiting an average star, somewhere on the fringes of a mildly interesting galaxy, the Milky Way. The galaxy itself is one among several others that makes up a bigger structure called the Local Group galaxy cluster. To scale it up further, the Milky Way galaxy, along with about 100,000 other galaxies, is part of the Laniakea Supercluster, which spans a distance of about 500 million light years.
These superclusters are among the largest structures in the universe, along with filaments (thread-like structures connecting different regions, if the threads could contain a few thousand galaxies!), and voids (immense spaces between filaments containing few galaxies or none at all).
The network formed by these clusters, filaments and voids in the universe is what is called the 'Cosmic Web'. Cosmologists seek to understand, among other things, the origin and evolution of this vast cosmic web that makes up our universe.
Most recently, there have been two major discoveries that have added to our existing knowledge of the universe: the Saraswati supercluster, and the detection of 25 Giant Radio Galxies (GRGs). Shishir Sankhyayan, a graduate student in the Physics Programme at IISER Pune, has been involved in both teams that made these discoveries.
Cosmology has been a fascination since childhood for Sankhyayan, and this lead him to obtain his Bachelor's degree from Lucknow University, and his Master's from Pune University, with a specialization in Astronomy and Astrophysics. For his PhD, Sankhyayan is interested in looking at large scale structures in the universe, their structure and their evolution. He studies astronomical and cosmological data available in the public domain. Using this data, he investigates the geometry and distribution of large scale structures and makes observations and certain inferences from these, which may help in the discovery of even more structures.
For example, data obtained from the Sloan Digital Sky Survey (SDSS), which collects information about the sources and intensity of radiation from different objects in the sky, lead to the discovery of the Saraswati supercluster. This structure, lying in the constellation of Pisces, spans about 600 million light years across (compare this to the Milky Way, whose size is about 150,000 light years), and consists of about 43 galaxy clusters, connected by filaments.
How to discover a supercluster: A crash course
Sankhyayan credits the discovery of Saraswati, to an observation made by IUCAA faculty member Joydeep Bagchi way back in 2002, when he (Bagchi) was studying a single cluster in that particular region of the sky. Says Sankhyayan, “Based on the data available at the time from radio, X-ray and optical telescopes, Bagchi hypothesised that there must be other masses surrounding this cluster, which would explain its particular structure or morphology. However, there simply wasn't enough data to substantiate it.”
Saraswati Super-cluster
    The distribution of galaxies, from Sloan Digital Sky Survey (SDSS), in and around Saraswati supercluster. It is clearly visible that the density of galaxies is very high in the central region of the image, that is the Saraswati supercluster region. The typical size of a galaxy here is around 250,000 light years. The galaxy sizes are increased for representation. (Image courtesy: Shishir Sankhyayan)
It took about 15 years for the SDSS to collect enough data for that particular region of the sky, for Sankhyayan and others to analyse, and make this discovery.
Sankhyayan explains, “It took us 3-4 years just to make sense of the data. We then had to refine it. Which is to say, we had to understand and remove certain theoretical and instrumental biases present in the data. Following this, we applied certain cluster-finding algorithms to the data to see whether it fits the existing parameters for what may be called a supercluster. There was also a lot of bookkeeping involved. We compared what we had with data from other papers and other catalogues. We were able to see that the region we had identified as having a high density of galaxies, also had a high density of clusters.  Likewise, the regions where we observed voids were also matching with previously catalogued data for this region. After performing several other checks, we were then confident that what we had discovered was indeed a supercluster.”
And this was introduced to the world as Saraswati.
What's in a name?
The paper in which this finding has been reported, elaborates on the name, Saraswati, as follows:
Saraswati is the ancient Indian goddess of knowledge, music, art, wisdom, and nature, muse of all creative endeavor. Historically, the Saraswati river is an important river goddess mentioned in the Rig’veda. Saraswati played an important role in Indian culture, since Vedic Sanskrit and the first part of the Rig’veda are regarded to have originated when the Vedic people lived on its banks, during the second millennium BCE. The Sanskrit name also means “ever flowing stream with many pools.” This may describe the present large-scale filamentary structure of galaxies located in the Zodiacal constellation of Pisces, having many clusters and groups moving and merging together.
Sankhyayan adds, “We call the Milky Way as Akash Ganga, because it appears like a river of stars in the night sky. Likewise, one may think of the supercluster as a river, or sea of galaxies. So, in keeping with this train of thought, we named it after the ancient river, Saraswati.”
The gravity of the situation
Saraswati is a very rare object, one of only 4-5 other superclusters observed so far. It is at a distance of 4 billion light years away from us, the second farthest supercluster detected so far. Considering that the universe is about 14 billion years old, this means that the light that we see coming from the supercluster, was emitted when the universe was about 10 billion years old. We are, in effect, looking at the past. And this discovery, it turns out, has raised several important questions regarding the origin and evolution of the universe.
Sankhyayan elaborates, “The existing cosmological models do not explain how such humongous structures could have formed at that stage of the universe. We believe that in the early universe, there was a more-or-less uniform distribution of energy. And very tiny fluctuations, of the order of one in a  100,000, lead to the formation of structures, which grew in size, aggregating more matter, because of gravity. Now, our existing models and simulations do not predict the existence of such massive objects (like Saraswati) at that age.”
Concepts like 'dark matter' and 'dark energy' are invoked to explain the expansion and structure-formation of the universe, and this discovery is likely to offer more insight into our understanding of dark energy, as well as force scientists to revisit the current models for the evolution of the universe.
So where do we go from here? “We look for more superclusters!”, replies Sankhyayan. “We are also going to study Saraswati further and obtain more data about the galaxies present in it, their distances from us, and so on. Once we have a better understanding of this, we can map out the structure and dynamics of the supercluster. This would be a good place to start, for testing the validity of existing hierarchical structure formation models of the universe.”
The Universe has more tricks up its sleeve 
When he wasn't busy discovering a supercluster, Sankhyayan was also involved in the recent discovery of 25 Giant Radio Galaxies (GRGs), extremely rare astrophysical objects (only about 300 of these have been discovered so far) that are about as large as 33 Milky Way-sized galaxies lined up end to end. They harbour supermassive black holes at their centres, which eject massive jets of energetic, charged particles. These appear like lobes when seen through a radio telescope, and are characteristic to GRGs. Their size as well as their very existence is still a mystery. Although about 200 candidate objects had been identified, of these, 25 have been confirmed as GRGs while the others are being investigated further.
Sankhyayan's contribution to this work, was the exploration of the regions surrounding these GRGs. “What this means,” he says, “is that we have looked at whether these GRGs exist in cosmological voids or clusters, or something intermediate to these. We hope to explore this further and are working on it right now. Also, the energetics and the dynamics of GRGs are not well understood. That is, we don't really know what is happening at the centre of these galaxies that jets that powerful are emanating from them.”
The universe certainly has no dearth of mysteries that need solving, and each discovery only seems to raise more interesting questions. To quote Douglas Adams again, “All you really need to know for the moment is that the universe is a lot more complicated than you might think, even if you start from a position of thinking it's pretty complicated in the first place.”
Here’s hoping that Shishir Sankhyayan unravels more mysteries of the universe, and poses even more questions. We wish him the very best for the future.

The discovery of Saraswati supercluster has been reported in the Astrophysical Journal (844(1):25). The team involved in this discovery are Joydeep Bagchi (IUCAA), Shishir Sankhyayan (IISER Pune), Prakash Sarkar (NIT Jamshedpur), Somak Raychaudhury (IUCAA), Joe Jacob (Newman College, Thodupuzha) and Pratik Dabhade (IUCAA).
The discovery of 25 Giant Radio Galaxies has been reported in the Monthly Notices of the Royal Astronomical Society (469(3):2886-2906). The team involved in this discovery includes Pratik Dabhade (IUCAA, Leiden University, Netherlands), Madhuri Gaikwad (NCRA-TIFR), Joydeep Bagchi (IUCAA), M. Pandey-Pommier (Univ Lyon, France), Shishir Sankhyayan (IISER Pune) and Somak Raychaudhury (IUCAA).
- by Harshini Tekur