“Saraswati”- one of the most massive large-scale structures in
the Universe discovered
Indian
astrophysicists identify mega-structure of galaxies 4 billion
light-years away - several scholars and faculty of Indian
Universities involved
The
distribution of galaxies, from Sloan Digital Sky Survey (SDSS),
in Saraswati supercluster. It is clearly visible that the
density of galaxies is very high in the Saraswati supercluster
region. The typical size of a galaxy here is around 250,000
light years. The galaxy sizes are increased for representation. (Downloadable
.pdf File and
.png File)
A team of astronomers from the
Inter University Centre for Astronomy & Astrophysics (IUCAA),
and Indian Institute of Science Education and Research (IISER),
both in Pune, India, and members of two other Indian
universities, have identified a previously unknown, extremely
large supercluster of galaxies located in the direction of
constellation Pisces. This is one of the largest known
structures in the nearby Universe, and is at a distance of 4,000
million (400 crore) light-years away from us.
This novel discovery is being published in the latest issue of
The Astrophysical Journal, the premier research journal of the
American Astronomical Society.
Large-scale structures in the Universe are found to be
hierarchically assembled, with galaxies, together with
associated gas, and dark matter, being clumped in clusters,
which are organized with other clusters, smaller groups,
filaments, sheets and large empty regions (“voids”) in a pattern
called the “Cosmic web” which spans the observable Universe.
Superclusters are the largest coherent structures in the Cosmic
Web. A Supercluster is a chain of galaxies and galaxy clusters,
bound by gravity, often stretching to several hundred times the
size of clusters of galaxies, consisting of tens of thousands of
galaxies. This newly-discovered ‘Saraswati’ supercluster, for
instance, extends over a scale of 600 million light-years and
may contain the mass equivalent of over 20 million billion suns.
When astronomers look far away, they see the Universe from long
ago, since light takes a while to reach us. The Saraswati
supercluster is observed as it was when the Universe was 10
billion years old.
The long-popular “Cold dark matter” model of the evolution of
the Universe predicts that small structures like galaxies form
first, which congregate into larger structures. Most forms of
this model do not predict the existence of large structures such
as the “Saraswati Supercluster” within the current age of the
Universe. The discovery of these extremely large structures thus
force astronomers into re-thinking the popular theories of how
the Universe got its current form, starting from a more-or-less
uniform distribution of energy after the Big Bang. In recent
years, the discovery of the present-day Universe being dominated
by “Dark Energy”, which behaves very differently from
Gravitation, might play a role in the formation of these
structures.
It is believed that galaxies are
formed mostly on the filaments and sheets that are part of the
cosmic web, and many of the galaxies travel along these
filaments, ending up in the rich clusters, where the crowded
environment switches off their star formation and aids in the
transformation of galaxies to disky blue spiral galaxies to red
elliptical galaxies. Since there is an extensive variation of
environment within a Supercluster, galaxies travel through these
varied environments during their “lifetime”. To understand their
formation and evolution, one needs to identify these
Superclusters and closely study the effect of their environment
on the galaxies. This is a very new research area- with the aid
of observations of new observational facilities, astronomers are
now beginning to understand galaxy evolution. This discovery
will greatly enhance this field of research.
“Saraswati” (or “Sarasvati”), a word that has
proto-Indo-European roots, is a name found in ancient Indian
texts to refer to the major river around which the people of the
ancient Indian civilization lived. It is also the name of the
celestial goddess who is the keeper of the celestial rivers. In
modern India, Saraswati is worshipped as the goddess of
knowledge, music, art, wisdom and nature – the muse of all
creativity.
Our own galaxy is part of a Supercluster called the Laniakea
Supercluster, announced in 2014 by Brent Tully at the University
of Hawaii and collaborators.
Interestingly, Somak Raychaudhury, currently Director of IUCAA,
Pune, who is a co-author of this paper, also discovered the
first massive Supercluster of galaxies on this scale (the
“Shapley Concentration”), during his PhD research at the
University of Cambridge. In his paper, published in the journal
‘Nature’ in 1989, he had named the supercluster after the
American astronomer Harlow Shapley, in recognition of his
pioneering survey of galaxies, from the Southern hemisphere, in
which this massive super-structure was first imaged, way back in
1932.
Joydeep Bagchi from IUCAA, the lead author of the paper and
co-author Shishir Sankhyayan (PhD scholar at IISER, Pune) said,
‘’We were very surprised to spot this giant wall-like
supercluster of galaxies, visible in a large spectroscopic
survey of distant galaxies, known as the Sloan Digital Sky
Survey (see figure above). This supercluster is clearly
embedded in a large network of cosmic filaments traced by
clusters and large voids. Previously only a few comparatively
large superclusters have been reported, for example the ‘Shapley
Concentration’ or the ‘Sloan Great Wall’ in the nearby universe,
while the ‘Saraswati’ supercluster is far more distant one. Our
work will help to shed light on the perplexing question; how
such extreme large scale, prominent matter-density enhancements
had formed billions of years in the past when the mysterious
Dark Energy had just started to dominate structure formation.’’
Two most massive clusters of
galaxies in the Saraswati supercluster : “ABELL 2631”
cluster (left) and “ZwCl 2341.1+0000” cluster (right). “ABELL
2631” resides in the core of the Saraswatisupercluster. The
Saraswati supercluster has a total of 43 clusters of galaxies.
(Downloadable .pdf File and .png File)
This project was supported by the University Grants Commission
of India, through its funding of IUCAA, and the Indo-French
Centre for the Promotion of Advanced Research (CEFIPRA)
programme.
