Researchers describe one of the most massive large-scale structures in the universe

Researchers describe one of the most massive large-scale structures in the universe

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. Credit: Inter University Centre for Astronomy
& Astrophysics (IUCAA)

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.

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. Credit: Inter University Centre for
Astronomy & Astrophysics (IUCAA)

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 , 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 , 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 , 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 formation.”

Explore further:

Astronomers find supercluster of galaxies near Milky Way

More information: Saraswati: An Extremely Massive ~ 200
Megaparsec Scale Supercluster. arxiv.org/abs/1707.03082

Journal reference: Astrophysical
Journal

Provided by: Inter University Centre for Astronomy
& Astrophysics (IUCAA)