doi:10.1038/nindia.2017.83 Published online 14 July 2017

New galaxy supercluster spotted

K. S. Jayaraman

The Saraswati supercluster.
Indian astronomers have announced the discovery a major concentration of galaxies located in the direction of the constellation Pisces, around four billion light years away from Earth. The discovery will be published in the Astrophysical Journal next week.

The astronomers are calling it the "Saraswati" supercluster after the Hindu goddess of knowledge and wisdom.

A team from the Inter University Centre for Astronomy & Astrophysics (IUCAA), and Indian Institute of Science Education and Research (IISER), both in Pune, along with members of two other Indian Universities identified the supercluster from an international project called Sloan Digital Sky Survey (SDSS). The digital survey, which started operation in 1998, uses a 2.5 m wide-angle spectroscopic optical telescope in New Mexico, USA. In the SDSS sky map, the Indian team identified an over-dense region — extending over a scale of 600 million light-years — and nick-named it Saraswati.

Saraswati comprises at least 43 massive galaxy clusters with a total mass of over 20 million billion suns, placing it among some of the largest and most massive superclusters known, the astronomers say.

Galaxies are the basic building blocks of the Universe – Earth being in a galaxy called Milky Way, which is a part of more than 54 galaxies. On the larger scale, galaxies are found in clusters and clusters form "superclusters" or over-dense regions. Milky Way, for instance, is part of the Laniakea supercluster.

The physical mechanism of supercluster formations is still not well understood. “Our work will help shed light on the perplexing question of 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," says Joydeep Bagchi, lead author of the paper.

The discovery forces astronomers into rethinking popular theories of how the Universe got its current form, starting from a more-or-less uniform distribution of energy after the Big Bang, adds Somak Raychaudhury, IUCAA director and co-author.

This is an important result, says Annapurni Subramaniam of the Indian Institute of Astrophysics in Bengaluru, not part of the study. "... it will help understand large scale structure formation in the early Universe," she told Nature India.

Abhas Mitra, former head of theoretical astrophysics at the Bhabha Atomic Research Centre in Mumbai agrees. "The discovery is an important contribution to observational astronomy from India, because, as of now, only a few superclusters are known," he told Nature India. "However it may be borne in mind that, it is extremely difficult to infer much physics, by inspecting a static sky map of SDSS galaxies." 

The authors admit in their paper that follow-up work is needed. They note, “The Saraswati supercluster and its environs in Stripe 82 region must be surveyed in greater depth with more galaxy redshifts taken on a wider scale for a better understanding of what physical processes were involved in the growth of such enormous cosmic structures in the distant universe.”