Earliest building blocks of Milky Way galaxy named Shakti, Shiva

New Delhi, Mar 22 : Earliest "building blocks" of our Milky Way galaxy have been identified from 12-13 billion years ago, which is very close in time to when the universe's first galaxies started to form, according to a new research.
Naming these groups of stars 'Shakti' and 'Shiva', astronomers said the findings are the equivalent of "finding traces of an initial settlement that grew into a large present-day city".
Milky Way is said to have formed by the merging of smaller galaxies, making way for "fairly large building blocks", according to the researchers.
When galaxies collide and their stellar populations mingle, most of the stars retain very basic properties, directly linked to the speed and direction of their origin galaxy, they explained.
In this study published in The Astrophysical Journal, the research team from Max Planck Institute for Astronomy, Germany, analysed stellar datasets and found that stars from the merging galaxies were crowded around two specific signatures of energy and angular momentum, or the rate at which a spinning object's rotating speed changes. Two different star groups were thus formed -- 'Shakti' and 'Shiva'.
The study's co-author Khyati Malhan named these two structures Shakti and Shiva, the latter one of the principal deities of Hinduism and the former a female cosmic force often portrayed as Shiva's consort.
The researchers found that the "like-minded" stars forming Shakti and Shiva, and coming from two different galaxies, had a higher angular momentum compared to the stars at the heart of the Milky Way.
The higher angular momentum observed was consistent with the stellar groups that had belonged to separate galaxies merging with the Milky Way, the team said.
Also, all these stars were low in metal content, signalling that they were formed a long time ago. Stars formed recently contain more of heavier metallic elements, they explained.
Therefore, their energy and angular momentum, along with low metallic content, comparable with that of the stars at the heart of Milky Way, makes 'Shakti' and 'Shiva' good candidates for some of the earliest ancestors of our Milky Way, the researchers said.
"Shakti and Shiva represent two of those early, massive progenitors that coalesced at high redshift - perhaps 12 gigayears ago - perhaps the last event from the protogalaxy before disk formation commenced," they wrote in their study. A gigayear has a billion years.
Shakti and Shiva might be the first two additions to the heart of the Milky Way, initiating its growth towards a large galaxy, said study co-author Hans-Walter Rix from Max Planck Institute for Astronomy.
For their analysis, the researchers used the data provided by the European Space Agency's Gaia satellite and combined it with the stellar datasets from the US Sloan Digital Sky Survey, having detailed information about the stars' chemical composition.
Launched in 2013, Gaia's dataset now includes positions, changes in the positions and distances for almost 1.5 billion stars within our galaxy, providing an ideal dataset for this kind of "big data galactic archeology", the team said.