Ancient stars "Shakti" and "Shiva" shaped the Milky Way galaxy

The Milky Way, as we see it today, is a colossal galaxy, a vast island of stars, gas, and dark matter. However, its beginnings were far more modest. Researchers have pinpointed two of the earliest building blocks that contributed to the Milky Way’s formation — stellar populations nicknamed Shakti and Shiva.

These ancient proto-galactic fragments, from 12 to 13 billion years ago, merged with our young galaxy, laying the foundation for the grand spiral we know as home.

Galactic ancestry

Investigating the origins of stars that migrated to the Milky Way from other galaxies is a complex endeavor. However, astronomers Khyati Malhan and Hans-Walter Rix of the Max Planck Institute for Astronomy tackled this challenge using two surveys:

• Gaia: The European Space Agency’s Gaia satellite has created a remarkably precise three-dimensional map of our galaxy. It provides detailed information on the positions, motions, and distances of almost 1.5 billion stars.
  
• Sloan Digital Sky Survey (SDSS): SDSS gathers in-depth spectra of stars, revealing their chemical makeup.

Stellar clues

When galaxies collide, there’s a massive reshuffling of stars. However, certain fundamental properties of stars can offer clues about their origins:

Stellar motion

Stars that were originally part of the same galaxy tend to retain similar patterns of motion. Astronomers track two key components:

• Energy: The total energy of a star’s motion gives hints about its interaction with the larger galaxy.
  
• Angular Momentum: Related to a star’s rotational motion, angular momentum offers clues about the galaxy in which it formed.

Chemical composition

Stars are born from clouds of gas that contain elements forged in earlier generations of stars. Older stars, born early in the universe’s history, have a lower abundance of heavy elements (known as “metals” to astronomers). Searching for stars with low metallicity pinpoints the earliest populations within a galaxy.

Identifying Shakti and Shiva in Milky Way

Malhan and Rix discovered distinct groups of stars with strikingly similar motion patterns and very low metallicity. These clusters, named Shakti and Shiva, are believed to be remnants of two very early galaxies absorbed by the nascent Milky Way billions of years ago.

“We observed that, for a certain range of metal-poor stars, stars were crowded around two specific combinations of energy and angular momentum,” explained Malhan.

Naming these ancient stellar groups Shakti and Shiva highlights the profound connection researchers feel with the cosmos.

Shakti, the embodiment of feminine power and creation in Hindu mythology, reflects the birth of galaxies. Shiva, the symbol of destruction and rebirth, mirrors the transformative mergers galaxies undergo.

Shakti and Shiva’s role in Milky Way formation

“Shakti and Shiva might be the first two additions to the ‘poor old heart’ of our Milky Way, initiating its growth towards a large galaxy,” said Rix. The “poor old heart” represents another group of ancient stars residing in the Milky Way’s center and formed during its earliest stages.

This imagery emphasizes the heart as a record of the Milky Way’s history, with Shakti and Shiva’s addition marking key growth milestones. Their integration billions of years ago spurred the Milky Way’s journey from a compact, young galaxy to the vast spiral we know.

Mergers like these not only built up the Milky Way‘s mass but shaped its structure. Understanding such events is key to tracing galactic evolution, revealing how collisions fueled growth and organization.

Technical details of Shakti and Shiva

Other technical details of Shakti and Shiva include:

Orbital dynamics and spatial distribution

Shakti and Shiva exhibit prograde orbits specifically located within the Milky Way’s solar circle. This unique positioning indicates their early integration into the galaxy’s complex dynamical system, suggesting a significant role in the Milky Way’s formative processes and evolving structure.

Contradictory diagnostics of origin

While their orbital characteristics and metallicities clearly identify Shakti and Shiva, a paradox emerges regarding their origin.

Their abundance patterns – particularly ratios of [Mg/Fe], [Al/Fe], and [Mg/Mn] against [Fe/H] – suggest a rapid enrichment process typically associated with in-situ formation within a galaxy’s main potential well.

However, the context of their discovery and overall properties point towards an accreted origin. This challenge underscores the complexities of early galaxy formation and how smaller proto-galactic fragments were incorporated into larger galaxies.

Mass estimates and galactic influence

Shakti and Shiva are each estimated to possess a mass 10 million times greater than our Sun. This significant mass indicates a substantial contribution to the Milky Way during its early development.

These mass estimates help quantify their impact on the Milky Way’s evolution, potentially affecting its mass distribution and influencing future star formation regions.

Shakti and Shiva: Deciphering the Milky Way

The discovery of Shakti and Shiva is just one piece in the intricate puzzle of our galaxy’s formation. Ongoing and future astronomical surveys promise even more detailed data about stellar motion and chemical composition.

This data will further illuminate our understanding of the Milky Way’s complex and dynamic history. This discovery offers a glimpse into the earliest stages of our galaxy’s formation, a testament to the power of astronomical observation and analysis.

The study is published in The Astrophysical Journal.

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