Astronomers find two of the Milky Way's earliest building blocks

Research scientists at the Max Planck Institute for Astronomy have published "good candidates for some of the earliest ancestors of our Milky Way" in the Astrophysical Journal.

Named "Shakti" and "Shiva," the two structures appear to be the remnants of two galaxies that merged with an early version of the Milky Way between 12 and 13 billion years ago, thus contributing to its initial growth.

The institute said in a statement on Thursday that the discovery was the "astronomical equivalent of archeologists identifying traces of an initial settlement that grew into a large present-day city."

So far, it is impossible to identify which stars came from which predecessor galaxy. However, most stars retain basic properties that are "directly linked to the speed and direction of the galaxy in which they originated," the institute said.

Using "basic physics," the researchers found large groups of stars with similar, unusual values for energy and angular momentum -- the momentum associated with orbital motion or rotation -- that indicates a possible merger remnant. The very low metal content of the stars was an "additional pointer" that suggested they already existed at the time of the early version of the Milky Way.

The Max Planck Institute said that thanks to the European Space Agency's (ESA) astrometry satellite Gaia, "large and high-quality data sets" were available. Launched in 2013, the satellite "revolutionized studies of the dynamics of stars" in the Milky Way, producing an increasingly accurate data set including positions, changes in position and distances for almost 1.5 billion stars.

The researchers have identified the two structures by combining data from the Gaia satellite with stellar spectra from the Sloan Digital Sky Survey, which provide detailed information about the stars' chemical composition.

New surveys are promising to deliver more relevant data. Both spectra and precise distances should "enable astronomers to make a firm decision on whether or not Shakti and Shiva are indeed a glimpse of our home galaxy's earliest prehistory," the institute said.