galaxies

Seeing a Mirror Image of the Milky Way From Billions of Years Ago

Ever wonder what our Milky Way Galaxy looked like in its early history? Astronomers using the Webb Telescope (JWST) found another galaxy that’s almost a mirror image of our galaxy as an infant. It’s nicknamed “The Sparkler”. That’s because it has about two dozen glittering globular clusters orbiting around it. There are also a few dwarf galaxies there, being swallowed up by the galaxy.

The JWST view shows the Sparkler as it looked when the Universe was only four billion years old or about a third of the Universe’s present age. That implies that this galaxy, like the Milky Way, began forming very early in cosmic history. If the Sparkler takes the same growth path that the Milky Way did through galaxy mergers and acquisitions, then it should grow the same way the Milky Way did. In about nine billion years, it could look very much like a twin of ours.

A Distant, Early Proxy for Our Milky Way

The Sparkler lies in the direction of the constellation Volans (in the southern hemisphere sky). It’s quite far away, at a redshift of z = 1.38. That’s about 9 billion light-years and puts it at a time a few billion years after the Big Bang. Like the Milky Way, the Sparkler didn’t spring up out of the cosmos fully formed. Both galaxies began as “overdensities” of matter (like clouds of neutral hydrogen) in the early universe. Think of them as galaxy “seeds” attracted together through their gravitational pull on each other. The globular clusters were born in some of those clumps, probably slightly before the galaxy’s birth. That’s why the stars in some globulars are older than their galaxies.

A color composite image of the oldest globular cluster in the Milky Way, obtained by the Gemini South Telescope in Chile. It’s about 12.8 billion years old and is one of the last surviving members of the clusters that formed the Milky Way’s inner bulge. Credit: Credit: Gemini Observatory/AURA/NSF; composite image produced by Mattia Libralato of Space Telescope Science Institute.

Next came an era of “mergers and acquisitions” as the infant Milky Way (and presumably the Sparkler, too) began cannibalizing nearby dwarf galaxies. That’s a big evolutionary step. It’s possible that at least half our own galaxy’s mass came from those mergers. Over time, all the material coalesced into the spiral disk where the Sun and most other stars exist today.

The Sparkler’s Future Compared to the Milky Way

Will the Sparkler follow the same evolutionary path as the Milky Way? From the JWST data, it seems that it will. Although it’s currently only a small fraction of the mass of the Milky Way—about 3%—that will change as it gobbles up other, smaller galaxies. Eventually, it will match the Milky Way’s mass in the present-day universe.

That’s pretty exciting because it gives astronomers a chance to understand what happened as our own galaxy evolved. “We appear to be witnessing, first hand, the assembly of this galaxy as it builds up its mass—in the form of a dwarf galaxy and several globular clusters,” said Professor Duncan Forbes of Swinburne University in Australia. He studied the galaxy and its clusters, along with Professor Aaron Romanowsky of San Jose State University in California. “We are excited by this unique opportunity to study both the formation of globular clusters and an infant Milky Way, at a time when the Universe was only 1/3 of its present age,” he said.

Forbes and Romanowsky used the JWST data to explore the ages and metallicities of several “sparkles” (compact sources) in and around the Sparkler galaxy. The aim was to study the metallicity across a range of compact star clusters surrounding the Sparkler. The scientists wanted to see if they resemble younger versions of the Milky Way’s globulars.

Clues to the Sparkler’s Early History in its Globular Clusters

The JWST observations of the Sparkler could also answer various questions about globular clusters and their formation, according to Romanowsky. “The origin of globular clusters is a long-standing mystery,” he said. “We are thrilled that JWST can look back in time to see them in their youth”.

It turns out that the Sparker’s clusters do have striking similarities to some of the Milky Way’s globulars. Several seemed to have formed very early and the stars are quite metal-rich. That seems to indicate a very rapid process of chemical enrichment in the early Universe.’ clusters, albeit younger versions of the Milky Way’s globulars. Several seemed to have formed very early and the stars are quite metal-rich. That seems to indicate a very rapid process of chemical enrichment in the early Universe.

A couple of clusters had stars that were slightly older and metal-poor than in other clusters. They probably belong to a low-mass satellite galaxy that’s being ingested into the Sparkler. The scene is very reminiscent of the Milky Way’s history of mergers throughout its life.

More Data Needed

To get a better understanding of the evolutionary state of the Sparkler and its clusters, the two scientists say that more observations of similar-type clusters around other distant galaxies in the universe are needed. That would help determine if the Sparkler is typical of the merger-style of galaxy evolution (similar to the Milky Way). If not, then the details of early galaxy evolution, chemical enrichment, mass growth, and star cluster formation might need some revision.

For More Information

Distant galaxy mirrors the early Milky Way
Reconstructing the genesis of a globular cluster system at a look-back time of 9.1 Gyr with the JWST
The Formation of the Galaxy (an educational guide)

Carolyn Collins Petersen

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