“The Sparkler” Might be Our Milky Way’s Long-Lost Twin

The Sparkler is a small galaxy residing 9 billion years ago in the adolescent universe. It has only one-hundredth of the Milky Way’s mass in stars; however, over the eons it will grow to become the size of our galaxy. Looking back in time at this developing galaxy is enabling astronomers examine a growth spurt similar to what our own galaxy might have experienced.

Discovered in one of the first James Webb Space Telescope (JWST) images to be released, the Sparkler is distorted into a stick shape via the gravitational lensing of a foreground galaxy cluster. Gold-tinged star clusters surround the galaxy — hence its nickname. Of the two-dozen objects, dubbed “sparkles,” five are likely to be globular clusters, dense clumps of ancient stars.

Lamiya Mowla (University of Toronto) and colleagues first analyzed the sparkles last fall, determining the clusters’ ages and chemical composition. They found that the Sparkler’s globular candidates are chemically enriched (what astronomers often term metal-rich), indicating several previous generations of stars had already lived and died within them. Other sparkles, which appear more extended than the compact globulars, are metal-poor.

Now, in a new study, Duncan Forbes (Swinburne University, Australia) and Aaron Romanowsky (San Jose State University and University of California, Santa Cruz) are comparing the Sparkler’s star clusters to those around galaxies closer to home — namely, our own — to understand the galaxy’s history. The study appears in the Monthly Notices of the Royal Astronomical Society.

The Milky Way hosts more than 150 known globular clusters, most of them orbiting well outside its spiral disk of stars. Some of these globulars probably formed early on; although their origins aren’t well understood, they must somehow have been incorporated into our infant galaxy as it came together. Other globulars joined the Milky Way as it gobbled up other galaxies and their accoutrement.

A second group of reference points are the globulars around the Milky Way’s largest satellite galaxy, the Large Magellanic Cloud (LMC). The LMC is about the same size now that the Sparkler was all those 9 billion years ago. Eventually, if the LMC is absorbed after multiple passes, its globulars would join the Milky Way’s collection.

A final point of comparison is the dwarf galaxy Gaia-Enceladus. It no longer exists — the Milky Way’s gravity tore it to shreds 9 billion years ago — exactly the timeframe at which we’re seeing the Sparkler. But while the galaxy was destroyed, its globulars survived to join our galaxy’s stellar halo. Other astronomers have been able to identify which ones originally belonged to Gaia-Enceladus dwarf.

Forbes and Romanowsky ultimately compared the ancient sparkles to dozens of globular clusters around the Milky Way and the LMC. They find that the globular sparkles are like the metal-rich globulars around the Milky Way, which probably formed with our galaxy and orbit close to its disk.

Two of the more extended sparkles, which are metal-poor, appear to be akin to the globulars around the LMC and Gaia-Enceladus. Forbes and Romanowsky thus speculate that these two objects might actually also be globular clusters, only they’re newly formed and belong to a dwarf galaxy that’s falling into the Sparkler.

“We appear to be witnessing, firsthand, the assembly of this galaxy as it builds up its mass, in the form of a dwarf galaxy and several globular clusters,” says Forbes.

Astronomers have used age and composition measurements to conduct a sort of archaeology in our own galaxy, deriving the merger history of the Milky Way. But such analysis is complicated enough when we have the reams of data on the Milky Way that several sky surveys have provided. For distant galaxies residing in the long-ago universe, measurements become more uncertain.

Another study, led by Adélaïde Claeyssens (Stockholm University), examined the same light from the candidate globular clusters around the Sparkler, and found that they’re metal-poor rather than metal-rich. She and colleagues are now working toward clarifying the differences in the measurements. Forbes and Romanowky acknowledge in their study that the ages and metallicities of these clusters “deserve further study.”

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