Astronomers from leading institutions have identified the oldest known spiral galaxy, BRI 1335-0417, which formed approximately 12.4 billion years ago—mere 1.4 billion years after the Big Bang. This groundbreaking discovery, detailed in Science, sheds light on the origins and evolution of galaxies like our Milky Way.
Galaxies fall into three primary categories: ellipticals, irregulars, and spirals. Ellipticals are vast, spheroidal clusters of billions of stars resembling enormous globular clusters. Irregulars lack any defined structure. Spirals, like our Milky Way, boast distinct features including a central bulge, disk, and winding arms.
The formation timeline of the earliest spirals has long puzzled scientists, but BRI 1335-0417 narrows the gap. Discovered serendipitously by Takafumi Tsukui of SOKENDAI University of Advanced Studies in Japan while reviewing ALMA archives, this galaxy stands as the most ancient spiral identified to date. For context, the overall oldest known galaxy, the irregular GN-z11, emerged just 400 million years post-Big Bang.
To the untrained eye, ALMA's image of BRI 1335-0417 may seem blurry, yet it reveals intricate details of this remote object. "I was thrilled to see such clear evidence of a rotating disk, spiral arms, and centralized mass in a distant galaxy—unlike anything in prior studies," Tsukui shares. "The ALMA data's exceptional quality initially made me mistake it for a nearby galaxy."
Remarkably massive for its era, BRI 1335-0417 spans a 15,000 light-year diameter—about one-third the Milky Way's size—while packing in a comparable total mass. Researchers propose it arose from a dramatic merger of smaller galaxies.
Its destiny may illuminate the evolution of spirals, which comprise roughly 72% of observable galaxies. While some theories posit spirals evolve into ellipticals, the mechanism remains elusive. Co-author Satoru Iguchi of Japan's National Astronomical Observatory adds, "Our Solar System resides in a Milky Way spiral arm. Understanding these ancient spirals reveals the birthplace of our cosmic neighborhood."
