A new class of galaxies so unusual that researchers have nicknamed them "platypus galaxies" — after the famously unclassifiable mammal — has been discovered using the James Webb Space Telescope. Like their namesake, which confounded 18th-century naturalists with its combination of mammalian, avian, and reptilian features, these galaxies combine properties that current theory says should be mutually exclusive.
The objects display the compact, bulge-dominated structure normally associated with old, quiescent elliptical galaxies that stopped forming stars billions of years ago. Yet they are also surrounded by large reservoirs of gas-rich material in rings and streams — exactly the raw material that should not be present in a galaxy that has already exhausted its star-forming fuel.
Upsetting Standard Galaxy Evolution
Standard models of galaxy evolution predict a clear one-way life cycle: galaxies form stars from gas clouds, exhaust their gas, and then gradually fade as their remaining stars age. Finding objects that appear to be both old and quiescent while also sitting inside fresh gas reservoirs implies that galaxies can re-acquire material late in their evolution — a possibility that current flagship simulations do not predict.
"These objects simply should not exist according to our best models," said Dr. Sirio Belli of the University of Bologna, a co-author of the study published in Nature Astronomy. "We think they may be catching gas from the cosmic web or from minor mergers. Either explanation requires us to substantially revise how we think galaxy evolution works."