Astronomers tend to divide galaxies into two broad categories: late-types and early-types. Late-types are young, full of new stars, and often have spiral structures, while early-types have an elliptical shape and they tend to be old with little star formation.
Every rule appears to have its exception, though, and it turns out that even dead ellipticals can have spiral structures where stars are forming. Jean Michel Gomes and Polychronis Papaderos, from the Institute of Astrophysics and Space Sciences in Portugal, and their colleagues have discovered three early-type galaxies sporting a spiral-like structure.
“According to our current view, grand design spiral-like features are associated with disc galaxies. These are, in general, regions of enhanced star formation,” Gomes said in a statement. “We were surprised to have discovered, for the first time in the optical, spiral-like structures in early-type galaxies, which we believed to have stopped forming stars in the last few billion years and should entirely lack spiral features.”
Early-types are formed from the merger of large spiral galaxies. The collision is messy but very slow; this is why galaxies evolve from the regular spiral structure of late-types to the more chaotic elliptical shape. But according to the research, presented at the 2nd Study of Emission-Line Galaxies with Integral-Field Spectroscopy (SELGIFS) Advanced School on Integral-Field Spectroscopic Data Analysis, ellipticals continue to acquire cold gas and to form stars even billions of years after the merger.
“This study provides further observational evidence for a still ongoing growth of some seemingly 'old and dead' early-type galaxies in the local universe, out of a reservoir of cold gas that feeds low-level star-forming activity in their periphery,” Papaderos added.
The work was published in the journal Astronomy & Astrophysics and is part of the Calar Alto Legacy Integral Field spectroscopy Area (CALIFA) survey that has so far been able to produce a 3D reconstruction of how stars and gas are distributed within a galaxy.
The goal is to address the several unknowns present in our current picture of galaxy evolution, and the discovery of spiral structures in early-type galaxies really tells us that there’s a lot more that we need to learn.