In spectral observations, which emission type would strongly indicate the presence of cold, star-forming material in a spiral but be largely absent in a large elliptical?
Answer
Radio emission from cold hydrogen gas (the 21-cm line)
Spiral galaxies show strong radio emission associated with cold hydrogen gas, a marker of star-forming material, whereas ellipticals are described as being remarkably silent in the radio wavelengths associated with this cold material.
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