What defines an active galaxy?

An active galaxy is characterized by significant emissions of energy that are detectable across a wide range of wavelengths, including radio, infrared, optical, ultraviolet, X-rays, and gamma rays. This high-energy output typically originates from a supermassive black hole at the galaxy's center, which accretes material and emits vast amounts of radiation as this material falls in. The presence of jets, which are streams of charged particles ejected at relativistic speeds, and intense emissions from the accretion disk contribute to the high-energy signatures observed in active galaxies.

In contrast, galaxies characterized by older stars or minimal change over time do not exhibit these energetic phenomena. Similarly, a galaxy with a dim light profile would be indicative of lower star formation rates and less energetic activity, distinguishing it from active galaxies that are noted for their brightness and variability across the electromagnetic spectrum. Thus, the defining feature of an active galaxy is its capacity to emit substantial energy across various wavelengths, making option C the correct choice.