The above figure shows sample far-IR spectra of some nearby galaxies. They were obtained with the Long Wavelength Spectrometer onboard the Infrared Space Observatory and are reprinted here with permission of J. Fischer of the Naval Research Laboratory. The majority of the energy from these galaxies emerges in the far-IR, and thus this type of spectrum is the best way to directly probe their activity.

The spectra show continuum emission from dust (the overall shape of the spectrum) and discrete spectral lines from atomic and molecular features. The shape of the continuum reveals the temperature of the emitting dust, while the spectral lines reveal the redshift (distance) of the galaxy, the types of stars it contains, and whether or not there is an active black hole (known as an Active Galactic Nucleus, or AGN) at its center.