NaD

• NaI doublet at λλ5890,5896Å reso- nant lines in the rest-frame optical galaxy spectra. This radiative transition arises from the neutral sodium atoms at temperatures T < 1000K. The transitionwhich gives rise to the doublet is from the ground-state configuration 3s to the energy levels of the 3p configuration composed by two stateswith total angularmomen- tum j=3/2 and j=1/2 (spin-orbit effect). The continuum photons emitted by the stellar component are absorbed by the NaI atoms and are re-emittedwhen the electrons in the 3p configuration de- cay spontaneously to the ground-state. As a resonant line, the NaI doublet can be observed both in absorption and in emission.

• 星系光谱中可见，在去除恒星的贡献之后，还trace中性的ISM，参见[1]