星系中的常见发射线

光学波段

[OII] doublet (3737A)
[OIII] (4959A and 5007A)
Balmer series (6563A, 4861A, 4340A, 4103A, ...).
[NII]6583A
发射线的线比可以区分不同的电离机制，金属丰度 BPT diagram,详细的工作可参见[1]
[NII]/[OII]: . intermediate mass stars are the main source of nitrogen en- richment in massive galaxies (Moll´a et al. 2015), the gas from their stellar winds should be, on average, extremely nitrogen- rich.

最强的线 [CII]158mu [OIII]88mu [OI]64mu，[OIII]来自HII区
The new [OIV]25.9μm/[OIII]88μm vs [NeIII]15.6μm/[NeII]12.8μm diagram is proposed as the best diagnostic to separate: i) AGN activity from any kind of star formation; and ii) low-metallicity dwarf galaxies from starburst galaxies.
the ([NeIII]15.6μm + [NeII]12.8μm)/([SIV]10.5μm + [SIII]18.7μm) ratio is proposed as a promising metallicity tracer to be used in obscured objects, where optical lines fail to accurately measure the metallicity.
[CII] 158$\mu$m fine-structure line，trace regions of active star formation and is the main coolant of the cold, neutral atomic medium.[3]

both [CI] and CO emission trace gas that is predominantly molecular, with a density n ~ 500-1000 cm−3, [CII] traces lower density material (n ~ 100 cm−3) [4]