Young, bright stars are usually embedded in clouds of gas which are photoionized by energetic photons from these stars. The H Balmer line of hydrogen at 6562.8 Åis the best tracer of star formation because in ionization-bounded HII regions, the Balmer emission line luminosities scale directly with the ionizing fluxes of the embedded stars. The H line can therefore be used to derive quantitative star formation rates in galaxies. However, beyond redshifts of z=0.20.3, H becomes redshifted to the near infrared where many strong sky emission lines are present.
UV radiation produced by young, massive stars shortward of 730 Åis sufficient to photoionize heavier elements such as neutral oxygen. The [OII] 3726-3729 Ådoublet is the strongest feature after H. [OII] equivalent widths are well correlated with H, but [OII] has on average half the flux as H. The [OII] doublet is the most prominent feature in the spectra of faint galaxies in the blue, and it is a prime candidate for tracing the internal kinematics of the ionized gas. The DS and DS transitions give rise to the 3726.1 Åand 3728.8 Åemission lines respectively.
At temperatures typical of star-forming regions (T10000-20000 K), the excitation energy between the two upper D levels and the lower S level is of the order of the thermal electron energy . The [OII] doublet is therefore closely linked to collisional excitations and de-excitations i.e. to the electronic density N [\protect\astronciteOsterbrock1974]. Since the two D levels have nearly the same excitation energy, the relative excitation rates of the two levels depend only on the ratio of collision strengths. In the low-density limit N 0, every collisional excitation will be followed by the emission of a photon since the radiative transition A coefficient will dominate the decay probability to the lower level. The relative excitation rates of the D and D levels are proportional to their statistical weights. Since the z-axis components of the total angular momenta J of the D and D levels have a six-fold and four-fold degeneracy respectively, the ratio of strengths of the two lines in this low-density limit is j/j = 1.5 . In the high density limit N, collisional excitation and de-excitations dominate and set up a Boltzmann population ratio. The relative populations of the two levels D and D are in the ratio of their statistical weights, and the relative strengths of the two lines are in the ratio
A and A are the DS and DS radiative transition probabilities [\protect\astronciteOsterbrock1974]. The [OII] intensity ratio measures the electron density best in the neighborhood of N 10 cm, and it could be used to measure electron density as function of position within intermediate redshift galaxies.