Estimating Electron Temperatures in Ionized Nebulae: The Direct Method and its Limitations

In this paper we examine the Direct Method for measuring electron temperatures in H ii regions, and the extent to which such measurements can provide meaningful information on the physical conditions in these regions. We discuss the limits to what can be inferred about electron temperatures from nebular emission line fluxes. We provide a new simplified method for estimating electron temperatures, including parameters that can be used to determine this from UV [O iii] and [O ii] oxygen lines observable in high-redshift objects using ground-based telescopes. We test this method on published UV high redshift observations and compare the results with reported electron temperatures.