Question

Why is the planetary nebula distance indicator (PNLF) considered a secondary distance indicator?

Answer 1

PNLF is not the direct way to observe the distances in-universe. It observes the distance by use of the [O III] λ5007 forbidden line. It can be used to determine distances to both spiral and elliptical galaxies despite their completely different stellar populations and is part of the Extragalactic Distance Scale.

This method is unbiased by metallicity because oxygen is a primary nebular coolant, any drop in its concentration raises the plasma's electron temperature and raises the number of collisional excitations per ion. This compensates for having a smaller number of emitting ions in the PNe resulting in little change in the λ5007 emissions. Consequently, a reduction in oxygen density only lowers the emergent [O III] λ5007 emission line intensity by approximately the square root of the difference in abundance.

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