Flux ratios of high-ionisation lines [NeV]$λ$3425 and [FeVII]$λ$6087
They looked at some special lines in the galactic light, kind of like how we can use binoculars to see different colors.
Technical definition
The comparison of the intensities of specific emission lines originating from neon V ($NeV) at 342.5 nanometers and iron VII ($FeVII) at 608.7 nanometers.