ultra-low luminosity galaxies

I spent part of the day carefully reading a paper by Wu on her analysis of the mid-IR and spectral properties of some extremely low luminosity galaxies in the SDSS. She finds that the deficit of PAH emission in low-luminosity star-forming galaxies is related to the metallicity and to the radiation fields in the galaxies, but both of these properties are closely related to one another and to many other properties, so the causal relations are not yet known, in my view.