The dark secrets of gaseous nebulae -- highlights from deep spectroscopy

In this contribution, I will briefly review the development of the theory of photoionized gaseous nebulae, highlighting some of the key events. I will then present some recent developments of deep spectroscopy of planetary nebulae (PNe) and H II regions, concentrating on observations of faint heavy element optical recombination lines (ORLs). I will show that there is strong evidence that nebulae contain another previously unknown component of cold (about 1,000 K), high-metallicity plasma, probably in the form of H-deficient inclusions embedded in the warm (about 10,000 K) diffuse nebula of "normal (i.e. near solar) composition". This cold gas emits essentially all the observed fluxes of heavy element ORLs, but is too cool to excite any significant optical or ultraviolet collisionally excited lines (CELs) and thus invisible via the latter. The existence of H-deficient gas in PNe and probably also in H II regions, not predicted by the current stellar evolution theory, provides a natural solution to the long-standing dichotomy between nebular plasma diagnostics and abundance determinations using CELs on the one hand and ORLs on the other, a discrepancy that is ubiquitously observed in Galactic and extragalactic PNe as well as H II regions.