Chemical composition and evolution of irregular and blue compact galaxies. The dawn of a thirty year journey - Commentary on: Lequeux J., Peimbert M., Rayo J. F., et al., 1979, A&A, 80, 155

The paper by J. Lequeux, M. Peimbert, J. Rayo, A. Serrano, and Silvia Torres-Peimbert (hereafter, LPRST) brings us back to the dawn of the studies on the evolution of late-type dwarfs and of galaxies in general. Thirty years ago, when they wrote their paper, a number of milestones had already been published on the chemical evolution of galaxies (e.g. Salpeter 1955; Schmidt 1959; Tinsley 1968; Cameron & Truran 1971; Talbot & Arnett 1971; Lynden-Bell 1975; Audouze & Tinsley 1976; Mayor 1976; Vigroux et al. 1976, online), but none of them dealt explicitly with late-type dwarf galaxies. Lequeux et al. (1979) set the stage for all subsequent studies of the evolution of irregular and blue compact dwarfs (BCDs). The term pioneer is often used (and sometimes misused) in the astronomical literature. In their case, pioneers is really what they were. The aims of their study were manifold and quite challenging for the time: a) derive the primeval abundance of helium by extrapolating the (He/H) vs (O/H) relation to zero oxygen abundance; b) estimate the primary fraction of 14N by studying the (N/O) vs (O/H) relation; c) derive the heavy element content vs the total galactic mass relation; d) derive the heavy element yield p(Z) by means of the (O/H) vs. Mgas/Mtot relation; e) put limits on the evolutionary properties of irregulars and BCDs by comparing the results of the previous items with the predictions of chemical evolution models. Quite an ambitious plan, considering the number of papers devoted to each of these topics in the last decades. “Everything you always wanted to know about dwarf galaxies, but were afraid to ask”, Woody Allen would have said. It is impressive to see how valid the results they achieved were, in spite of the poor tools available at the time, but isn’t this exactly what pioneers do? Consider, for instance, their analysis of the nitrogen evolution, aimed at checking the revolutionary suggestion of a partial primary origin of nitrogen done by Edmunds & Pagel (1978) and Alloin et al. (1979) on the basis of observations of Galactic and extragalactic HII regions. To this aim, LPRST observed the HII regions in six late-type dwarfs and computed chemical evolution models to compare the resulting abundances. Their chemical evolution models were run assuming the instantaneous recycling approximation, an oversimplifying assumption (as they were aware of), especially for an element mostly produced by intermediate-mass, relatively longlived stars. No appropriate tables with nucleosynthesis yields for low and intermediate mass stars had been published yet (the Renzini & Voli 1981, paper, another of the A&A most cited ones, was still to come), Arnett (1978) had just published his fundamental paper on massive star yields and the Chiosi & Caimmi (1979) version with mass loss was just about to appear. Yet, Lequeux et al. were able to put limits on the fraction of primary nitrogen, later confirmed by models computed when stellar lifetimes and appropriate yields could be taken into account (e.g. Matteucci & Tosi 1985; Diaz & Tosi 1986, online). On the primordial helium abundance derived from the extrapolation of the HII region abundances in metal-poor galaxies, the debate has been quite hot for decades between the groups who favored a low value close to Yp = 0.23, similar to the one obtained by LPRST (e.g. Pagel et al. 1992; Olive et al. 1997; Peimbert et al. 2000, and references therein), and those who favored a value higher than 0.24 (e.g. Izotov et al. 1994, online). Only the advent of the deus ex-machina WMAP and its incredibly tight constraints on the products of Big Bang nucleosynthesis (Spergel et al. 2003) has ended the quarrel and induced people to converge on Yp = 0.248 (Izotov et al. 2007; Peimbert et al. 2007). Another aspect that, in my opinion, made this paper so interesting for the readers, and still makes it among the most cited ones in the whole lifetime of A&A, is the combination of good observational data, interpretation tools, and theoretical modeling: a template that authors should always try to adopt. Equally excellent was the choice of the target galaxies, three BCDs (I Zw 18, II Zw 40, and I Zw 70), and three irregulars (NGC 4449, NGC 6822, and IC 10), objects that have become the pet galaxies of many of us, thanks to their intriguing aspects and importance for trying to understand how dwarfs evolve and what the differences and similarities among their various types are. LPRST correctly admitted that their analysis could not establish whether BCDs have star formation (SF) histories significantly different from those of dwarf irregulars, but they felt that the only real difference is in the present SF rate. This is indeed what we think today (e.g. Tosi 2009, and references therein). They also favored the hypothesis that neither BCDs nor irregulars are young systems and that they all contain old stellar populations. We can now tell that they got it right: all the late-type dwarfs observed at telescopes with enough power