4.6. Potential Evolutionary Transitions
Fornax is the second most luminous dSph galaxy in the Local Group. The young age of its youngest measurable population (~ 200 Myr, Grebel & Stetson 1999) is astonishing considering its lack of gas. Just a few hundred Myr ago Fornax would have been classified as a dIrr. What caused Fornax to lose all of its gas after some 13 Gyr of continuous, decreasing star formation is not clear.
The presence of intermediate-age populations in some of the more distant Galactic dSphs, the possible detection of associated gas in the surroundings of several of them, indications of substantial mass loss discussed elsewhere in this paper, morphological segregation, common trends in relations between their integrated properties, and the apparent correlation between star formation histories and Galactocentric distance all seem to support the idea that low-mass dIrrs will eventually evolve into dSphs if their environment fosters this evolution. DSphs may be the natural final phase of low-mass dIrrs, and the type distinction may be artificial. The six dSph companions of M31 span a similar range in distances as the Milky Way dSphs (Grebel & Guhathakurta 1999). A study of whether their detailed star formation histories (not yet available) show a comparable correlation with distance from M31 would provide a valuable test of the suggested impact of environment.
The mass (traced by the luminosity) of a dwarf galaxy plays a major role in its evolution as indicated by the good correlation between luminosity and mean metallicity (e.g., Caldwell 1999). The observed lack of rotation in dSphs requires that its hypothesized low-mass dIrr progenitor must have gotten rid of its angular momentum, which may occur through substantial mass loss. However, this scenario does not account for the existence of isolated dSphs such as Tucana. Alternatively, the progenitor may have had very little rotation to begin with. Either way, the subsequent fading must have been low since otherwise dIrrs and dSphs would not follow such a fairly well-defined common relation. Several authors (e.g., Mateo 1998) suggested that the luminosity-metallicity relation is instead bimodal with separate loci for dIrrs and dSphs in the sense that at a given luminosity a dIrr tends to be more metal-poor than a dSph, excluding evolutionary transitions. Hunter, Hunsberger, & Roye (2000) go a step further and suggest that a number of Local Group dIrrs might have formed as ancient tidal dwarfs that lack dark matter, are essentially non-rotating, and contribute to the increased scatter in the absolute magnitude-mean metallicity relationship for MB < -15 mag.