Are interactions the primary triggers of star formation in dwarf galaxies

We investigate the assumption that the trigger of star formation in dwarf galaxies is interactions with other galaxies, in the context of a search for a ‘primary’ trigger of a first generation of stars. This is cosmologically relevant because the galaxy formation process consists not only of the accumulation of gas in a gravitational potential well but also of the triggering of star formation in this gas mass, and also because some high-z potentially primeval galaxy blocks look like nearby star-forming dwarf galaxies. We review theoretical ideas proposed to account for the tidal interaction triggering mechanism and present a series of observational tests of this assumption using published data. We also show results of a search in the vicinity of a composite sample of 96 dwarf late-type galaxies for interaction candidates showing star formation. The small number of possible perturbing galaxies identified in the neighbourhood of our sample galaxies, along with similar findings from other studies, supports the view that tidal interactions may not be relevant as primary triggers of star formation. We conclude that interactions between galaxies may explain some forms of star formation triggering, perhaps in central regions of large galaxies, but they do not seem to be significant for dwarf galaxies and, by inference, for first-time galaxies forming at high redshifts. Intuitive reasoning, based on an analogy with stellar dynamics, shows that conditions for primary star formation triggering may occur in gas masses oscillating in a dark-matter gravitational potential. We propose this mechanism as a plausible primary trigger scenario, which would be worth investigating theoretically.