A Nonlinear-Open-System Model for Star Formation in Spiral Galaxies

Abstract

We re-examine the discovery of short star-forming duration in late-type galaxies by Tomita et al. (1996 PASJ 48, 285) and propose another interpretation about their results. According to their discussion, in spiral galaxies, especially in early-type spirals, star-formation eras continue for no more than several times 107) years. Moreover, they showed the number distribution of star-formation activities in spiral galaxies and concluded that the dispersion of the distribution is at least 1.5 orders of magnitude. From the above results, we interpret that galaxies must have repetitive star-formation histories. This is because the dispersions easily emerge if recurrent star formations take place. Here, we also note that their results are derived mainly from the data expressing information of the cold component of the interstellar medium (ISM). Turning now to theoretical studies, it is known that a galaxy can exchange its phase of ISM recurrently as a nonlinear open system. Previously, this model was restricted to classes of dwarf galaxies, since there have been no observational evidence indicating cyclic phenomena in an extent of giant galaxies before Tomita et al. We must note that the period of the cycle is about several times 107 years. This period of a cyclic model of ISM is also determined by the evolution of the cold component of ISM. Thus, we comment that both the observational and theoretical periods of cyclic star formation are of the same order. This coincidence indicates that spiral galaxies are nonlinear open systems.