Self-Organized Large-Scale Coherence in Simulations of Galactic Star Formation

It is often assumed that galaxies cannot generate large-scale coherent star- forming activity without some organizing agent, such as spiral density waves, bars, large-scale instabilities, or external perturbations due to encounters with other galaxies. We present simulations of a simple model of star formation in which local spatial couplings lead to large-scale coherent, and even synchronized, patterns of star formation without any explicit propagation or any separate organizing agent. At a given location, star formation is assumed to occur when the gas velocity dispersion falls below a critical value dependent on the density. Young stars inject energy into the gas in their neighborhood, increasing the velocity dispersion and inhibiting the instability. A dissipation function continually "cools" the gas. The stability of this local inhibitory feedback model is examined both analytically and numerically. A large number of two-dimensional simulations are used to examine the effect of spatial couplings due to energy injection into neighboring regions. We find that several distinct types of behavior can be demarcated in a phase diagram whose parameter axes are the density (assumed constant in most models) and spatial coupling strength. These "phases" include, with decreasing density, a spatially homogeneous steady state, oscillatory "islands," traveling waves of star formation or global synchronization, and scattered "patches" of star formation activity. The coherence effects are explained in terms of the ability of the energy injected near a star formation site to introduce phase correlations in the subsequent cooling curves of neighboring regions.