Abstract
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.