Formation of cavities, filaments, and clumps by the nonlinear development of thermal and gravitational instabilities in the interstellar medium understellar feedback

Based on our high-resolution two-dimensional hydrodynamical simulations, we propose that large cavities may be formed by the nonlinear development of the combined thermal and gravitational instabilities, without need for stel lar energy injection in a galaxy modeling the Large Magellanic Cloud (LMC). Our numerical model of star formation allows us to follow the evolution of the blast waves due to supernovae in the inhomogenous, multiphase, and tur bulent-like media self-consistently. Formation of kiloparsec-scale inhomoge neity, such as cavities as seen in the observed H I map of the LMC, is supp ressed by frequent supernovae (the average supernova rate for the whole dis k is similar to 0.001 yr(-1)). However, the supernova explosions are necess ary for the hot component (T-g > 10(6)-10(7) K). Position-velocity maps sho w that kiloparsec-scale shells/arcs formed through nonlinear evolution in a model without stellar energy feedback have kinematics similar to explosive phenomena, such as supernovae. We also find that dense clumps and filament ary structure are formed as a natural consequence of the nonlinear evolutio n of the multiphase interstellar medium (ISM). Although the ISM on a small scale looks turbulent-like and transient, the global structure of the ISM i s quasi-stable. In the quasi-stable phase, the volume filling factor of the hot, warm, and cold components are similar to 0.2, similar to 0.6, and sim ilar to 0.2, respectively. We compare observations of H I and molecular gas of the LMC with the numerically obtained H I and CO brightness temperature distributions. The morphology and statistical properties of the numerical H I and CO maps are discussed. We find that the cloud mass spectra of our m odels represent a power-law shape, but their slopes change between models w ith and without the stellar energy injection. We also find that the slope d epends on the threshold brightness temperature of CO.