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On the Origin of the H I Holes in the Interstellar Medium of Dwarf Irregular Galaxies

We suggest that large H I holes observed in the interstellar medium (ISM) of galaxies such as the Large Magellanic Cloud (LMC), the Small Magellanic Cloud (SMC), and Holmberg II (Ho II, DDO 50, UGC 4305) can form as the combined result of turbulence coupled with thermal and gravitational instabilities. We investigate this problem with three-dimensional hydrodynamic simulations, taking into account cooling and heating processes and the action of the self-gravity of the gas. We construct an algorithm for radiative transfer to postprocess the simulated data and build emission maps in the 21 cm neutral hydrogen line. With this approach, we are able to reproduce the structure of the shells and holes as observed in regions of the ISM where no stellar activity is detected. In order to quantify the comparison of our synthetic maps to the observations, we calculate the physical scale-autocorrelation length relation (L-Lcr relation) both on the synthetic H I maps and the H I map of Ho II. The L-Lcr relation shows a linear increase of the autocorrelation length with the physical scale up to the scale of energy injection and flattens for larger scales. The comparison of the L-Lcr relation between the observations and the synthetic maps suggests that turbulence is driven in the ISM of Ho II on large scales (~6 kpc). The slope of the L-Lcr relation in the linear regime in Ho II is better reproduced by models where turbulence is coupled with a low-efficiency cooling of the gas. These results demonstrate the importance of the interplay between turbulence and the thermodynamics of the gas for structure formation in the ISM. Our analysis can be used to determine the scale on which kinetic energy is injected into the ISM of dwarf irregular galaxies and to derive, in a first approximation, the cooling rate of the gas.