It is clear that both radiative and mechanical feedback energies can dominate evolutionary processes in star-forming galaxies, although we still lack understanding of important aspects in the feedback processes. The extreme range in scale over which feedback has influence is especially remarkable. In addition, it is also possible to investigate and parameterize chemical feedback with analogous methods (e.g., Oey 2000, 2003).
Radiative feedback is responsible for the nebular emission-line diagnostics and tracers of star formation, as well as the WIM component of the ISM. On a detailed level, the line diagnostics still need improved calibrations and photoionization modeling to increase their utility and parameter space. The H II LF is emerging as a quantitative diagnostic of global star formation in galaxies, and reveals a universal law in the stellar membership function for massive stars (equation 6). The ionization and energy budget of the WIM, while apparently tied to massive stars, remains to be better understood. Ultimately, the escape of ionizing radiation beyond the parent galaxies bears upon the intergalactic environment and reionization of the early Universe.
Mechanical feedback indisputably drives shell structures that are ubiquitous in the ISM: SNRs, stellar wind-driven bubbles, and superbubbles. The standard, adiabatic model for the evolution of the bubbles and superbubbles is broadly consistent with a variety of empirical evidence; yet, quantitatively, a number of outstanding problems remain. The late evolution of the shells is especially enigmatic and critical for understanding the role of feedback in generating the HIM, as well as the global properties of the ISM. Spatial correlations of interstellar shells with star-forming regions still need to firmly establish the physical processes and interactions related to feedback. Preliminary studies of the size and velocity distributions of H I shells appear to confirm a dominant role for mechanical feedback in structuring the ISM. This analytic analysis can be extended to parameterize the interstellar porosity, implying a critical threshold for the outflow of superwinds, heavy elements, and ionizing radiation.
I am grateful to the conference organizers and STScI for supporting in part this contribution to the Symposium.