Observations of Stellar Feedback: Tracers, Diagnostics, Measurements, and Physical Interpretation

Stellar feedback is central to galaxy evolution, yet the coupling of radiation, stellar winds, and supernovae to the multi-phase interstellar medium remains uncertain. This review assesses this as a problem of inference and frames the discussion around four qualitative axes of ignorance: observational diagnostics are indirect; stellar population synthesis models need to be ground-truthed before they can be used to infer feedback quantities; recent high-redshift observations have opened a new regime that potentially requires re-calibrating; and stellar feedback is interdependent of the affected environment. The inference chain of feedback, which requires conversions from observables to physical quantities and from those quantities to an interpretation of how energy and momentum are transferred to the surrounding medium, is examined across the principal observational diagnostics of stellar feedback: from ionised gas and mechanically-driven bubbles, to giant molecular cloud lifetimes, galactic outflows, and Lyman-continuum escape. This addresses the overarching question of how much observations actually allow us to know about the link between the feedback-driving massive stars and the feedback-driven interstellar medium. Further, we discuss how the assumptions that go into stellar population synthesis models—along with environmental (e.g., metallicity, dust content) and galaxy properties (e.g., stellar mass, gas content)—can affect feedback measurements, as well as how recent high-redshift observations are testing calibrations established in the nearby universe. The emerging picture is that the interpretation of a feedback measurement requires control over both the inference chain and the physical context in which the measurement is made; consequently, feedback should be contextualised, meaning that measurements must be interpreted in terms of the feedback mechanism, gas phase, environment, and spatial–temporal scale. Progress in the field will depend on measuring feedback-related quantities across the full range of star-forming systems, and more importantly, on obtaining observations (“real” as well as synthetic from postprocessed simulations) that match the same tracer and spatial scale across the same systems.