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The ionization parameter U is potentially useful for measuring radiationpressure feedback from massive star clusters, as it reflects theradiation-to-gas-pressure ratio and is readily derived from mid-infrared lineratios. We consider several effects which determine the apparent value of U inHII regions and galaxies. An upper limit is set by the compression of gas byradiation pressure. The pressure from stellar winds and the presence of neutralclumps both reduce U for a given radiation intensity. The most intenselyirradiated regions are selectively dimmed by internal dust absorption ofionizing photons, inducing observational bias on galactic scales. We explorethese effects analytically and numerically, and use them to interpret previousobservational results.  We find that radiation confinement sets the upper limit log_10 U = -1 seen inindividual regions. Unresolved starbursts display a maximum value of ~ -2.3.While lower, this is also consistent with a large portion of their HII regionsbeing radiation dominated, given the different technique used to interpretunresolved regions, and given the bias caused by dust absorption. We infer thatmany individual, strongly illuminated regions cannot be dominated by stellarwinds, and that even when averaged on galactic scales, shocked wind pressurescannot be large compared to radiation pressure. Therefore, most HII regionscannot be adiabatic wind bubbles. Our models imply a metallicity dependence inthe physical structure and dust attenuation of radiation-dominated regions,both of which should vary strongly across a critical metallicity of aboutone-twentieth solar.

Ionization Parameter as a Diagnostic of Radiation and Wind Pressures in H II Regions and Starburst Galaxies