Exploring Case B I: The Footprints of Hidden Radiation Fields in H I Balmer Decrements

Abstract

Theory predicts that H I emission from photoionized clouds should lie close to the Case B limit established by Menzel and Baker, and observations of nearby nebulae have largely confirmed this expectation. However, recent observations of more distant high-redshift systems have revealed a small fraction with striking departures from Case B. What additional physics might be at work in these objects? This paper is part of a series exploring the physical processes that can produce such departures. In Paper I (D26), we show that Case B remains remarkably robust over a broad range of physical conditions, consistent with observations of nearby nebulae. Here, we explore one additional ingredient: a soft, non-ionizing radiation field. Such a continuum can strongly perturb the Balmer spectrum. The largest effects occur for radiation fields resembling log(Teff K)\sim 3.2-4.2 blackbodies, where photoexcitation from the metastable 2s level alters the hydrogen level populations and subsequent radiative cascade. The resulting Halpha/Hbeta and Hgamma/Hbeta ratios follow a different trajectory from dust reddening, allowing the two effects to be distinguished using multiple Balmer lines. In unresolved systems, a cool source located near the line-emitting gas may provide an additional radiation field distinct from the hot stars responsible for ionization. Balmer decrements may therefore carry information about otherwise hidden components of the radiation field surrounding the emitting gas.