Discovery of the Hybrid Response of Photoionized Gases

Abstract Photoionized gases are prevalent throughout the Universe. In such gases, the ion concentration typically exhibits two response modes to radiation: a positive response in the low-ionization state and a negative response in the high-ionization state. Here, we report the discovery of a widespread misalignment at the boundary between the above two response modes and identify a third mode—the hybrid response—through time-dependent photoionization simulations. This phenomenon arises from the asynchrony among the ionization rate, recombination rate, and ion column density. Among these, only the ionization rate can respond instantaneously to changes in radiation. Consequently, the initial rate of change in the column density of N i ion is given by − N i I i + N i −1 I i −1 . However, this quantity is typically nonzero at the peak of N i , leading to a misalignment between the boundaries of positive and negative responses. Such hybrid effects introduce additional complexity in the interpretation of gas properties, highlighting the need for further investigation.