Observational evidence of inverted level population of hydrogen atoms in ionized gas

ABSTRACT Electron temperature is one of the key parameters in ionized gas. The intensity ratio of the observed hydrogen radio recombination lines (RRLs) to the free–free continuum of the corresponding frequency is often used to calculate the electron temperature in ionized gas. Two assumptions, local thermal equilibrium (LTE) and optically thin for both RRLs and continuum, were normally used for such calculation. However, these two assumptions may not be true in most cases resulting in wrong electron temperature. In this work, the observational data of H93$\alpha$ in the mini-starburst region, W49A, are used to find the evidence of inverted level population of hydrogen atoms in ionized gas. With an improved method for diagnosing inverted level population of hydrogen atoms using line-to-continuum ratio of RRLs, evidence of inverted level populations of hydrogen atoms in ionized gas was found in three regions of W49A. Inverted level population of hydrogen should be considered for calculating electron temperature with RRLs in ionized gas. The electron temperature in ionized gas derived with the observations of RRLs and free–free continuum under the canonical optically thin LTE assumption might not be a good approximation.