Properties of Interstellar Medium in the S0 Galaxy NGC 1222: Evidence for Shock-enhanced Line Emission

Abstract In this paper, we present a comprehensive study on the properties of the interstellar medium in NGC 1222, a star-forming early-type merging galaxy that forms a triple system, using optical and far-infrared spectroscopic, and multiband photometric data. The fit to the spectral energy distribution reveals a high dust content in the galaxy, with a dust-to-stellar mass ratio of M dust / M ⋆ ∼ 3.3 × 10 −3 that is 40–90 times larger than the mean value of local S0 galaxies. By comparing the observed optical emission line ratios to shock models, we suggest that a merger-induced shock, which is further supported by the higher-than-average [O i ] 63 μ m- and [C ii ] 158 μ m-to-PAH ratios, plays a role in heating the gas in NGC 1222. We also show evidence for gas inflow by analyzing the kinematic properties of NGC 1222.