On the Doublet Flux Ratio of Mg II Resonance Lines in and Around Galaxies

Observations of metallic doublet emission lines, particularly Mg II 2796, 2803, provide crucial information for understanding galaxies and their circumgalactic medium. This study explores the effects of resonant scattering on the Mg II doublet lines and the stellar continuum in spherical and cylindrical geometries. Our findings show that under certain circumstances, resonance scattering can cause an increase in the doublet flux ratio and the escaping flux of the lines beyond what are expected in optically thin spherical media. As expected, the doublet ratio is consistently lower than the intrinsic ratio when the scattering medium is spherically symmetric and dusty. However, if the scattering medium has a disk shape, such as face-on disk galaxies, and is viewed face-on, the doublet ratio is predicted to be higher than two. These results may provide a valuable insight regarding the complexity of the shape and orientation of distant, spatially-unresolved galaxies. The importance of the continuum-pumped emission lines and expanding media is discussed to understand various observational aspects, including doublet flux ratios, which can be lower than 1.5 or higher than two, as well as symmetric or asymmetric line profiles. It is also discussed that the diffuse warm neutral medium would be an essential source of Mg II emission lines.