A Far-ultraviolet Fluorescent Molecular Hydrogen Emission Map of the Milky Way Galaxy

Abstract

We present the far-ultraviolet (FUV) fluorescent molecular hydrogen (H 2 ) emission map of the Milky Way Galaxy obtained with FIMS/SPEAR covering ∼76% of the sky. The extinction-corrected intensity of the fluorescent H 2 emission has a strong linear correlation with the well-known tracers of the cold interstellar medium (ISM), including color excess E(B–V) , neutral hydrogen column density N (H i ), and H α emission. The all-sky H 2 column density map was also obtained using a simple photodissociation region model and interstellar radiation fields derived from UV star catalogs. We estimated the fraction of H 2 ( f H2 ) and the gas-to-dust ratio (GDR) of the diffuse ISM. The f H2 gradually increases from <1% at optically thin regions where E(B–V) < 0.1 to ∼50% for E(B–V)  = 3. The estimated GDR is ∼5.1 × 10 21 atoms cm −2 mag −1 , in agreement with the standard value of 5.8 × 10 21 atoms cm −2 mag −1 .