SIMBAD references

Context. There are significant amounts of H₂ in the Milky Way. Due to its symmetry H₂ does not radiate at radio frequencies. CO is thought to be a tracer for H₂; however, CO is formed at significantly higher opacities than H₂. Thus, toward high Galactic latitudes significant amounts of H₂ are hidden and are called CO-dark.
Aims. We demonstrate that the dust-to-gas ratio is a tool for identifying locations and column densities of CO-dark H₂.
Methods. We adopt the hypothesis of a constant E(B-V)/N_H ratio, independent of phase transitions from HI to H₂. We investigate the Doppler temperatures T_D, from a Gaussian decomposition of HI4PI data, to study temperature dependences of E(B-V)/N_HI.
Results. The E(B-V)/N_HI ratio in the cold HI gas phase is high in comparison to the warmer phase. We consider this as evidence that cold HI gas toward high Galactic latitudes is associated with H₂. Beyond CO-bright regions, for T_D≤1165K we find a correlation (N_HI+2N_H2)/N_HI∝-logT_D. In combination with a factor X_CO=4.0x10²⁰cm^–2/(K.km/s) this yields N_H/E(B-V)∼5.1 to 6.7x10²¹cm^–2mag^–1 for the full sky, which is compatible with X-ray scattering and UV absorption line observations.
Conclusions. Cold HI with T_D≤1165K contains on average 46% CO-dark H₂. Prominent filaments have T_D≤220K and typical excitation temperatures T_ex∼50K. With a molecular gas fraction of ≥61% they are dominated dynamically by H₂.