H I filaments are cold and associated with dark molecular gas. HI4PI-based estimates of the local diffuse CO-dark H2 distribution

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 H I 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 H I gas phase is high in comparison to the warmer phase. We consider this as evidence that cold H I gas toward high Galactic latitudes is associated with H₂. Beyond CO-bright regions, for T_D ≤ 1165 K we find a correlation (N_HI + 2N_H2)/N_HI ∝-logT_D. In combination with a factor X_CO = 4.0 × 10²⁰ cm^-2 (K km s^-1)^-1 this yields N_H/E(B-V) ~ 5.1 to 6.7 × 10²¹ cm^-2 mag^-1 for the full sky, which is compatible with X-ray scattering and UV absorption line observations.
Conclusions: Cold H I with T_D ≤ 1165 K contains on average 46% CO-dark H₂. Prominent filaments have T_D ≤ 220 K and typical excitation temperatures T_ex ~ 50 K. With a molecular gas fraction of ≥61% they are dominated dynamically by H₂.

FITS files for Figs. 11-13 are only available at the CDS via anonymous ftp to http://cdsarc.u-strasbg.fr (ftp://130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/cat/J/A+A/639/A26