H I filaments are cold and associated with dark molecular gas
P. M. W. Kalberla, J. Kerp, U. Haud
Abstract
Context. There are significant amounts of H 2 in the Milky Way. Due to its symmetry H 2 does not radiate at radio frequencies. CO is thought to be a tracer for H 2 ; however, CO is formed at significantly higher opacities than H 2 . Thus, toward high Galactic latitudes significant amounts of H 2 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 H 2 . Methods. We adopt the hypothesis of a constant E ( B − V )∕ N H ratio, independent of phase transitions from H I to H 2 . 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 H 2 . Beyond CO–bright regions, for T D ≤ 1165 K we find a correlation ( N HI + 2 N H2 )∕ N HI ∝−log T D . In combination with a factor X CO = 4.0 × 10 20 cm −2 (K km s −1 ) −1 this yields N H ∕ E ( B − V ) ~ 5.1 to 6.7 × 10 21 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 H 2 . 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 H 2 .