H I filaments are cold and associated with dark molecular gas

HI4PI-based estimates of the local diffuse CO–dark H₂ distribution^★

¹ Argelander-Institut für Astronomie, Auf dem Hügel 71, 53121 Bonn, Germany
e-mail: pkalberla@astro.uni-bonn.de
² Tartu Observatory, University of Tartu, 61602 Tõravere, Tartumaa, Estonia

Received: 28 January 2020
Accepted: 30 April 2020

Abstract

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.