Volume 639, July 2020
|Number of page(s)||15|
|Section||Interstellar and circumstellar matter|
|Published online||06 July 2020|
H I filaments are cold and associated with dark molecular gas
HI4PI-based estimates of the local diffuse CO–dark H2 distribution★
Argelander-Institut für Astronomie,
Auf dem Hügel 71,
2 Tartu Observatory, University of Tartu, 61602 Tõravere, Tartumaa, Estonia
Accepted: 30 April 2020
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)∕NH ratio, independent of phase transitions from H I to H2. We investigate the Doppler temperatures TD, from a Gaussian decomposition of HI4PI data, to study temperature dependences of E(B−V)∕NHI.
Results. The E(B−V)∕NHI 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 TD ≤ 1165 K we find a correlation (NHI + 2NH2)∕NHI ∝−logTD. In combination with a factor XCO = 4.0 × 1020 cm−2 (K km s−1)−1 this yields NH∕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 TD ≤ 1165 K contains on average 46% CO–dark H2. Prominent filaments have TD ≤ 220 K and typical excitation temperatures Tex ~ 50 K. With a molecular gas fraction of ≥61% they are dominated dynamically by H2.
Key words: ISM: clouds / ISM: structure / ISM: molecules / dust, extinction
FITS files for Figs. 11–13 are only available at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (188.8.131.52) or via http://cdsarc.u-strasbg.fr/viz-bin/cat/J/A+A/639/A26
© ESO 2020
Current usage metrics show cumulative count of Article Views (full-text article views including HTML views, PDF and ePub downloads, according to the available data) and Abstracts Views on Vision4Press platform.
Data correspond to usage on the plateform after 2015. The current usage metrics is available 48-96 hours after online publication and is updated daily on week days.
Initial download of the metrics may take a while.