[Paper Review] HI filaments are cold and associated with dark molecular gas. HI4PI based estimates of the local diffuse CO-dark H2 distribution

P. M. W. Kalberla, J. Kerp+1 more|arXiv (Cornell University)|Apr 30, 2020

Astrophysics and Star Formation Studies70 references6 citations

TL;DR

This paper proposes that cold H i filaments (T_D ≤ 1165 K) in the Milky Way's diffuse interstellar medium are associated with CO-dark molecular hydrogen (H₂), using Doppler temperatures from HI4PI data to correct for unaccounted H₂ in dust-to-gas ratios. It finds that cold H i gas correlates with H₂, with 46% of total H in the CNM being CO-dark H₂, and filaments with T_D ≤ 220 K have molecular fractions ≥61%, indicating H₂ dominance.

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 called CO-dark. Aims. We demonstrate that the dust-to-gas ratio is a tool to identify 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 HI to H2. We investigate the Doppler temperatures TD, from a Gaussian decomposition of HI4PI data, to study temperature dependencies of E(B-V)/NHI. Results. The E(B-V)/NHI ratio in the cold HI gas phase is high in comparison to the warmer one. We consider this as evidence that cold HI gas toward high Galactic latitudes is associated with H2. Beyond CO-bright regions we find for TD < 1165 K a correlation (NHI + 2NH2 )/NHI prop -log T_D. In combination with a factor XCO = 4.0 10 20 cm^-2 (K km s^-1 )-1 this yields for the full-sky NH /E(B-V) sim 5.1 to 6.7 10^21 cm^-2 mag^-1, compatible with X-ray scattering and UV absorption line observations. Conclusions. Cold HI with T_D < 1165 K contains on average 46% CO-dark H2. Prominent filaments have TD < 220 K and typical excitation temperatures Tex sim 50 K. With a molecular gas fraction of > 61% they are dominated dynamically by H2.

Motivation & Objective

• To identify and quantify CO-dark molecular hydrogen (H₂) in the diffuse interstellar medium using dust extinction and H i data.
• To determine whether cold H i gas phases correlate with hidden H₂, especially at high Galactic latitudes.
• To develop a temperature-dependent correction for the dust-to-gas ratio that accounts for unobserved H₂ in H i column density measurements.
• To assess the dynamical dominance of H₂ in cold, filamentary H i structures.
• To validate the correction using X_CO = 4.0×10²⁰ cm⁻² (K km s⁻¹)⁻¹ and compare with X-ray and UV observations.

Proposed method

• Gaussian decomposition of HI4PI data to derive Doppler temperatures (T_D) for H i components.
• Use of the E(B-V)/N_HI ratio as a proxy for dust-to-gas ratio, assuming linearity with total N_H even during H i to H₂ phase transitions.
• Empirical derivation of a correction function f_c(T_D) that adjusts E(B-V)/N_HI to estimate E(B-V)/(N_HI + 2N_H2), accounting for CO-dark H₂.
• Application of the X_CO factor (4.0×10²⁰ cm⁻² (K km s⁻¹)⁻¹) to convert CO luminosity to H₂ column density in CO-bright regions.
• Cross-checking with FIR emission and UV absorption data to validate the dust-gas correlation.
• Statistical analysis of 74% of the sky outside CO-bright regions to derive average H₂ fractions and temperature thresholds.

Experimental results

Research questions

• RQ1 Is cold H i gas (T_D ≤ 1165 K) associated with hidden molecular hydrogen not traced by CO?
• RQ2 Does the E(B-V)/N_HI ratio systematically increase with decreasing T_D, indicating unaccounted H₂?
• RQ3 What is the average molecular gas fraction f^N_H2 = 2N_H2 / N_H in the cold neutral medium (CNM) outside CO-bright regions?
• RQ4 How do the physical properties of H i filaments (T_D, T_ex, f^N_H2) relate to their H₂ dominance and dust association?
• RQ5 Can the f_c(T_D) correction be extended to CO-bright regions with a standard X_CO factor?

Key findings

• The E(B-V)/N_HI ratio increases with decreasing T_D, indicating that cold H i gas (T_D ≤ 1165 K) is associated with unaccounted molecular hydrogen.
• A temperature-dependent correction f_c(T_D) is derived, allowing estimation of the true dust-to-gas ratio including CO-dark H₂.
• On average, 46% of the total hydrogen column density in the CNM outside CO-bright regions is in the form of CO-dark H₂.
• Filaments with T_D ≤ 220 K have excitation temperatures T_ex ≈ 50 K and molecular gas fractions f^N_H2 ≥ 61%, indicating H₂ dominance.
• The derived N_H/E(B-V) ratio of 5.1 to 6.7×10²¹ cm⁻² mag⁻¹ is consistent with X-ray scattering and UV absorption line observations.
• The f_c(T_D) correction significantly reduces systematic deviations in the dust-to-gas ratio at high Galactic latitudes, supporting its use for correcting foreground extinction.