Molecular hydrogen in filaments at high Galactic latitudes

TL;DR

This work tests whether H2 traces CNM filaments at high Galactic latitudes by correlating H2 absorption with Hi and FIR filaments identified as Hessian caustics. It combines stellar and FUSE H2 data with Hi/FIR filament maps to show that nearly all H2 lines of sight intersect Hi filaments and that most H2 velocities align with HI filament velocities, while FIR associations are more uncertain due to line-of-sight confusion. The results indicate that H2 formation is enhanced in dense CNM filaments, reinforcing a filamentary CNM–H2 connection and suggesting that the CNM in filaments provides favorable conditions for H2 formation. The findings support a broadened view of the CNM–H2 transition in the diffuse ISM and motivate further high-resolution HI and H2 absorption studies to refine the filamentary picture.

Abstract

Context. Neutral atomic hydrogen (HI) absorption lines can be used to probe the cold neutral medium (CNM) at high Galactic latitudes. Cold HI with a significant optical depth from the GASKAP-HI survey is found to be located predominantly if not exclusively within filamentary structures that can be identified as caustics with the Hessian operator. Most of these HI filaments (57%) are also observable in the far-infrared (FIR) and trace the orientation of magnetic field lines. Aims. We considered whether molecular hydrogen (H2) might also be preferentially associated with CNM filaments. Methods. We analyzed 241 H2 absorption lines against stars and determined whether the lines of sight intersected HI or FIR filaments. Using Far Ultraviolet Spectroscopic Explorer (FUSE) H2 data in the velocity range -50 < vLSR < 50 km/s, we traced 65 additional H2 lines for filamentary HI and FIR structures in velocity and probed the H2 absorption for coincidences in position and velocity. Results. For 305 out of 306 positions, the lines of sight with H2 absorption intersect HI filaments. In 120 cases, there is also evidence for a correlation with dusty FIR filaments. All of the 65 available sight lines with known velocities intersect HI filaments. In 64 cases, the H2 velocities are consistent with HI filament velocities. For FIR filaments, an agreement is found for only 13 out of 14 H2 absorption lines. Conclusions. For the majority of H2 absorption lines, there is evidence that H2 is associated with cold HI filaments. Evidence of an association with FIR filaments is less compelling. Confusion along the line of sight limits the detectability of FIR filaments. For a comparable degree of UV excitation in the disk and lower Galactic halo, the formation rate of H2 appears to be enhanced in HI filaments with increased CNM densities.

Figures (6)

• Figure 1: Average filament coverage fractions $F_c$ for FIR and Hi filaments within a single velocity channel at Galactic latitudes $|b| > 10\degr$.
• Figure 2: Comparison of velocities from H$_2$ absorption and filaments in Hi and FIR. Top: Direct comparison between $v_\mathrm{H2}$ and filament velocities $v_\mathrm{HI}$ (black) or $v_\mathrm{FIR}$ (red) as defined in Sect. \ref{['Filaments']}. Bottom: Histogram of the velocity deviations $v_\mathrm{H2} - v_\mathrm{fil}^{\mathrm{HI}}$ for Hi and FIR filaments. H$_2$ data are taken from Wakker2006. The dotted blue curve represents a reference distribution with $\sigma v_\mathrm{turb}^{\mathrm{HI}} = 2.48$ km s$^{-1}$.
• Figure 3: Comparison of velocities from H$_2$ absorption and filaments in Hi and FIR. Top: Direct comparison between $v_\mathrm{H2}$ and filament velocities $v_\mathrm{HI}$ (black) or $v_\mathrm{FIR}$ (red) as defined in Sect. \ref{['Filaments']}. Bottom: Histogram of the velocity deviations $v_\mathrm{H2} - v_\mathrm{fil}^{\mathrm{HI}}$ for Hi and FIR filaments. H$_2$ data are taken from Gillmon2006a. The dotted blue curve represents a reference distribution with $\sigma v_\mathrm{turb}^{\mathrm{HI}} = 2.48$ km s$^{-1}$.
• Figure 4: H$_2$ fraction $\mathrm{log}f_\mathrm{H2}$ derived by Gillmon2006a vs. observed velocity deviations $v_\mathrm{H2} - v_\mathrm{fil}$. The filament velocities are $v_\mathrm{HI}$ (black) or $v_\mathrm{FIR}$ (red), as defined in Sect. \ref{['Filaments']}.
• Figure 5: H$_2$ column densities $\mathrm{log}N_\mathrm{H2}$ derived by Wakker2006 vs. observed velocity deviations $v_\mathrm{H2} - v_\mathrm{fil}$. The filament velocities are $v_\mathrm{HI}$ (black) or $v_\mathrm{FIR}$ (red), as defined in Sect. \ref{['Filaments']}.