Abstract
Context. The atomic phase of the interstellar medium plays a key role in the formation process of molecular clouds. Due to the line-of-sight confusion in the Galactic plane that is associated with its ubiquity, atomic hydrogen emission has been challenging to study. Aims. We investigate the physical properties of the “Maggie” filament, a large-scale filament identified in H I emission at line-of-sight velocities, vLSR ~−54 km s−1. Methods. Employing the high-angular resolution data from The H I/OH Recombination line survey of the inner Milky Way (THOR), we have been able to study H I emission features at negative vLSR velocities without any line-of-sight confusion due to the kinematic distance ambiguity in the first Galactic quadrant. In order to investigate the kinematic structure, we decomposed the emission spectra using the automated Gaussian fitting algorithm GAUSSPY+. Results. We identify one of the largest, coherent, mostly atomic H I filaments in the Milky Way. The giant atomic filament Maggie, with a total length of 1.2 ± 0.1 kpc, is not detected in most other tracers, and it does not show signs of active star formation. At a kinematic distance of 17 kpc, Maggie is situated below (by ≈500 pc), but parallel to, the Galactic H I disk and is trailing the predicted location of the Outer Arm by 5−10 km s−1 in longitude-velocity space. The centroid velocity exhibits a smooth gradient of less than ±3 km s−1 (10 pc)−1 and a coherent structure to within ±6 km s−1. The line widths of ~10 km s−1 along the spine of the filament are dominated by nonthermal effects. After correcting for optical depth effects, the mass of Maggie’s dense spine is estimated to be 7.2−1.9+2.5 × 105 M⊙. The mean number density of the filament is ~4 cm−3, which is best explained by the filament being a mix of cold and warm neutral gas. In contrast to molecular filaments, the turbulent Mach number and velocity structure function suggest that Maggie is driven by transonic to moderately supersonic velocities that are likely associated with the Galactic potential rather than being subject to the effects of self-gravity or stellar feedback. The probability density function of the column density displays a log-normal shape around a mean of ⟨NH I⟩ = 4.8 × 1020 cm−2, thus reflecting the absence of dominating effects of gravitational contraction. Conclusions. While Maggie’s origin remains unclear, we hypothesize that Maggie could be the first in a class of atomic clouds that are the precursors of giant molecular filaments.
Highlights
Stars form in the cold, dense interiors of molecular clouds
We investigate the physical properties of the “Maggie” filament, a large-scale filament identified in H i emission at line-of-sight velocities, local standard of rest (LSR) ∼ −54 km s−1
What is the molecular gas fraction of Maggie and are there signatures of molecular cloud formation? To address this, we investigated the molecular lines 12CO, 13CO, and C18O (J = 1–0) using the Milky Way Imaging Scroll Painting (MWISP; Su et al 2019) survey
Summary
Stars form in the cold, dense interiors of molecular clouds. The physical properties of these clouds set the initial conditions under which star formation takes place. A key question in understanding the star formation process as part of the global interstellar matter cycle addresses the formation of large-scale molecular clouds out of the diffuse atomic phase of the interstel-. The ISM has a hierarchical structure and facilitates the formation of filaments that are governed by the Galactic potential on a large scale. Soler et al (2020) present a network of H i filaments already evident in the diffuse atomic phase of the ISM that is structured mostly parallel to the Galactic plane. Maggie is shown to be a highly elongated filamentary cloud (see Fig. 1) extending over ∼4◦ on the sky in Galactic longitude. Given its central velocity of LSR ≈ −54 km s−1 and assuming circular motion, Maggie is located approximately 17 kpc away from us and has a length of more than 1 kpc
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