2022A&A...657A...1S

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 HI emission at line-of-sight velocities, v_LSR ~-54 km/s. Methods. Employing the high-angular resolution data from The HI/OH Recombination line survey of the inner Milky Way (THOR), we have been able to study HI emission features at negative v_LSR 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 HI 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 HI disk and is trailing the predicted location of the Outer Arm by 5-10 km/s in longitude-velocity space. The centroid velocity exhibits a smooth gradient of less than ±3km/s/(10pc) and a coherent structure to within ±6km/s. The line widths of ∼10km/s 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 x 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 <N_HI> = 4.8 x 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.