The “Maggie” filament: Physical properties of a giant atomic cloud

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.

The Galactic dynamics revealed by the filamentary structure in atomic hydrogen emission

“…However, the characterization of the emission in terms of filaments highlights a large number of elongated structures that are reminiscent of the plumes in the HI emission toward the outskirts of nearby spiral galaxies (Walter et al 2008). Recent studies have identified prominent elongated structures in the HI emission, for example, the 1.1-kpc-long Cattail filament with roughly 6.5 × 10 4 M (Li et al 2021) and the 1.2-kpc-long Magdalena filament with approximately 7.2 × 10 5 M (Syed et al 2022). Our results indicate that these elongated structures are the norm toward the outer Galaxy.…”

Mapping the Thermal Condensation of Diffuse H i in the North Celestial Pole Loop

“…Modeling the emission spectrum requires approximating the total brightness temperature emitted by the gas that would be observed in the absence of the background radio source. This is usually achieved by linear interpolation of the emission spectra in the area surrounding the source (e.g., Heiles & Troland 2003a;Murray et al 2015Murray et al , 2018Syed et al 2021). Specifically, we calculated the average spectrum taken in an annulus centered on the source with an inner radius r 1.2 in = ¢ and an outer radius r 3 out = ¢ (i.e., 4 and 10 pixels of size 18″, respectively).…”

“…Syed et al 2021). Rearranging Equation (9) to use the entries in the table leads to σ nt ≈ 1 km s −1 , as entered in Table3.…”

HI filaments as potential compass needles? Comparing the magnetic field structure of the Small Magellanic Cloud to the orientation of GASKAP-HI filaments

“…High spatial resolution observations have revealed that the H gas in the Milky Way is organised into highly filamentary structures (e.g., McClure-Griffiths et al 2006;Clark et al 2014;Martin et al 2015;Kalberla et al 2016;Blagrave et al 2017;Soler et al 2020;Skalidis et al 2022;Campbell et al 2022;Soler et al 2022;Syed et al 2022). Upon comparisons with starlight and dust polarisation data, it has been found that the elongation of these slender (with presumed widths of 0.1 pc) H filaments is often aligned with their ambient magnetic field orientations (McClure-Griffiths et al 2006;Clark et al 2014Clark et al , 2015Martin et al 2015;Kalberla et al 2016;Clark & Hensley 2019, see Skalidis et al 2022 for a counter-example).…”