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

¹ Max-Planck-Institut für Astronomie, Königstuhl 17, 69117 Heidelberg, Germany
e-mail: syed@mpia.de
² University of Vienna, Department of Astrophysics, Türkenschanzstrasse 17, 1180 Vienna, Austria
³ Harvard Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA, 02138, USA
⁴ Chalmers University of Technology, Department of Space, Earth and Environment, 412 93 Gothenburg, Sweden
⁵ Department of Physics and Astronomy, The University of Calgary, 2500 University Drive NW, Calgary AB T2N 1N4, Canada
⁶ Jet Propulsion Laboratory, California Institute of Technology, 4800 Oak Grove Drive, Pasadena, CA 91109, USA
⁷ Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
⁸ Universität Heidelberg, Zentrum für Astronomie, Institut für Theoretische Astrophysik, Albert-Ueberle-Str. 2, 69120 Heidelberg, Germany
⁹ Universität Heidelberg, Interdisziplinäres Zentrum für Wissenschaftliches Rechnen, INF 205, 69120 Heidelberg, Germany
¹⁰ Argelander-Institut für Astronomie, Auf dem Hügel 71, 53121 Bonn, Germany
¹¹ Centre for Astrophysics and Planetary Science, University of Kent, Canterbury CT2 7NH, UK
¹² National Radio Astronomy Observatory, PO Box O, 1003 Lopezville Road, Socorro, NM 87801, USA
¹³ Department of Physics, Indian Institute of Science, Bengaluru 560012, India
¹⁴ I. Physik. Institut, University of Cologne, Zülpicher Str. 77, 50937 Cologne, Germany
¹⁵ Jodrell Bank Centre for Astrophysics, Department of Physics and Astronomy, University of Manchester, Oxford Road, Manchester M13 9PL, UK
¹⁶ Astrophysics Research Institute, Liverpool John Moores University, IC2, Liverpool Science Park, 146 Brownlow Hill, Liverpool L3 5RF, UK

Received: 7 May 2021
Accepted: 1 November 2021

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, v_LSR ~−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 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 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 ⟨N_H 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.