Abstract
We present a study of the filamentary structure in the emission from the neutral atomic hydrogen (HI) at 21 cm across velocity channels in the 40′′ and 1.5-km s−1 resolution position-position-velocity cube, resulting from the combination of the single-dish and interferometric observations in The HI/OH/recombination-line survey of the inner Milky Way. Using the Hessian matrix method in combination with tools from circular statistics, we find that the majority of the filamentary structures in the HI emission are aligned with the Galactic plane. Part of this trend can be assigned to long filamentary structures that are coherent across several velocity channels. However, we also find ranges of Galactic longitude and radial velocity where the HI filamentary structures are preferentially oriented perpendicular to the Galactic plane. These are located (i) around the tangent point of the Scutum spiral arm and the terminal velocities of the Molecular Ring, around l ≈ 28° and vLSR ≈ 100 km s−1, (ii) toward l ≈ 45° and vLSR ≈ 50 km s−1, (iii) around the Riegel-Crutcher cloud, and (iv) toward the positive and negative terminal velocities. A comparison with numerical simulations indicates that the prevalence of horizontal filamentary structures is most likely the result of large-scale Galactic dynamics and that vertical structures identified in (i) and (ii) may arise from the combined effect of supernova (SN) feedback and strong magnetic fields. The vertical filamentary structures in (iv) can be related to the presence of clouds from extra-planar HI gas falling back into the Galactic plane after being expelled by SNe. Our results indicate that a systematic characterization of the emission morphology toward the Galactic plane provides an unexplored link between the observations and the dynamical behavior of the interstellar medium, from the effect of large-scale Galactic dynamics to the Galactic fountains driven by SNe.
Highlights
The diffuse neutral atomic hydrogen (HI) is the matrix within which star-forming clouds reside and the medium that takes in the energy injected by stellar winds, ionizing radiation, and supernovae (SNe, see, for example, Kulkarni & Heiles 1987; Dickey & Lockman 1990; Kalberla & Kerp 2009; Molinari et al 2014)
We present a study of the filamentary structure in the emission from the neutral atomic hydrogen (HI) at 21 cm across velocity channels in the 40 and 1.5-km s−1 resolution position-position-velocity cube, resulting from the combination of the single-dish and interferometric observations in The HI/OH/recombination-line survey of the inner Milky Way
Using the Hessian matrix method in combination with tools from circular statistics, we find that the majority of the filamentary structures in the HI emission are aligned with the Galactic plane
Summary
The diffuse neutral atomic hydrogen (HI) is the matrix within which star-forming clouds reside and the medium that takes in the energy injected by stellar winds, ionizing radiation, and supernovae (SNe, see, for example, Kulkarni & Heiles 1987; Dickey & Lockman 1990; Kalberla & Kerp 2009; Molinari et al 2014). The structure of the HI emission in small velocity intervals has revealed a multitude of filamentary (slender or thread-like) structures, which were first identified in the earliest extended observations (see, for example, Weaver & Williams 1974; Heiles & Habing 1974; Colomb et al 1980) Many of these filaments are curved arcs that appear to be portions of small circles on the sky. The diameters of these arc structures change with velocity in the manner expected for expanding shells (Heiles 1979, 1984; McClure-Griffiths et al 2002) These observations constitute clear evidence of the injection of energy into the ISM
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