On the Formation of Density Filaments in the Turbulent Interstellar Medium

Abstract

This study is motivated by recent observations on ubiquitous interstellar density filaments and guided by modern theories of compressible magnetohydrodynamic (MHD) turbulence. The interstellar turbulence shapes the observed density structures. As the fundamental dynamics of compressible MHD turbulence, perpendicular turbulent mixing of density fluctuations entails elongated density structures aligned with the local magnetic field, accounting for low-density parallel filaments seen in diffuse atomic and molecular gas. The elongation of low-density parallel filaments depends on the turbulence anisotropy. When taking into account the partial ionization, we find that the minimum width of parallel filaments in the cold neutral medium and molecular clouds is determined by the neutral-ion decoupling scale perpendicular to magnetic field. In highly supersonic MHD turbulence in molecular clouds, both low-density parallel filaments due to anisotropic turbulent mixing and high-density filaments due to shock compression exist.

How primordial magnetic fields shrink galaxies

“…While current simulations are able to generate realistic galaxies by resolving the multi-phase structure of the ISM and account for a variety of important physical processes (e.g Hopkins et al 2014;Kimm et al 2015;Grisdale et al 2017); they generally neglect magnetic fields, despite magnetic energy being in equipartition with turbulent energy and comparable or above thermal energy, even at high redshifts (z 2, Bernet et al 2008;Wolfe et al 2008;Mulcahy et al 2014;Mao et al 2017;Mulcahy et al 2017). Accordingly, magnetic fields should affect the ISM multi-phase structure and dynamics (Moss et al 2007;Villagran & Gazol 2017;Xu et al 2019;Körtgen et al 2019). They could destabilise spiral arms (Inoue & Yoshida 2018) or reduce the number of star formation sites in the ISM (Hennebelle & Iffrig 2014).…”

The role of Galactic H II regions in the formation of filaments

“…One of their most striking properties is their typical inner width of 0.1 pc that seems to be quasiuniversal (André et al 2016;Arzoumanian et al 2019) and which could result from their formation process (Federrath 2016; André 2017). However, this point is still debated and high-resolution observations are needed to solve the question of filament width universality (Henshaw et al 2017;Ossenkopf-Okada & Stepanov 2019;Xu et al 2019). The study of filaments has attracted much attention being the original matter and host zone of star formation.…”

Scintillation Arc Brightness and Electron Density for an Analytical Noodle Model

“…These small-scale fluctuations may scatter electrons, providing the resistance required for reconnection. A plausible minimum scale for turbulence is the ion inertial length, where electron and ion fluids decouple (Spangler & Gwinn 1990;Xu et al 2019). On the other hand, Lithwick & Goldreich (2001) suggest that the minimum widths of sheets or filaments should be about the ion cyclotron radius.…”

“…Physically, the noodles may take the form of filaments ("spaghetti") or sheets ("lasagna"). Magnetized plasmas often contain over-or under-dense sheets or filaments (Cornwell et al 1989;Armstrong et al 1990;Desai et al 1994;Grall et al 1997;Xu et al 2019) These density variations give rise to changes in refractive index. Such structures are expected to appear at small scales in reconnection sheets, as discussed in detail in Section 1.3 of Paper I.…”