Formation of H I clouds in shock-compressed interstellar medium: physical origin of angular correlation between filamentary structure and magnetic field

Journal Article · Sat Dec 10 00:00:00 EST 2016 · Astrophysical Journal

DOI:https://doi.org/10.3847/0004-637X/833/1/10· OSTI ID:22868394

Inoue, Tsuyoshi ^[1]; Inutsuka, Shu-ichiro ^[2]

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1. Division of Theoretical Astronomy, National Astronomical Observatory of Japan, Osawa 2-21-1, Mitaka, Tokyo 181-0015 (Japan)
2. Department of Physics, Graduate School of Science, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8602 (Japan)

Recent observations of the neutral Galactic interstellar medium showed that filamentary structures of H i clouds are aligned with the interstellar magnetic field. Many interesting applications are proposed based on the alignment, such as measurement of magnetic field strength through the Chandrasekhar–Fermi method and removal of foreground dust emissions for the detection of inflationary polarized emission in the cosmic microwave background radiation. However, the physical origin of the alignment remains to be explained. To understand the mechanism, we examine the formation of H i clouds triggered by shock compression of the diffuse warm neutral medium using three-dimensional magnetohydrodynamic simulations. We show that the shock-compressed medium of density n∼1 cm{sup −3} evolves into H i clouds with n∼50 cm{sup −3} via thermal instability consistent with previous studies. We apply a machine vision transformation developed by Clark et al. to the simulated column density structures to measure angle correlation between filamentary structures of H i clouds and magnetic field. We find that the orientation of H i filaments depends on the environmental turbulent velocity field, particularly on the strength of shear strain in the direction of the magnetic field, which is controlled by the angle between the shock propagation direction and upstream magnetic field. When the strain along the magnetic field is weak, filamentary components of H i clouds lie perpendicular to the magnetic field. However, the filaments have come to align with the magnetic field, if we enhance the turbulent strain along the magnetic field or if we set turbulence in the preshock medium.

• OSTI ID: 22868394
• Journal Information: Astrophysical Journal, Vol. 833, Issue 1; Other Information: Country of input: International Atomic Energy Agency (IAEA); ISSN 0004-637X
• Country of Publication: United States
• Language: English

Related Subjects

79 ASTROPHYSICS
COSMOLOGY AND ASTRONOMY
ANGULAR CORRELATION
BACKGROUND RADIATION
COMPRESSION
COMPUTERIZED SIMULATION
COSMIC DUST
DENSITY
DETECTION
EMISSION
INSTABILITY
INTERSTELLAR MAGNETIC FIELDS
MAGNETOHYDRODYNAMICS
ORIENTATION
RELICT RADIATION
REMOVAL
THREE-DIMENSIONAL CALCULATIONS
TRANSFORMATIONS
TURBULENCE
VELOCITY