Nature of striation in 21 cm channel Maps: velocity caustics

Abstract

The alignment of striated intensity structures in thin neutral hydrogen (HI) spectroscopic channels with Galactic magnetic fields has been observed. However, the origin and nature of these striations are still debatable. Some studies suggest that the striations result solely from real cold-density filaments without considering the role of turbulent velocity fields in shaping the channel’s intensity distribution. To determine the relative contribution of density and velocity in forming the striations in channel maps, we analyze synthetic observations of channel maps obtained from realistic magnetized multi-phase HI simulations with thermal broadening included. We vary the thickness of the channel maps and apply the Velocity Decomposition Algorithm to separate the velocity and density contributions. In parallel, we analyze GALFA-HI observations and compare the results. Our analysis shows that the thin channels are dominated by velocity contribution, and velocity caustics mainly generate the HI striations. We show that velocity caustics can cause a correlation between unsharp-masked HI structures and far-infrared emission. We demonstrate that the linear HI fibers revealed by the Rolling Hough Transform (RHT) in thin velocity channels originate from velocity caustics. As the thickness of channel maps increases, the relative contribution of density fluctuations in channel maps increases and more RHT-detected fibers tend to be perpendicular to the magnetic field. Conversely, the alignment with the magnetic field is the most prominent in thin channels. We conclude that similar to the Velocity Channel Gradients (VChGs) approach, RHT traces magnetic fields through the analysis of velocity caustics in thin channel maps.

Magnetic Fields in Giant Filaments Probed by the Velocity Gradient Technique: Regular Magnetic Field Interrupted by Magnetization Gaps

“…The VGT (González-Casanova & Lazarian 2017; Hu et al 2018; is the main analysis tool used in this work, and was developed based on the anisotropy of MHD turbulence (Goldreich & Sridhar 1995) and fast turbulent reconnection theories (Lazarian & Vishniac 1999). To extract the velocity information from the position-position-velocity (PPV) cube of the spectral line, the thin velocity channels Ch(x, y) were employed, in which the width of a thin velocity channel is less than the velocity dispersion of the spectral lines observed along the individual line of sight (Hu et al 2023). We use velocity-resolved maps to study the structure of the magnetic field, using Equations (1), (2), and (3), where the velocity-resolved map is the PPV cube of the spectral line, and the results are valid as long as the velocity channel is smaller than the velocity dispersions along the individual lines of sight (the cloud is well resolved in velocity).…”

“…Recently, a new method, called the velocity gradient technique (VGT; González-Casanova & Lazarian 2017;Hu et al 2023), has emerged, offering a fresh approach to estimating magnetic field orientation by analyzing velocity-resolved maps that trace the gas. The VGT is based on the theory of the anisotropy of magnetohydrodynamic (MHD) turbulence, where turbulent eddies tend to stretch along the magnetic field lines (Goldreich & Sridhar 1995; Lazarian & Vishniac 1999).…”

“…By examining the alignment between the magnetic field orientation and the velocity gradient of the turbulent eddies, it becomes possible to infer the magnetic field through the motion of these eddies. The validity of this approach has been demonstrated through MHD simulations and observations of turbulent media (Hu et al 2019a(Hu et al , 2019c(Hu et al , 2023Zhao et al 2022). Taking advantage of this technique, we measure the magnetic fields of six giant filaments in the Milky Way and study the relation between evolution and the magnetic fields in these giant filaments.…”