Distribution of Velocity Gradient Orientations: Mapping Magnetization with the Velocity Gradient Technique

Abstract

Recent developments of the Velocity Gradient Technique (VGT) show that the velocity gradients provide a reliable tracing of magnetic field direction in turbulent plasmas. In this paper, we explore the ability of velocity gradients to measure the magnetization of interstellar medium. We demonstrate that the distribution of velocity gradient orientations provides a reliable estimation of the magnetization of the media. In particular, we determine the relation between Alfvenic Mach number M A in the range of M A ∈ [0.2, 1.7] and properties of the velocity gradient distribution, namely, with the dispersion of velocity gradient orientation as well as with the peak to base ratio of the amplitudes. We apply our technique for a selected GALFA-HI region and find the results consistent with the expected behavior of M A . Using 3D MHD simulations we successfully compare the results with our new measure of magnetization that is based on the dispersion of starlight polarization. We demonstrate that, combined with the velocity dispersion along the line of sight direction, our technique is capable to delivering the magnetic field strength. The new technique opens a way to measure magnetization using other gradient measures such as synchrotron intensity gradients (SIGs) and synchrotron polarization gradients (SPGs).

Magnetic field morphology in interstellar clouds with the velocity gradient technique

“…The VGT thus has an advantage for studying magnetic fields, especially for molecular clouds at low Galactic latitudes when the foreground and background polarization contributions are important. Other differences include the difference of the accumulation of the signal along the line of sight for gradients and polarization, which is especially important in the case of super-Alfvenic (M A > 1) turbulence 31 . However, the good correlation between the VGT and the polarization measurement of magnetic field orientations shown in Table 1 demonstrates that all these factors are sub-dominant for the clouds at hand.…”

“…However, Regions of gravitational collapse are expected to turn the direction of velocity gradients by 90 • with respect to magnetic fields 46 , which we will discuss in the following section. Lazarian et al (2018) 31 µ is the expectation of the relative angle between the un-rotated gradients and the magnetic field derived from Planck polarization. The uncertainty is given by the standard error of the mean, i.e.…”

“…Table 1 illustrates the good correspondence between the two values. The advantage of the VGT compared to the traditional DCF technique is its ability to measure the not only mean magnetization but also a detailed distribution of the magnetization using a self-consistent algorithm 31 , which is important for better understanding of star formation and other key astrophysical processes.…”

“…The VGT has been numerically tested for a wide range of column densities from diffuse transparent gas 19,28,29 to molecular self-absorbing dense gas 30 . It was shown to be able to provide both the orientations of the magnetic field as well as a measure of media magnetization 31,32 . The technique has been used to study magnetic field in diffuse H I 19,20,31,33 and was shown to be complementary to other methods of tracing magnetic field 21 .…”

“…While some of the structure in the spectroscopic data may not be due to MHD turbulence, recent studies of VGT show the way of identifying these situations and using them to study other important interstellar physics, e.g. the gravitational collapse of a cloud of interstellar matter 31 .…”