Measuring the Alfvénic Nature of the Interstellar Medium: Velocity Anisotropy Revisited

Abstract

The dynamics of the interstellar medium (ISM) are strongly affected by turbulence, which shows increased anisotropy in the presence of a magnetic field. We expand upon the Esquivel & Lazarian method to estimate the Alfvén Mach number using the structure function anisotropy in velocity centroid data from position-position-velocity maps. We utilize 3D magnetohydrodynamic (MHD) simulations of fully developed turbulence, with a large range of sonic and Alfvénic Mach numbers, to produce synthetic observations of velocity centroids with observational characteristics such as thermal broadening, cloud boundaries, noise, and radiative transfer effects of carbon monoxide. In addition, we investigate how the resulting anisotropy-Alfvén Mach number dependency found in Esquivel & Lazarian (2011) might change when taking the second moment of the position-position-velocity cube or when using different expressions to calculate the velocity centroids. We find that the degree of anisotropy is related primarily to the magnetic field strength (i.e. Alfvén Mach number) and the line-of-sight orientation, with a secondary effect on sonic Mach number. If the line-of-sight is parallel to up to ≈45 deg off of the mean field direction, the velocity centroid anisotropy is not prominent enough to distinguish different Alfvénic regimes. The observed anisotropy is not strongly affected by including radiative transfer, although future studies should include additional tests for opacity effects. These results open up the possibility of studying the magnetic nature of the ISM using statistical methods in addition to existing observational techniques.

Velocity Gradients as a Tracer for Magnetic Fields

“…These intrinsic properties of the eddies imprinted by the Alfvénic turbulence imply not only the condition of a preferential direction along the local magnetic field, but also that the eddy velocity depends the size of the eddy. The effects of the anisotropy of the velocity fluctuations on the turbulent medium have been described by analyzing intensity anisotropies of spectral line cube velocity channels Burkhart et al 2014;Esquivel et al 2015;Kandel et al 2016b), correlations of velocity centroids (Esquivel & Lazarian 2005;Federrath et al 2010;Kandel et al 2016a), the bispectrum (Burkhart et al 2009), and higher order statistical moments (Kowal et al 2007), as well as using Principal Component Analysis (PCA) (Heyer et al 2008). All these techniques, however, require that the statistical samples that limit their ability to trace magnetic fields over sufficiently small patches of the sky.…”

“…The most popular of them are the first moments of spectral line (see Münch & Wheelon 1958;Kleiner & Dickman 1985;O'dell & Castaneda 1987;Miesch et al 1999). Velocity centroids were also suggested as a means of measuring anisotropy of turbulence using velocity correlations (Esquivel & Lazarian 2005Burkhart et al 2014). A theoretical elaboration of the latter technique using the analytical description of PPV (Lazarian & Pogosyan 2000, 2008 was obtained in Kandel et al (2016a).…”

Damping of Alfvén Waves by Turbulence and Its Consequences: From Cosmic-Ray Streaming to Launching Winds

“…tral lines (see Chepurnov et al 2010Chepurnov et al , 2015 or synchrotron emission (see Lazarian & Pogosyan 2012, 2016, while the Alfven Mach number M A can be obtained using anisotropy studies with spectral lines (see Esquivel & Lazarian 2005, Burkhart et al 2014, Esquivel, Lazarian & Pogosyan 2015 or synchrotron studies (see Lazarian & Pogosyan 2012, 2016, Herron et al 2016. In particular the variations of L and M A with the distance from the observer can be obtained using multifrequency polarization studies as explained in Lazarian & Pogosyan (2016).…”

The Phase Coherence of Interstellar Density Fluctuations

“…Probability distribution functions of column densities, including different measures obtained with them, e.g., skewness and kurtosis (Burkhart et al 2009(Burkhart et al , 2010, Tsalis statistics (Esquivel & Lazarian 2010;Toffelmire, Burkhart & Lazarian 2011), and dispersion (Burkhart & Lazarian 2012), were considered as tools to obtain the properties of turbulence from observations. Together with the techniques for the anisotropy of turbulence studies Esquivel & Lazarian 2011;Heyer et al 2008;Burkhart et al 2014) and intermittency studies (Padoan et al 2004;Kowal et al 2007), they present an impressive toolbox for quantitative studies of interstellar turbulence.…”