JWST imaging of the Pleiades: anisotropy of turbulence in the cold neutral medium

TL;DR

This work uses JWST/NIRCam imaging of PAH emission in the Pleiades CNM at sub-parsec scales ($\sim 0.2~\mathrm{mpc}$) to probe the anisotropy of MHD turbulence in the cold neutral medium. Through careful data cleaning and a two-dimensional Fourier analysis, the authors extract highly anisotropic PAH power spectra and identify an isotropic CIB residual, modeling it with both ON-OFF and anisotropy-based approaches. The PAH spectra follow power laws with indices around $-3.5$ near Merope and $-3$ in more distant regions, and the anisotropy orientation aligns with Planck-derived magnetic-field directions, indicating an anisotropic turbulent cascade in the CNM. The study links the observed turbulence to the radiation-stellar environment of the Pleiades and develops a framework for integrating JWST observations with MHD turbulence theory, while highlighting the need for targeted simulations to fully interpret the 3D anisotropic density structure.

Abstract

Interstellar medium studies rely on magnetohydrodynamic (MHD) turbulence as a framework for interpretation. In this context, the statistical characterization of interstellar observations is of prime importance. We open a new perspective on diffuse interstellar matter by analyzing James Webb Space Telescope (JWST) observations of the Pleiades nebula with NIRCam. These observations are remarkable in that they provide a microscope view at the cold neutral medium (CNM) with a spatial resolution of 0.2 mpc (40 au). A two-dimensional Fourier analysis is used to characterize the structure of PAH emission in regions near and far from the Pleiades star Merope. To produce maps of the interstellar emission, stars and galaxies are filtered out. The final step in the data cleaning involves subtracting a component, in Fourier space, which we infer to be a residual of the near-infrared cosmic background. The PAH emission power spectra are highly anisotropic. They are well fitted with a break-free power-law, suggesting that we do not observe a specific scale for energy dissipation. Power-law indices are -3.5 near Merope and -3 in the more distant field. The magnetic field orientation, as derived from the Planck dust polarization data, aligns with the PAH anisotropy. The power anisotropy is constant across scales. These findings are discussed in relation to interstellar turbulence that may be driven by the Pleiades stars. The JWST observations of the Pleiades offer a new viewpoint for comparing observations and theoretical models, as they examine physical scales at which turbulence in the CNM is subsonic and decoupled from the thermal instability. The observations may indicate that the turbulent energy cascade in the CNM is anisotropic.

Figures (21)

• Figure 1: Overview of the F335M observations in the Pleiades. Top left: cutout from the digitized Palomar Sky Survey (POSS II, $\lambda_{eff}\sim480~\rm{nm}$). Top right: three color composition from Spitzer/IRAC (red: 8.0 $\mu$m, green 5.8 $\mu$m, blue 3.6 $\mu$m). Merope saturates the IRAC detectors and causes the dark cross. Bottom: F335M observations at both positions in our JWST/NIRCam program. Field 2 is on the left and Field 1 on the right. Note that the color scale is not the same in both images. The footprint of the Spitzer/IRAC image is shown in the POSS II image and those of the JWST/NIRCam images are shown in the Spitzer/IRAC image as rectangles. The red and blue stars on the rightmost JWST image indicate the positions of Merope and PQ Tau. The four black square boxes display the location of the square maps analyzed in this paper. The Cut 1 images are north of the Cut 2 images.
• Figure 2: JWST images of diffuse sky emission. The panels show the average image of the four pairs we analyse; the top row shows Field 1 images, and the bottom row Field 2 images. The Cut 1 images are to the left and the Cut 2 images to the right. The mean values of each image are set to zero. The center positions and the position angles of the vertical axes of the images can be found in Table \ref{['tab:coordinates']}.
• Figure 3: Power spectra fo Field 1 Cut 1 at different steps of the data cleaning. The top spectrum is that of the initial JWST image with the stars, their spikes, and galaxies. The bottom one is that of the image at the end of cleaning. The dashed lines show the spectra of the maps of stars, spikes and galaxies subtracted to the JWST image. The wavenumber is expressed in units of $\rm pix^{-1}_{JWST}$ in the bottom axis and au on the top axis.
• Figure 4: Cross-power spectra for the four JWST images. Distinct colors and symbols identify the spectra of the images listed in the top right-hand corner. The spectra are corrected for beam attenuation. Due to the large dynamic range of the four spectra, the statistical error bars are too small to be visible on the plot.
• Figure 5: Angular dependence of the power spectrum amplitude for the Field 1 image Cut 1. The angular dependence is plotted for 5 $k$ values listed in the top right corner. The position angle (horizontal axis) is measured modulo $\pi$ with respect to the Celestial North and positive to the east. The dashed lines show the fits of the Gaussian model in Eq. \ref{['eq:PS_model']} to the profiles.