A Stratification in Magnetic Field Structures: The Radio Outflow in NGC 4151

TL;DR

This study uses VLA polarization observations of NGC 4151 at 3 and 10 GHz to map kpc-scale magnetic fields in a nearby radio-quiet AGN. The data reveal a stratified outflow consisting of a jet spine (B ⟂ outflow), a surrounding sheath (B ∥ outflow), and a magnetized wind with toroidal/helical fields, accompanied by RM gradients indicative of a helical field. The wind is massive but kinetically weak, with a mass outflow of ~$0.01$–$0.03\ M_\odot\,\mathrm{yr^{-1}}$ and kinetic power of ~$(3-10)\times10^{40}$ erg s$^{-1}$, resulting in a coupling efficiency of ~$(10^{-4}-0.005)$ relative to L$_{bol}$, suggesting limited galaxy-scale feedback but clear magnetically driven processes in a radio-quiet AGN. Overall, the work demonstrates that radio-quiet AGN can host magnetically dominated jets and winds, challenging the view that strong jets are exclusive to radio-loud systems and highlighting the role of magnetic fields in shaping AGN outflows.

Abstract

The nature of radio outflows in radio-quiet AGN remains poorly understood. In this study, we present kpc-scale polarization observations of the Seyfert galaxy NGC\,4151 using the Karl G. Jansky Very Large Array (VLA) in B-array at 3 and 10 GHz. We find that the inferred magnetic (B-) field structures show a stratification: a `spine-sheath'-like structure, with fields perpendicular to the jet direction in the `spine' and parallel in the `sheath', is observed in the higher resolution (0.5 arcsec) image at 10 GHz. In addition, a `wind'-like component with B-fields perpendicular to the radio outflow is observed in the 3 GHz image (resolution 2 arcsec); this feature is prominent along the `receding' (eastern) jet direction. Rotation measure (RM) ranges from $-230$ to 250 rad m$^{-2}$ over the polarized regions, indicating a low-electron-density ($10^{-2}-10^{-3}$ cm$^{-3}$) tenuous medium surrounding the source causing Faraday rotation. A {tentative} RM gradient of $+75$ to $-25$ rad m$^{-2}$ is observed transverse to the northern `wind' component, while a similar gradient with opposite sign is seen across the southern `wind' component, suggestive of a helical magnetic field threading the outflow. Based on an analysis of the available radio and X-ray data, we conclude that the stratified radio outflow in NGC 4151 is magnetically-driven. The bi-conical radio `wind' is found to be massive ($1050-3200 M_\odot$) with a high mass outflow rate ($0.01-0.03$ M$_\odot$ yr$^{-1}$) but low in kinetic power ($<0.01$% of L$_{\rm{bol}}$), making it less impactful for galactic-scale feedback. Our study suggests that radio-quiet AGN may also host magnetically dominant jets and winds, even while their jets are smaller and weaker compared to radio-loud AGN.

Figures (8)

• Figure 1: Top Left: 3 GHz image of NGC 4151 at a resolution of $\sim$ 2 arcsec, obtained using the VLA B-array. The polarization vectors are shown in red ticks with 6 $\arcsec$ of length proportional to a fractional polarization of 10%. The contour levels shown in black are at 5$\sigma\times$ ($\pm1$, 2, 4, 8, 16, 32, 64, 128, 256) with $\sigma=80~\mu$Jy beam$^{-1}$. The gray-scale varies from $-0.8$ to 80 mJy beam$^{-1}$ in logarithmic scale. The dark horizontal line marks the distance scale of $\sim$ 350 pc (4 arcsec). The synthesized beam shown at the bottom left corner of the image is of size 2.1 arcsec $\times$ 1.7 arcsec at a PA of 85$\degr$. Top Right: 10 GHz image of NGC 4151 at a resolution of $\sim$ 0.5 arcsec obtained using the VLA B-array. The polarization vectors are shown in red ticks with 2 arcsec proportional to a fractional polarization of 50%. The contour levels shown in black are at 3$\sigma\times$ ($\pm1$, 2, 4, 8, 16, 32, 64, 128, 256, 512) with $\sigma=8~\mu$Jy beam$^{-1}$. The gray-scale varies from $-0.07$ to 2 mJy beam$^{-1}$ in logarithmic scale. The synthesized beam is of size 0.7 arcsec $\times$ 0.6 arcsec at a PA of 89$\degr$. Bottom Left & Right: Inferred magnetic-field vectors in the NGC 4151 outflow at 3 (left) and 10 GHz (right) by rotating EVPAs by 90$\degr$. The B-field shows a stratification and is found to be similar to what is predicted for a jet 'spine', jet 'sheath' and a magnetically driven 'wind' Laing2014Mehdipour2019. North is to the top and East to the left in this and all subsequent figures.
• Figure 2: Left: The in-band 3 GHz RM image, blanked by an RM error of 20 rad m$^{-2}$. The 3 GHz contours are shown in black, the 10 GHz contours in red. The blue and the yellow dashed lines indicate the transverse slices taken across the jet. Right: The RM and RM error values are shown with a solid line and a shaded region, respectively, corresponding to the slices drawn on the left image. The slices taken from bottom to top on the left figure are represented from left to right on the right figure.
• Figure 3: Top left: RM image of NGC 4151, using polarization data at 3 GHz, 9 GHz and 11 GHz, shown in color. The contours from 10 GHz data (Figure \ref{['fig:3GHzimage']}, right) are overplotted in black. The lowest contour from 3 GHz data has been shown in red. The RM values are shown from $-200$ to $+200$ rad m$^{-2}$. Top right: The RM error image shown in colour with values ranging from 0 to 40 rad m$^{-2}$. Bottom left: The depolarization image of NGC 4151 with values ranging from 0 to 1. The contours from 3 GHz data (Figure \ref{['fig:3GHzimage']}, left) are overplotted in black. Bottom right: The depolarization error image shown in colour with values ranging from 0 to 0.15.
• Figure 4: Left: Spectral index image made from 3 GHz and 10 GHz total intensity images. The spectral index values are shown in colour with values ranging from $-2$ to 0. The uniformly weighted 3 GHz Stokes I image has been overlaid with black contours with levels similar to Figure \ref{['fig:3GHzimage']}, left panel. The synthesized beam is of size 1.7 arcsec $\times$ 1.4 arcsec at a PA of 75$\degr$. Right: The spectral index error image in colour with values ranging from $0$ to $0.2$.
• Figure 5: Values of $N_{\rm H}$ and the radio loudness parameter, $R$, estimated for NGC 4151 (blue bar) superimposed to the anti-correlation (black line) found for radio-loud sources Mehdipour2019. The dashed line simply extends the correlation to our estimate for NGC 4151.