The impact of ram pressure on the radio spectral index and magnetic field of NGC 4522: A high-resolution VLA continuum study

TL;DR

This study uses high-resolution VLA observations at $3\ \mathrm{GHz}$ and $10\ \mathrm{GHz}$ to dissect how ram-pressure stripping in NGC 4522 reshapes its radio continuum. By decomposing thermal and nonthermal components and mapping the spectral index and polarized emission, the authors identify distinct physical regimes: (i) a main disk with moderate thermal contribution and relatively flat $\alpha$, (ii) an outer disk where ICM shocks re-accelerate cosmic-ray electrons, flattening the nonthermal spectrum, and (iii) extraplanar clouds with unusually flat or even positive $\alpha$ due to enhanced thermal emission and aging nonthermal electrons aided by ICM–ISM mixing. The polarized emission is highly asymmetric, indicating magnetic-field compression and/or draping on the leading edge, while depolarization and turbulent fields reduce polarization within the disk. The results demonstrate multiple mechanisms by which the ICM interacts with the ISM, and highlight the necessity of high-frequency observations to properly separate thermal and nonthermal contributions in ram-pressure–stripped galaxies. Together with targeted simulations and MUSE data, this work advances our understanding of how cluster environments influence galactic magnetized gas and cosmic-ray populations.

Abstract

We present high-resolution Very Large Array (VLA) continuum observations at S-band ($3$ GHz, $560$ pc scale) and X-band ($10$ GHz, $200$ pc scale) of the ram-pressure-stripped Virgo galaxy NGC 4522, to investigate the characteristics of its radio continuum, spectral index, and magnetic field under the influence of the intracluster medium (ICM). The total radio continuum shows an asymmetry that extends northwest, mirroring the HI gas distribution, but showing distinct features in the extraplanar regions. The spectral index steepens systematically from $α\sim-0.6$ in the main disk to $α\sim-1.1$ in the outer disk. We find that the spectral index behavior of the outer disk is mainly due to an ICM shock that can re-accelerate electrons and a significant reduction of thermal emission. Intriguingly, extraplanar clouds exhibit exceptionally flat spectral indices ($α\sim-0.2$ to $0$), resulting from a combination of significantly enhanced thermal emission and pronounced spectral aging of the non-thermal component. Although some of these regions correlate with H$α$, others do not. We propose that the mixing between the ICM and interstellar medium (ISM) is an alternative mechanism that enhances thermal emission independently of star formation. Polarized continuum emissions are highly asymmetric, preferentially distributed along the ICM wind side, and the polarization fraction increases radially outward from the galactic midplane, indicating that the polarized emission is strongly influenced by the ICM wind. Our results show how and where the ICM substantially affects the ISM, and also demonstrate that high-frequency observations are crucial for analyzing the radio continuum of ram pressure stripping galaxies.

Figures (15)

• Figure 1: A composite map showing H$\alpha$ (red contours, Boselli_2018_vestige), ALMA $^{12}$CO(1-0) (yellow contours, 2018ApJ...866L..10L), H1 (blue contours, 2009AJ....138.1741C), and optical (background, HST WFC F814W) of NGC 4522. This galaxy has a highly asymmetric H1 disk with extraplanar molecular gas and associated star-forming regions. The black dashed box indicates the region that is the focus of this study.
• Figure 2: Top: A composite map of S-band total continuum intensity (cyan contours) overlaid with H$\alpha$ (red contours), H1 (blue contours), and optical (background, HST WFC F814W) of NGC 4522. The contour levels of the S-band continuum are (4, 7, 10, 20, 35, 50, 90) $\times$$6$$\mu$Jy beam$^{-1}$. The synthesized beam of $7\farcs8\times6\farcs0$ is shown in the bottom-left corner. Compared to the stellar disk, the S-band continuum emission appears asymmetric, extending toward the northwest, similar to the H1 gas distribution. Bottom: Same as the top panel but for the X-band total continuum convolved to the S-band resolution. The contour levels of the X-band continuum are (4, 6, 8, 20, 35, 50) $\times$$5.5$$\mu$Jy beam$^{-1}$. The convolved synthesized beam of $7\farcs8\times6\farcs0$ is shown in the bottom-left corner. At this resolution and sensitivity, the X-band continuum is also extended toward the northwest. The expected direction of the ICM wind is indicated by the black arrow.
• Figure 3: Same as \ref{['fig:ngc4522_total_S']}, but overlaid with ALMA $^{12}$CO(1-0) emission, (yellow contours, 2018ApJ...866L..10L) instead of H1.
• Figure 4: Spectral index map between the S-band and X-band total continuum. H1 and H$\alpha$ are shown as blue and red contours, respectively. Only pixels where the convolved X-band emission exceeds $4\times\sigma_{\mathrm{rms,convol}}$ ($\sigma_{\mathrm{rms,convol}}\approx5.5\mu$Jy beam$^{-1}$) are displayed. Six regions of interest (R1 -- R6) are indicated by green circles. The spectral index generally steepens from the midplane to large scale heights, but increases again in the extraplanar regions, where very complex variations and extremely flat spectral indices are observed. The inset shows the spectral index distribution, with orange and blue histograms representing the extraplanar region (R3 -- R6 and their vicinity) and the rest of the galaxy, respectively. Overall, the spectral index is slightly flatter ($-1.2$ to $0.1$) than that reported by 2004AJ....127.3375V from L- and C-band data ($-2$ to $-0.7$).
• Figure 5: Top: S-band polarized continuum intensity (magenta contours) and apparent magnetic field vectors (yellow pseudo-vectors, uncorrected for Faraday rotation) overlaid on the optical image. Blue and cyan contours represent H1 and the S-band total continuum, respectively. The contour levels of the polarized emission are (3.5, 4.5, 7) $\times~6\mu$Jy beam$^{-1}$. Polarized emission is highly asymmetric and mainly concentrated near the ICM wind front (southeast region). The polarization angles are roughly parallel to the galactic disk in this area. Bottom: Same as the top panel but for the X-band. The contour levels of the polarized emission are (4, 7) $\times~ 1.5\mu$Jy beam$^{-1}$. As in the S-band, X-band polarized emission also appears near the ICM wind front and extends toward the northeast. The magnetic field vectors are nearly parallel to the H1 distribution, suggesting that the ISM in this region is strongly influenced by the ICM. Note that the X-band total-intensity map in this panel is shown at its native resolution ($2\farcs8\times2\farcs2$).