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 to analyze how ram pressure from the intracluster medium influences the radio spectral index and magnetic fields in galaxy NGC 4522, revealing asymmetries, spectral steepening, and enhanced thermal emission.

Contribution

It provides detailed high-resolution radio continuum data of NGC 4522, demonstrating the effects of ram pressure on spectral index, thermal emission, and magnetic fields, highlighting the importance of high-frequency observations.

Findings

Spectral index steepens from -0.6 to -1.1 from disk center to outer regions.

Extraplanar clouds show flat spectral indices due to thermal emission and spectral aging.

Polarized emission is asymmetric and influenced by ICM wind.

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 $\alpha\sim-0.6$ in the main disk to $\alpha\sim-1.1$ 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 ($\alpha\sim-0.2$ 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$\alpha$, 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.