MeerKAT observations of the spiral galaxy NGC 2997 in the S band. Detection of high dynamo modes

TL;DR

This study uses MeerKAT's S-band radio observations and Fourier analysis of Faraday rotation measures to reveal multiple large-scale magnetic field modes in the spiral galaxy NGC 2997, demonstrating advanced magnetic field mapping techniques.

Contribution

First application of Fourier analysis to RM data in a spiral galaxy, revealing multiple magnetic modes and demonstrating MeerKAT's enhanced capabilities for magnetic field studies.

Findings

Detection of three large-scale magnetic field modes in NGC 2997.

No field reversal at 3 kpc radius, contrary to previous reports.

Identification of even modes consistent with dynamo theory predictions.

Abstract

We seek to exploit the expanded observational range of the MeerKAT radio telescope with the new S-band receivers (2.0-2.8 GHz). To showcase its enhanced capabilities, we conducted new S-band observations of the galaxy NGC 2997 in full polarization. The S band is ideal for studying magnetic fields in spiral galaxies due to the weak Faraday depolarization. Performing a rotation measure (RM) synthesis allowed us to measure Faraday RMs in the galaxy, a signature of regular magnetic fields. A fast Fourier transform (FFT) algorithm was used to study the various azimuthal modes found in the RM data of the galaxy. The RM synthesis analysis indicates the direction of the magnetic field along the line of sight throughout the entire disk. Leveraging the sensitivity and high resolution provided by MeerKAT's S-band capability, this study achieves an unprecedented level of detail of the magnetic field structure. Our sector-based analysis of the RMs across azimuthal regions reveals the existence of modes of the large-scale magnetic field in NGC 2997. The variations in the RM values along the azimuthal angle reveal smoothly changing phase shifts between the rings, without the previously reported field reversal at about 3 kpc radius between the central region and disk. In this work, for the first time, a Fourier analysis has been applied to RM data averaged in sectors of rings in the disk plane of a spiral galaxy. Our Fourier analysis of the RM map shows three different large-scale field modes detected in the disk of NGC 2997. After applying a geometric modification, even multiples of the first mode were detected, as predicted from theoretical studies of dynamo action in a spiral galaxy with symmetric spiral structure. Our new method opens up new possibilities for investigating magnetic fields in spiral galaxies.