MeerKAT observations of the spiral galaxy NGC 2997 in the S band

Detection of high dynamo modes

¹ Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
² Thüringer Landessternwarte, Sternwarte 5, 07778 Tautenburg, Germany
³ SKA Observatory, Jodrell Bank, Lower Withington, Macclesfield SK11 9FT, United Kingdom
⁴ South African Radio Astronomy Observatory, Liesbeek House, River Park, Cape Town 7705, South Africa
⁵ National Radio Astronomy Observatory, 520 Edgemont Road, Charlottesville, VA 22903, USA
⁶ William H. Miller III Department of Physics & Astronomy, Johns Hopkins University, 3400 North Charles Street, Baltimore, MD 21218, USA
⁷ Jodrell Bank Centre for Astrophysics, Department of Physics and Astronomy, The University of Manchester, Manchester M13 9PL, UK
⁸ National Radio Astronomy Observatory, P.O. Box O 1003 Lopezville Road, Socorro, NM 87801, USA
⁹ Center for Astrophysics | Harvard & Smithsonian, 60 Garden Street, Cambridge, MA 02138, USA
¹⁰ Centre for Radio Astronomy Techniques and Technologies (RATT), Department of Physics and Electronics, Rhodes University, Makhanda 6140, South Africa
¹¹ Institute for Radioastronomy, National Institute of Astrophysics (INAF IRA), Via Gobetti 101, 40129 Bologna, Italy

^⋆ Corresponding author: adamas@mpifr-bonn.mpg.de

Received: 22 April 2025
Accepted: 27 August 2025

Abstract

Aims. 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.

Methods. For the data calibration procedure, we utilized the Max Planck MeerKAT Galactic Plane Survey (MMGPS) pipeline, capable of performing full-Stokes calibration, self-calibration, and imaging of MeerKAT data. 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.

Results. The final radio maps in total and polarized intensity are characterized by root mean square (rms) noise of 11 μJy beam⁻¹ at a resolution of about 4″, enabling a detailed examination of the galaxy. The total radio intensity map reveals the spiral arm structure associated with star-forming regions and the inner ring around the nucleus. Additionally, the galaxy exhibits strong polarized emission indicative of a large-scale ordered magnetic field. This ordered field traces the optical spiral structure of the disk in the south, whereas we see larger pitch angles in the north. 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. Further refinement approaches would involve computing the RMs, while correcting for the inclination of the disk and considering the position angle of the major axis. 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.

Conclusions. 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.