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

TL;DR

MeerKAT S-band polarization observations of NGC 2997 reveal the galaxy's large-scale magnetic field with high detail. RM synthesis and sector-based RM analysis identify three azimuthal dynamo modes (m = 0, 2, 4) after geometric correction, consistent with dynamo theory for a spiral with symmetric arms. The analysis finds no evidence for a field reversal near 3 kpc, challenging previous reports. The work demonstrates a powerful method combining RM synthesis, sector averaging, and Fourier analysis to study galactic dynamos in external galaxies.

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.

Figures (12)

• Figure 1: Total radio intensity maps of NGC 2997 detected in the S band with MeerKAT. The scales are in Jy beam$^{-1}$. Left: Total radio intensity applying a robust 0 weighting during the cleaning process. This map shows an rms of $11\,\mu\rm{Jy\,beam}^{-1}$ at a resolution of 12.8" x 9.8", with a position angle of -33.5$^\circ$. The beam size is shown at the bottom left corner. Right: Total radio intensity applying robust -1 weighting during the cleaning process to show the inner ring of the central region of the galaxy. The rms of this map is $1.5\,\mu\rm{Jy\,beam}^{-1}$ at a resolution of 4.6" x 3.7", with a position angle of -34$^\circ$.
• Figure 2: Total flux density of different observations of NGC 2997 compared to the MeerKAT S-band observations. The points represent observations listed in Table \ref{['flux_literature_table']}. Blue circles in this plot correspond to studies that fully recover the extended emission of the source. These data points were used to estimate the spectral index of the total flux densities. This linear fit is represented by the black dashed line. Orange triangles represent observations that did not fully recover the extended emission of the source. These data points were not included in the fit. Inset: Total intensity emission (robust 0 weighting) in 8 spectral windows. The maps in each spectral window were smoothed to the resolution at the lowest frequency and corrected with the corresponding primary beam model. The blue circles represent the flux density integrated over the whole galaxy, integrated out to a radius of 250$^{\prime\prime}$. The red circle represents the integrated flux density over the whole bandwidth. The rms per spectral window is about $40\,\mu$Jy beam$^{-1}$ which gives a flux density error of about 1 mJy, similar to the symbol size. The general uncertainty of the absolute calibration scale of about 3% is not included in this plot.
• Figure 3: Left: Polarized intensity of NGC 2997 in color scale using robust parameter of 2 for the weighting and contours, corresponding to the total radio intensity. The scales are in Jy beam$^{-1}$. The contour levels are (3, 5, 8, 16, 32, and 50) $\times\,15.5\,\mu\rm{Jy\,beam}^{-1}$ at a resolution of $32^{\prime\prime}\times26^{\prime\prime}$. The beam size is shown at the bottom left corner of the image. The intrinsic polarization angles are rotated by 90$^\circ$ to show the orientations of the ordered magnetic field on the plane of the sky. Right: Polarized intensity of NGC 2997 in color scale using robust 0 weighting at a resolution of $15.2^{\prime\prime}\times11.6{^{\prime\prime}}$.
• Figure 4: Degree of polarization map of NGC 2997. The color scale indicates the percentage of polarized signal. The beam size of $16^{\prime\prime} \times 16^{\prime\prime}$ is represented at the bottom left corner with a black circle. The thermal contribution of the total intensity data has not been removed. An estimate of the thermal emission with the help of an IR image at $24\,\mu$m would be possible, but is beyond the scope of this paper.
• Figure 5: Left: Total radio intensity contours on top of a color scale map of the RMs after applying RM synthesis. The scales are in $\rm{rad\,m}^{-2}$. The contour levels are (3, 5, 8, 16, 32, and 50) $\times\,15.5\,\mu\rm{Jy\,beam}^{-1}$ at a resolution of $32^{\prime\prime}\times26^{\prime\prime}$. The foreground RM of our Galaxy has not been subtracted. Right: Same total intensity contours as in the left panel on top of the RM error map. The beam size is shown at the bottom left corner of each image. A cut-off of $5\times$ rms noise is applied to all these maps. Regions close to the minor axis of the disk are excluded due to unrealistic RM values.