Low radio frequency observations of seven nearby galaxies with GMRT

TL;DR

This study uses GMRT observations at 0.33 GHz to analyze the radio spectra of seven nearby galaxies, revealing frequency-dependent cosmic ray propagation effects and variations in radio-FIR correlation.

Contribution

It provides the first detailed spatially resolved non-thermal spectral analysis at 0.33 GHz for these galaxies and explores cosmic ray propagation mechanisms affecting radio-FIR correlation.

Findings

Spectral indices are flatter at galaxy centers and steepen outward.

Radio-FIR correlation exponent varies with frequency, being flatter at 0.33 GHz.

Cosmic ray propagation models suggest streaming instability explains most observations.

Abstract

We have observed seven nearby large angular sized galaxies at 0.33 GHz using GMRT with angular resolution of $\sim10''$ and sub-mJy sensitivity. Using archival higher frequency data at 1.4 or $\sim$6 GHz, we have then determined their spatially resolved non-thermal spectrum. As a general trend, we find that the spectral indices are comparatively flat at the galaxy centres and gradually steepen with increasing galactocentric distances. Using archival far infrared (FIR) MIPS 70 ${\mu} m$ data, we estimate the exponent of radio-FIR correlation. One of the galaxy (NGC 4826) was found to have an exponent of the correlation of $\sim1.4$. Average exponent from 0.33 GHz data for the rest of the galaxies was 0.63$\pm$0.06 and is significantly flatter than the exponent 0.78$\pm$0.04 obtained using 1.4 GHz data. This indicates cosmic ray electron (CRe) propagation to have reduced the correlation between FIR and 0.33 GHz radio. Assuming a model of simple isotropic diffusion of CRe, we find that the scenario can explain the frequency dependent cosmic ray electron propagation length scales for only two galaxies. Invoking streaming instability could, however, explain the results for the majority of the remaining ones.