GMRT 333 MHz observations of 6 nearby normal galaxies

TL;DR

This study presents GMRT 333 MHz observations of six nearby galaxies, analyzing their thermal and nonthermal emission components, spectral indices, and cosmic ray electron properties at kiloparsec scales.

Contribution

It provides high-resolution 333 MHz radio data for nearby galaxies and characterizes their thermal fractions and cosmic ray electron spectral indices.

Findings

Thermal fraction at 333 MHz is less than 5% in five galaxies.

Nonthermal spectral index is steeper in outer regions with low thermal fraction.

High thermal regions have a flatter spectral index around -0.78.

Abstract

We report Giant Meterwave Radio Telescope (GMRT) continuum observations of six nearby normal galaxies at 333 MHz. The galaxies are observed with angular resolutions better than ~20" (corresponding to a linear scale of about 0.4 - 1 kpc). These observations are sensitive to all the angular scales of interest, since the resolution of the shortest baseline in GMRT is greater than the angular size of the galaxies. Further, for five of these galaxies we show that at 333 MHz, the mean thermal fraction is less than 5%. Using archival data at about 1 GHz, we estimate the mean thermal fraction to be about 10% at that frequency. We also find that the nonthermal spectral index is generally steeper in regions with low thermal fraction and/or located in the outer parts of the galaxy. In regions of high thermal fraction, the nonthermal spectral index is flatter, and has a narrow distribution peaking at ~ -0.78 with a spread of 0.16, putting stringent constraints on the physical conditions for generation, diffusion and energy losses of cosmic ray electrons at scales of ~ 1 kpc.