Radio Continuum Halos of 7 Nearby Large Galaxies using uGMRT

TL;DR

This study uses deep uGMRT radio observations to reveal extensive, previously undetected radio halos and diffuse emissions in 7 nearby large galaxies, providing new insights into their magnetic and cosmic ray structures.

Contribution

It presents the first detection of significant diffuse radio halos in two galaxies and compares halo sizes across frequencies, revealing spectral steepening and larger halos at lower frequencies.

Findings

Detected larger radio halos at 0.4 GHz than at 1.5 GHz.

First-time detection of diffuse emissions in NGC3344 and NGC3627.

Spectral index maps show steepening up to -1.5 in galaxy halos.

Abstract

We present the results of deep radio observations of 7 nearby large galaxies observed using the upgraded Giant Metrewave Radio Telescope (uGMRT) 0.3-0.5 GHz receivers with an angular resolution of $\sim$10 arcsec. The achieved sensitivities of these observations range from $\approx$15 to 50 $\mu$Jy/beam which is $\approx$3-4 factor lower than the previous observations at these frequencies. For 2 galaxies (NGC3344 and NGC3627) with moderate inclination angles, significant diffuse emissions are seen for the first time. Detected radio halos in the vertical direction are significantly larger in our 0.4 GHz maps than compared to the observations at $\sim$1.5 GHz for 4 nearly edge-on galaxies - NGC3623, NGC4096, NGC4594, and NGC4631. For these 4 galaxies, significantly larger halos are also detected along the galaxy disk. For NGC3623 and NGC4594, we could detect elongated radio disks which was not seen before. We also present new uGMRT images of NGC3344 and NGC3623 at 1.3 GHz and a new VLA image of NGC3627 at 1.5 GHz. We fitted an exponential function to the flux densities along different cross-cuts and found a significantly wider distribution at 0.4 GHz uGMRT images than compared to the high-frequency images at $\sim$1.5 GHz. Using maps at 0.144, 0.4, and $\sim$1.5 GHz, we made spectral index maps of the 7 sample galaxies and found steepening of the spectrum up to a value of $\sim$ -1.5 in the halo regions of the galaxies.