LOFAR Detection of a Low-Power Radio Halo in the Galaxy Cluster Abell 990

TL;DR

This paper reports the first LOFAR detection of a low-power radio halo in the galaxy cluster Abell 990, a less-massive and low-luminosity system, expanding the known population of such halos.

Contribution

It presents the discovery of a faint radio halo in a low-mass, low-luminosity galaxy cluster using LOFAR, demonstrating the potential to find more in similar systems.

Findings

Detected a radio halo with a size of ~700 kpc and flux density of 20.2 mJy.

The cluster is not undergoing a major merger but is slightly disturbed.

The cluster is among the least luminous known to host a radio halo.

Abstract

Radio halos are extended ($\sim{\rm Mpc}$), steep-spectrum sources found in the central region of dynamically disturbed clusters of galaxies. Only a handful of radio halos have been reported to reside in galaxy clusters with a mass $M_{500}\lesssim5\times10^{14}\,M_\odot$. In this paper we present a LOFAR 144 MHz detection of a radio halo in the galaxy cluster Abell 990 with a mass of $M_{500}=(4.9\pm0.3)\times10^{14}\,M_\odot$. The halo has a projected size of $\sim$700$\,{\rm kpc}$ and a flux density of $20.2\pm2.2\,{\rm mJy}$ or a radio power of $1.2\pm0.1\times10^{24}\,{\rm W\,Hz}^{-1}$ at the cluster redshift ($z=0.144$) which makes it one of the two halos with the lowest radio power detected to date. Our analysis of the emission from the cluster with Chandra archival data using dynamical indicators shows that the cluster is not undergoing a major merger but is a slightly disturbed system with a mean temperature of $5\,{\rm keV}$. The low X-ray luminosity of $L_{X}=(3.66\pm0.08)\times10^{44}\,{\rm ergs\,s}^{-1}$ in the 0.1--2.4 keV band implies that the cluster is one of the least luminous systems known to host a radio halo. Our detection of the radio halo in Abell 990 opens the possibility of detecting many more halos in poorly-explored less-massive clusters with low-frequency telescopes such as LOFAR, MWA (Phase II) and uGMRT.