Suzaku Detection of Thermal X-Ray Emission Associated with the Western Radio Lobe of Fornax A

TL;DR

This study used Suzaku X-ray observations to detect and analyze thermal X-ray emission associated with the western radio lobe of Fornax A, revealing a significant thermal plasma component with implications for the lobe's physical properties.

Contribution

First detailed analysis of thermal plasma emission in Fornax A's radio lobe using Suzaku data, clarifying its spatial distribution and physical characteristics.

Findings

Detected ~1 keV thermal plasma emission in the lobe

Estimated total thermal gas mass of ~10^{10} solar masses

Thermal energy density comparable to magnetic and non-thermal electron energy densities

Abstract

We present the results of X-ray mapping observations of the western radio lobe of the Fornax A galaxy, using the X-ray Imaging Spectrometer (XIS) onboard the Suzaku satellite with a total exposure time of 327 ks. The purpose of this study is to investigate the nature and spatial extent of the diffuse thermal emission around the lobe by exploiting the low and stable background of the XIS. The diffuse thermal emission had been consistently reported in all previous studies of this region, but its physical nature and relation to the radio lobe had not been examined in detail. Using the data set covering the entire western lobe and the central galaxy NGC 1316, as well as comparison sets in the vicinity, we find convincingly the presence of thermal plasma emission with a temperature of ~1 keV in excess of conceivable background and contaminating emission (cosmic X-ray background, Galactic halo, intra-cluster gas of Fornax, interstellar gas of NGC 1316, and the ensemble of point-like sources). Its surface brightness is consistent with having a spherical distribution peaking at the center of the western lobe with a projected radius of ~12 arcmin. If the volume filling factor of the thermal gas is assumed to be unity, its estimated total mass amounts to ~10^{10} M_sun, which would be ~10^{2} times that of the central black hole and comparable to that of the current gas mass of the host galaxy. Its energy density is comparable to or larger than those in the magnetic field and non-thermal electrons responsible for the observed radio and X-ray emission.