MWA and VLA Observations of Diffuse Radio Lobes in M 87

TL;DR

This paper presents detailed radio observations of M87's diffuse lobes, analyzing their structure, spectrum, magnetic field, age, and energy sources, suggesting past enhanced AGN activity powered the lobes.

Contribution

It provides the first comprehensive multi-frequency analysis of M87's radio lobes, estimating their physical properties and proposing a historical AGN activity enhancement as the energy source.

Findings

Lobes have sharp edges and filaments with spectral indices -1.2 to -0.8.

Magnetic field strength in lobes is about 10 microGauss.

Lobe age is estimated at 30-50 million years.

Abstract

This study investigates the projected, quasi-symmetric $\sim\rm46\,kpc$-scale diffuse radio lobes surrounding the giant elliptical galaxy M\,87, utilizing well-sampled wideband ($\rm 60\,MHz-10.55\,GHz$) observations from MWA and VLA, supplemented by data from LOFAR and Effelsberg. The observed structures feature sharp edges and filaments, with nearly uniform and moderately steep spectral indices ($\alpha$, mostly within $-1.2\leq\alpha\leq-0.8$), indicating turbulence. Well-sampled radio spectra for the lobes' diffuse region are derived using the continuous injection (CI) model (with $\alpha_{\rm inj}\simeq-0.86$ and $\nu_{\rm b}\simeq1.72\rm\,GHz$), and for its three localized regions using the impulsive injection model (e.g., JP model). From energy equipartition analysis, we estimate the typical magnetic field strength in the lobes' diffuse region to be $B_{\rm eq}\simeq10\,\mu\rm G$. The age of the lobes is estimated as $\sim30-50\,\rm~Myr$, based on lifetimes derived from the CI and JP models and sound crossing time. Outflow powers of $\sim(0.2-2)\times10^{44}\,\rm erg\,s^{-1}$ for the lobes' diffuse components and $\sim(1-11)\times10^{44}\,\rm erg\,s^{-1}$ for the whole source are calculated. With this power assessment, we conclude that the galactic stellar wind has a negligible effect, the active galactic nucleus (AGN)-driven jet can provide the necessary energy for the whole system. Furthermore, we argue that while the wind driven by current AGN activity is unlikely to power the lobes' diffuse components, an average enhancement of AGN activity by a factor of $\sim 10^2$ over the past $\sim 30-50$ Myr remains plausible.