M87 at metre wavelengths: the LOFAR picture

TL;DR

This paper presents the first high-resolution LOFAR observations of M87 at frequencies down to 20 MHz, revealing a well-confined radio halo with detailed spectral analysis supporting a continuous injection model and estimating the halo's age and jet power.

Contribution

First high-resolution low-frequency images of M87 at 20 MHz, combined with spectral analysis, providing new insights into the source's magnetic field, age, and energetics.

Findings

No new extended emissions detected, source confined by intra-cluster medium pressure.

Continuous injection model best fits the data, indicating ongoing particle supply.

Halo age estimated at approximately 40 million years, with jet power of 6-10 x 10^44 erg/s.

Abstract

M87 is a giant elliptical galaxy located in the centre of the Virgo cluster, which harbours a supermassive black hole of mass 6.4x10^9 M_sun, whose activity is responsible for the extended (80 kpc) radio lobes that surround the galaxy. The energy generated by matter falling onto the central black hole is ejected and transferred to the intra-cluster medium via a relativistic jet and morphologically complex systems of buoyant bubbles, which rise towards the edges of the extended halo. Here we present the first observations made with the new Low-Frequency Array (LOFAR) of M87 at frequencies down to 20 MHz. Images of M87 were produced at low radio frequencies never explored before at these high spatial resolution and dynamic range. To disentangle different synchrotron models and place constraints on source magnetic field, age and energetics, we also performed a detailed spectral analysis of M87 extended radio-halo using these observations together with archival data. We do not find any sign of new extended emissions; on the contrary the source appears well confined by the high pressure of the intra-cluster medium. A continuous injection of relativistic electrons is the model that best fits our data, and provides a scenario in which the lobes are still supplied by fresh relativistic particles from the active galactic nuclei. We suggest that the discrepancy between the low-frequency radio-spectral slope in the core and in the halo implies a strong adiabatic expansion of the plasma as soon as it leaves the core area. The extended halo has an equipartition magnetic field strength of ~10 uG, which increases to ~13 uG in the zones where the particle flows are more active. The continuous injection model for synchrotron ageing provides an age for the halo of ~40 Myr, which in turn provides a jet kinetic power of 6-10x10^44 erg/s.