Secondary Radio and X-ray Emissions from Galaxy Mergers

TL;DR

This paper models secondary emissions from galaxy mergers caused by cosmic-ray interactions, explaining observed radio and X-ray fluxes and constraining physical parameters of the merging systems.

Contribution

It introduces a model linking cosmic-ray acceleration and secondary emissions to observed fluxes, enabling parameter constraints in galaxy mergers.

Findings

The model explains radio and X-ray emissions in NGC 660 and NGC 3256.

Constraints on gas mass, shock velocity, magnetic field, and CR spectral index are derived.

A single-zone model with specific spectral index fits observations of both galaxies.

Abstract

Shocks arising in galaxy mergers could accelerate cosmic-ray (CR) ions to TeV-PeV energies. While propagating in the intergalactic medium, these CRs can produce high-energy neutrinos, electron-positron pairs and gamma-rays. In the presence of intergalactic magnetic fields, the secondary pairs will radiate observable emissions through synchrotron radiation and inverse Compton scattering. In this paper, we demonstrate that these emissions can explain the radio and X-ray fluxes of merging galaxies such as NGC 660 and NGC 3256. Using our model in combination with the observations, we can constrain the gas mass, shock velocity, magnetic field and the CR spectral index $s$ of these systems. For NGC 660 a single-zone model with a spectral index $2.1\lesssim s\lesssim2.2$ is able to reproduce simultaneously the radio and X-ray observations, while a simple one-zone scenario with $s\sim2$ can describe the radio and a large fraction of X-ray observations of NGC 3256. Our work provides a useful approach for studying the dynamics and physical parameters of galaxy mergers, which can play an important part in future multi-messenger studies of similar and related extragalactic sources.