The Virgo Cluster as a Case Study for Ancient Cosmic Ray Halos in Radio and X-rays

Abstract

Virgo is the nearest cool core (CC) galaxy cluster, and is archetypal of the class of X-ray weak CCs. Despite the X-ray continuum implying a large cooling rate ($\gtrsim 10\,{\rm M_{\odot}\,yr^{-1}}$), measurements of cooler gas phases, dust, and star formation show almost no cooling outside the M87 nucleus. It is also known that M87$^{\ast}$ is a prodigious source of cosmic ray (CR) leptons observed in radio and $γ$-rays, and recent radio observations have shown that the M87 radio halo is low-frequency and its spectral index steepens rapidly away from the injection region+jets, consistent with an aging, low-energy CR population. We demonstrate that coupling X-ray and radio to newer Faraday rotation limits on the magnetic field, this implies the CRs within the CC radius ($\lesssim 40$kpc) must contribute significantly to the total pressure and soft X-ray emission, through a combination of Coulomb heating the gas and inverse-Compton (IC) scattering microwave background photons to $\sim$keV. We show this can explain the apparent correspondence between jet/radio power and X-ray cooling luminosity, and why the X-ray inferred cooling rate appears much larger than otherwise observed. This would also explain why the central X-ray inferred metallicity at $\lesssim 10\,$kpc appears to be suppressed relative to the UV/optical inferred metallicity (in a wavelength-dependent manner). Compared to more extreme radio+X-ray sources like Perseus, we find the observations require a softer 'escaping' CR spectrum (escaping or accelerated outside $\gtrsim 1\,$kpc from M87$^{\ast}$).