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

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 (10, M,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 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 (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 about 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 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 1, kpc from M87).