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 (≳ 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 (≲ 40kpc) 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 ∼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 ≲ 10kpc 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 ≳ 1kpc from M87∗).
Related
- The MIni-Halos in MIni-Clusters (MIMIC) project. New insights into the origin of mini-halos from a LOFAR, uGMRT, and MeerKAT study of diffuse radio emission in Abell 496 and ZwCl 235
- Radio Halos in Galaxy Clusters as unveiled by the SKA telescope
- Discovering supernovae in the Virgo Cluster during the LSST era
- The Next Generation Virgo Cluster Survey (NGVS). II. A Catalog of Galaxies in the Virgo Cluster
- Debiasing Local Cluster Infall Reveals H0: Application to the Virgo Cluster
BibTeX
@misc{hopkins2026virgo,
title = {{The Virgo Cluster as a Case Study for Ancient Cosmic Ray Halos in Radio and X-rays}},
author = {Philip F. Hopkins and Emily M. Silich and Jack Sayers and Sam B. Ponnada and Isabel S. Sands},
howpublished = {arXiv},
year = {2026},
month = oct,
doi = {10.48550/arxiv.2610.03881},
eprint = {2610.03881},
archivePrefix = {arXiv},
url = {https://arxiv.org/abs/2610.03881},
note = {CC BY 4.0},
}
RIS
TY - UNPB
TI - The Virgo Cluster as a Case Study for Ancient Cosmic Ray Halos in Radio and X-rays
AU - Philip F. Hopkins
AU - Emily M. Silich
AU - Jack Sayers
AU - Sam B. Ponnada
AU - Isabel S. Sands
PY - 2026
DA - 2026/10/02
PB - arXiv
DO - 10.48550/arxiv.2610.03881
UR - https://arxiv.org/abs/2610.03881
AB - 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 (≲ 40kpc) 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 ∼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 ≲ 10kpc 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 ≳ 1kpc from M87^∗).
ER -