Luminous “dark” Halos

If cold, dense clouds of gas make up the dark matter, then physical collisions between clouds must occur. These cause strong, radiative shocks to propagate through the cold gas, with the startling implication that all “dark” halos should be luminous. The expected luminosity is a strong function of halo velocity dispersion, and must contribute a large fraction of the observed X-ray emission from clusters of galaxies. Far from being excluded by the data, two pieces of evidence suggest that this model is correct: we find a luminosity-temperature correlation of the form L ∝ T , as seen in recent analyses; furthermore the anticipated spectra have substantially more power at low energies than isothermal bremsstrahlung spectra, and may account for the observed “excess” EUV emission from clusters. These successes are remarkable considering that the theory has no free parameters or ad hoc elements. We predict that the X-ray satellite Chandra will resolve the Virgo cluster into 10 point-like, transient X-ray sources. Subject headings: dark matter — galaxies: clusters — galaxies: halos