Creation of the X-ray cavity jet and its radio lobe in M87/Virgo with cosmic rays: Relevance to relic radio sources

Young cavities in the X-ray-emitting hot gas in galaxy clusters are often filled with radio synchrotron emission from cosmic rays. However, in the M87/Virgo cluster, where cavities are less prominent, X-ray observations show a 30 kpc long nearly radial filament of relatively cooler gas that projects from the cluster core into a large (40 kpc) radio lobe. We describe the dynamical relationship between these two very dissimilar observations with gas dynamical calculations that include the dynamical effects and spatial diffusion of cosmic rays. After cosmic rays inflate the cavity, they diffuse through the cavity walls, forming a much larger lobe. The cavities, which are most visible just after they have formed (in about 20 Myr), require a total cosmic ray energy that is more than 10 times larger than that usually assumed, E = 4PV. During the relatively brief cavity lifetime, a jetlike, low-entropy thermal filament is formed in the buoyant flow and moves at high subsonic velocities through the cavity center and beyond. After 100 million years, long after the cavity has disappeared, the relatively dense filament extends to 20-30 kpc and the cosmic rays have diffused into a quasi-spherical lobe 40 kpc in diameter. These computed X-ray and radio features agree well with those observed in M87/Virgo and resemble those in other "relic'' cluster radio sources such as Abell 13 and Abell 133. Eventually, the filament falls back and shocks at the center of the cluster (perhaps stimulating the famous nonthermal M87 jet), and only the aging radio lobe remains.