An adiabatic, X-ray emitting cavity in the galaxy/group NGC 4636 and a new stellar mass-to-light ratio for the central galaxy

An X-ray analysis revisits deep Chandra observations of the flamboyantly disturbed atmosphere in the galaxy/group NGC 4636. A known pair of faint cavities about 2-3.5 kpc from the centre is visible in azimuthally averaged Chandra data. These may be the first known cavities containing X-ray observable gas and which also appear as perfectly adiabatic perturbations. Radial gas density, temperature, and pressure profiles are all lowered in adiabatic ratios in the cavity, but the radial entropy profile is almost exactly matched with two power laws having classic logarithmic slopes, 0.77 and 1.1, but with no cavity feature. Adiabatically inflated thermal gas inside cavities is an unmistakable signature of expansion due to cosmic ray pressure. Although the gas pressure P is lower inside the cavities, the cosmic ray pressure P-c + P = P-0 allows the total internal pressure P-0 to be in hydrostatic equilibrium with the local atmosphere. Cosmic ray and gas pressures inside the cavities are comparable, P-c/P less than or similar to 11. Adiabatic cavities similar to those in NGC 4636 may be common. Chandra X-ray observations also allow a determination of the stellar mass-to-light ratio in the central galaxy by comparing the total mass profile determined from X-ray data with mass models of the stars from optical photometry and NFW dark haloes. The mass-to-light ratio Upsilon(V) = 7.77 required to match the X-ray total mass profile differs significantly from that found from the stellar velocity dispersion within the half-light radius.