Spatial Diffusion of X-Ray Emission Lines in the M87 Cooling Flow; Evidence for Absorption

Recent XMM-Newton observations of the cooling flow in M87 indicate sharply decreasing oxygen, iron, and silicon abundances within ~5 kpc of the galactic center. This result is unexpected since stellar mass loss and Type Ia supernovae are expected to produce pronounced central abundance maxima for all three elements. However, it has been suggested that many of the strong X-ray lines are optically thick and diffuse to larger radii in the cooling flow before escaping, falsifying the central abundances. We verify with radiation transfer calculations that this effect does indeed occur in the M87 cooling flow but that it is insufficient to account for the M87 observations. We suggest that some source of continuous opacity is required to reduce the central X-ray line emission, perhaps by warm gas at T~10⁵-10⁶ K. The radial surface brightness profiles of X-ray resonance lines are also sensitive to turbulence in cooling flows, which reduces the line center optical depths considerably. Turbulence may provide sufficient energy to continuously heat the warm absorbing gas.