OPTICAL CORONAL EMISSION-LINES FROM COOLING FLOWS IN ELLIPTIC GALAXIES AND GALAXY CLUSTERS

We calculate the predicted luminosities, surface brightness profiles, and line shapes of optical coronal emission lines for models of cooling flows in galaxy clusters and in individual elliptical galaxies. We find that the coronal line luminosities of cooling flows depend primarily on the total rate at which gas is cooling. The surface brightness profiles of the lines show where gas is cooling below X-ray emitting temperatures. In homogeneous cooling flow models in which all of the gas flows into the central regions, the coronal lines are very centrally concentrated, coming mainly from a limb-brightened shell lying just within the sonic radius of the inflow. For homogeneous models, the central surface brightnesses of the stronger coronal lines from nearby clusters with large cooling rates are expected to be approximately 6 x 10(-17) ergs cm-2 s-1 arcsec-2. For inhomogeneous models in which gas cools out of the flow at large radii, the surface brightness profiles of the coronal lines are more extended, and the central surface brightness is much lower (typically 6 x 10(-18) ergs cm-2 s-1 arcsec-2). The homogeneous and inhomogeneous models also have very different line shapes. The integrated line profiles for the homogeneous models are very wide and flat, and the central line profiles are double-peaked. On the other hand, the inhomogeneous models have nearly Gaussian line profiles. The predicted line fluxes from nearby clusters are fairly close to the present observational limits. With slightly improved observations, it should be possible to detect optical coronal emission lines in many cooling flow clusters. If the lines are not detected, it would establish unambiguously that gas is not cooling at the rates inferred from the X-ray observations. If the lines are detected, their spatial distribution will show directly where the cooling gas is being deposited.