CORONAL LINE EMISSION IN CLUSTER COOLING FLOWS

We report one marginal detection (PKS 0745-191) and four nondetections (A2199, 2A0335 + and A1795) of the coronal emission line [Fe X] 6374 angstrom radiated by approximately 10(6) K gas in the central regions of massive cooling flow clusters (mass cooling rates of M greater-than -or-equal-to 100 M. yr-1). Except for the nondetection of [Fe X] in A1 795, these observations are consistent with and more sensitive than pr evious upper limits for these and similar clusters of galaxies. We dis cuss in detail the specific difficulties in detecting this emission li ne against the starlight and the approximately 10 K emission-line regi on of the central cD. The [Fe X] emission directly probes the radiativ e behavior of cooling gas in the central 10 kpc of the cluster, which X-ray telescopes cannot yet spatially resolve. The [Fe X] detection in PKS 0745 - 191 cannot be explained by any plausible photoionization o r ps heating model for cluster gas, except for the standard cooling ga s picture. The [Fe X] luminosity measures (1) the cooling rate of ps i n the centers of these clusters, and (2) the amount of photoionizing U V radition that is generated by cooling gas. Such UV radiation can pho toionize and heat the luminous, cool (approximately 10(4) K) filaments . The level of the detection and upper limits reported here suggest th at a fraction (less than or similar to 30%) of the rate of mass coolin g observed by X-ray telescopes over extents of 100 kpc occurs in the c entral 10 kpc, and thus within the central cD itself. In addition, the [Fe X] strength is consistent with models for photoionizing the 10(4) K nebular filaments by cooling hot ps (e.g., self-ionized cooling fil aments, turbulent mixing layers, or stripped interstellar medium photo ionized by cooling gas), although upper limits a few times more sensit ive than the limits presented here would challenge the simplest of the se models.