Search for Coronal Emission Lines in Cooling Flow Clusters with the Keck 10 Meter Telescope

Abstract

Using the Low Resolution Imaging Spectrograph on the Keck 10 m telescope, we have searched for the coronal emission line [Fe X] 6374 Å produced by the gas at temperatures of 105 106 K in the central several 10 kpc regions of two massive cooling flow clusters: Abell 1795 and PKS 0745-191. We did not detect an significant [Fe X] 6374 Å emission in either cluster. The 1 σ surface brightness limits for the [Fe X] 6374 Å line in the central 75 h^(-1)_(50) kpc region of Abell 1795 and in the central 30 h^(-1)_(50) kpc region of PKS 0745-191 are 10^(-18) ergs s^(-1) cm^(-2) arcsec^(-2) and 7 × 10^(-19) ergs s^(-1) cm^(-2) arcsec^(-2), respectively. Our upper limits of the [Fe X] 6374 Å surface brightness in both clusters are about a factor of 10 better than the previous measurements. These results are consistent with the theoretically predicted [Fe X] 6374 Å surface brightness by the popular inhomogeneous models (Sarazin & Graney 1991), in which gas cools out of the flow at a range of radii, leaving the hot gas distributed as r. The upper limits on mass cooling rate derived from our measurement are also consistent with the cooling rates derived from X-ray data for these two clusters. Our results are contradictory to the claimed significant detection in Abell 1795 by Anton et al. (1991) and the 3 σ marginal detection in PKS 0745-191 reported by Donahue & Stocke (1994). In addition, we also measured upper limits on the [Fe X]/Hα ratio in both clusters, which provide strong constraints on various theoretical models of nebulae heating for production of the luminous low-ionization emission lines by cool (˜10^4 K) nebular filaments. The [Fe X]/Hα (R < 15 kpc) ratios in Abell 1795 and PKS 0745 - 191 are less than 0.6% and 0.13%, respectively.