FUSE observations of cooling-flow gas in the galaxy clusters A1795 and A2597

We present far-ultraviolet spectroscopy of the cores of the massive cooling-flow clusters Abell 1795 and 2597, obtained with the Far Ultraviolet Spectroscopic Explorer. As the intracluster gas cools through 3 x 10(5) K, it should emit strongly in the O VI lambda lambda 1032, 1038 resonance lines. We report the detection of O VI lambda 1032 emission in A2597, with a line flux of (1.35 +/- 0.35) x 10(-15) ergs cm(-2) s(-1), as well as detection of emission from C III lambda 977. A marginal detection of C III lambda 977 emission is also reported for A1795. These observations provide evidence for a direct link between the hot (10(7) K) cooling-flow gas and the cool (10(4) K) gas in the optical emission line filaments. Assuming simple cooling-flow models, the O VI line flux in A2597 corresponds to a mass deposition rate of similar to 40 M-. y(-1) within the central 36 kpc. Emission from O VI j1032 was not detected in A1795, with an upper limit of 1.5 x 10(-15) ergs cm(-2) s(-1), corresponding to a limit on the mass cooling-flow rate of (M) over dot (28 kpc) <28 M-. yr(-1). We have considered several explanations for the lack of detection of O VI emission in A1795 and the weaker than expected flux in A2597, including extinction by dust in the outer cluster and quenching of thermal conduction by magnetic fields. We conclude that a turbulent mixing model, with some dust extinction, could explain our O VI results while also accounting for the puzzling lack of emission by Fe XVII in cluster cooling flows.