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 Spect roscopic Explorer. As the intracluster gas cools through 3 x 10(5) K, it sh ould 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 fl ux 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 em ission 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-fl ow models, the O VI line flux in A2597 corresponds to a mass deposition rat e 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 exp ected flux in A2597, including extinction by dust in the outer cluster and quenching of thermal conduction by magnetic fields. We conclude that a turb ulent mixing model, with some dust extinction, could explain our O VI resul ts while also accounting for the puzzling lack of emission by Fe XVII in cl uster cooling flows.