FUSE Observations of Cooling‐Flow Gas in the Galaxy Clusters A1795 and A2597

Abstract

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]105 K, it should emit strongly in the O VI jj1032, 1038 resonance lines. We report the detection of O VI j1032 emission in A2597, with a line Ñux of (1.35^0.35)]10~15 ergs cm~2 s~1, as well as detec- tion of emission from C III j977. A marginal detection of C III j977 emission is also reported for A1795. These observations provide evidence for a direct link between the hot (107 K) cooling-Ñow gas and the cool (104 K) gas in the optical emission line filaments. Assuming simple cooling-Ñow models, the O VI line flux in A2597 corresponds to a mass deposition rate of D40 M_ yr~1 within the central 36 kpc. Emission from O VI j1032 was not detected in A1795, with an upper limit of 1.5]10~15 ergs cm~2 s~1, corresponding to a limit on the mass cooling-flow rate of M kpc)\28 yr~1. We have considered 0 (28 M_ 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 Ðelds. 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.