2001ApJ...553L.125B

Cooling flow models for the hot gas in elliptical galaxies predict that gas is cooling at a rate of ∼1 M_☉.yr^–1, yet there is little evidence for this phenomenon beyond the X-ray wave band. If hot gas is cooling, it will pass through the 3x10⁵ K regime and radiate in the O VI λλ1032, 1038 ultraviolet lines, which can be detected with the Far-Ultraviolet Spectroscopic Explorer (FUSE), and here we report on FUSE observations of the X-ray bright early-type galaxies NGC 1404 and NGC 4636. In NGC 1404, the O VI doublet is not detected, implying a cooling rate less than 0.3 M_☉.yr^–1, which is below the predicted values from the cooling flow model of 0.4-0.9 M_☉.yr^–1. In NGC 4636, both O VI lines are clearly detected, indicating a cooling rate of 0.43±0.06 M_☉.yr^–1, which falls within the range of values from the cooling flow prediction, 0.36-2.3 M_☉.yr^–1, and is closest to the model where the production of the cooled gas is distributed through the galaxy. The emission-line widths, 44±15 km.s^–1, are close to the Doppler broadening value (30 km.s^–1), indicating that the flow is quiescent rather than turbulent and that the flow velocity is less than 30 km.s^–1.