The Detection of a Cooling Flow Elliptical Galaxy from O [CSC]vi[/CSC] Emission

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Abstract
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 3 × 105 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.
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