Early-type galaxies often contain a hot X-ray emitting interstellar medium ( 3-8 \times 10 ^ { 6 } K ) with an apparent radiative cooling time much less than a Hubble time .
If unopposed by a heating mechanism , the gas will radiatively cool to temperatures \lesssim 10 ^ { { 4 } } K at a rate proportional to L _ { X } / T _ { X } , typically 0.03-1 M _ { \odot } yr ^ { -1 } .
We can test if gas is cooling through the 3 \times 10 ^ { 5 } K range by observing the OVI doublet , whose luminosity is proportional to the cooling rate .
Here we report on a study of an unbiased sample of 24 galaxies , obtaining Far Ultraviolet Spectroscopic Explorer spectra to complement the X-ray data of ROSAT and Chandra .
The OVI line emission was detected in about 40 % of the galaxies and at a luminosity level similar to the prediction from the cooling flow model .
There is a correlation between { \dot { M } } _ { OVI } and { \dot { M } } _ { X } , although there is significant dispersion about the relationship , where the OVI is brighter or dimmer than expected by a factor of three or more .
If the cooling flow picture is to be retained , this dispersion requires that cooling flows be time-dependent , as might occur by the activity of an AGN .
However , of detected objects , those with the highest or lowest values of { \dot { M } } _ { OVI } / { \dot { M } } _ { X } are not systematically hot or cool , as one might predict from AGN heating .