On VI in Elliptical Galaxies: Indicators of Cooling Flows

Early-type galaxies often contain a hot X-ray-emitting interstellar medium [(3-8)×10⁶ 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 <~10⁴ K at a rate proportional to L_X/T_X, typically 0.03-1 M_solar yr^-1. We can test whether gas is cooling through the 3×10⁵ K range by observing the O VI 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 O VI 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 M˙_OVI and M˙_X, although there is significant dispersion about the relationship, where the O VI is brighter or dimmer than expected by a factor of 3 or more. If the cooling flow picture is to be retained, then 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 M˙_OVI/M˙_X are not systematically hot or cool, as one might predict from AGN heating.