Radio Emission and the Hot Interstellar Medium of Early-Type Galaxies

We report the results of the analysis of a sample of 84 elliptical and S0 galaxies, observed in X-rays with the Einstein Observatory and in radio continuum at 5 GHz. Radio flux densities are both from published surveys and from our own VLA observations, which result in some of the lowest radio powers yet reported for early-type galaxies. In agreement with previous reports, we find that radio structures extending beyond the optical radius are found only in galaxies with 5 GHz radio power greater than ~ 10^29.5^ ergs s^-1^ Hz^-1^. We find that radio and X-ray luminosities are correlated, although with a large intrinsic scatter. This suggests that more than one mechanism may be at play. To explore possible connections between the hot X-ray-emitting interstellar medium (ISM) and the radio emission, we have searched for correlations of the latter with the X-ray to optical flux ratio, which can be considered as an indicator of the gaseous component. We find a correlation between core radio power and the X-ray to optical ratio, suggesting a connection between the hot ISM and nuclear radio sources, and pointing to accreting cooling flows as the fuel for the radio sources. We also notice that, for the same radio core power, extended radio lobes tend to be associated with galaxies with relatively smaller X-ray to optical ratios, pointing to the importance of the hot ISM in disrupting the radio jets and confining extended radio structures. Comparison of the data with the theoretical prediction of Soker and Sarazin can explain the lack of very extended radio structures for sources fainter than ~10^29.5^ ergs s^-1^ Hz^-1^ at 5 GHz with the disruption of radio jets at the sonic radius of typical galaxy cooling flows. Comparison of the present sample with bright 3CR radio galaxies reveals a continuum of X-ray and radio properties, and suggests that nuclear black holes in radio-faint galaxies might be less massive than those in powerful radio galaxies, if the nuclear activity is indeed due to accretion onto a collapsed object, unless there is some impediment to nuclear accretion in normal galaxies. We also find that, while thermal confinement might affect radio sources in clusters, the powerful isolated FR 2 radio sources are likely to be expanding adiabatically.