2006ApJ...653L...9D

We have observed four X-ray-underluminous groups of galaxies using the Giant Metrewave Radio Telescope (GMRT). The groups NGC 524, 720, 3607, and 4697 are underluminous in relation to the extrapolation of the L_X-T relation from rich clusters and do not show any evidence of current AGN activities that can account for such a departure. The GMRT observations carried out at low frequencies (235 and 610 MHz) were aimed at detecting low surface brightness, steep-spectrum sources indicative of past AGN activities in these groups. No such radio emissions were detected in any of these four groups. The corresponding upper limits on the total energy in relativistic particles is ∼3x10⁵⁷ ergs. This value is more than a factor of 100 less than that required to account for the decreased X-ray luminosities (or enhanced entropies) of these four groups in the AGN-heating scenario. Alternatively, the AGN activity must have ceased ∼4 Gyr ago, allowing the relativistic particles to diffuse out to such a large extent (∼250 kpc) that their radio emission could have been undetected by the current observations. If the latter scenario is correct, the ICM was preheated before the assembly of galaxy clusters.