We present new Giant Metrewave Radio Telescope observations at 235 MHz and
610 MHz of 18 X-ray bright galaxy groups. These observations are part of an
extended project, presented here and in future papers, which combines
low-frequency radio and X-ray data to investigate the interaction between
central active galactic nuclei (AGN) and the intra-group medium (IGM). The
radio images show a very diverse population of group-central radio sources,
varying widely in size, power, morphology and spectral index. Comparison of the
radio images with Chandra and XMM-Newton X-ray images shows that groups with
significant substructure in the X-ray band and marginal radio emission at >= 1
GHz host low-frequency radio structures that correlate with substructures in
IGM. Radio-filled X-ray cavities, the most evident form of AGN/IGM interaction
in our sample, are found in half of the systems, and are typically associated
with small, low- or mid-power double radio sources. Two systems, NGC5044 and
NGC4636, possess multiple cavities, which are isotropically distributed around
the group center, possibly due to group weather. In other systems the
radio/X-ray correlations are less evident. However, the AGN/IGM interaction can
manifest itself through the effects of the high-pressure medium on the
morphology, spectral properties and evolution of the radio-emitting plasma. In
particular, the IGM can confine fading radio lobes in old/dying radio galaxies
and prevent them from dissipating quickly. Evidence for radio emission produced
by former outbursts that coexist with current activity is found in six groups
of the sample.Comment: Accepted for publication in the Astrophysical Journal Supplement
Series, 26 pages, 18 figures. A version with high-quality figures is
http://www.astro.umd.edu/~simona/giacintucci_hr.pd