1.Introduction

1. Central radio galaxies in groups: cavities, bubbles and the history of AGN heatingS.Giacintucci1,2, T. Venturi¹, J.Vrtilek 2, L. David², S.Raychaudhuri3, R.Athreya41. INAF, IRA, Bologna, Italy 2. Harvard-Smithsonian CfA, Cambridge, MA, USA 3. U. Birmingham, UK 4. NCRA, Pune, India Tab. 1Galaxy group list. The 1.4 GHz flux density is from the NVSS. 2. Goals of the project and selected sample The main goal of our project is to study the properties of the central radio source in galaxy groups over a broad frequency range. In particular, we want to examine the AGN effects at various phases of activity, and study the geometry, timescales and physical mechanisms of the energy injection. Our main tool is the study of the spectral properties of the central radio galaxies, by means of the total and point – to - point spectrum. This will allow us to obtain estimates of the radiative ageof the emitting electrons, which will be examined in the light of the X-ray properties of the group environment. The progressive loss of particle energy produces a significant steepening of the radio spectrum, so that evidence of a former AGN activity (possibly related to presence of ghost cavities) may be reflected only at low frequencies. To this aim, we selected 18 galaxy groups (Tab.1) by means of a combination of X-ray and radio criteria. All groups are well-studied in the X-ray band, with high quality data from Chandra and/or XMM-Newton, and show, whether in the radio or X-rays or both, structure suggestive of strong AGN interaction with the intragroup gas. We have been observing our sample at 610 MHz, 327 MHz and 235 MHz with the Giant Metrewave Radio Telescope (GMRT, Pune, India). The observational status is reported in Table 1. 16/18 groups have been observed at 610 MHz; the completion of the 235 MHz observations will be performed in August 2008; the completion of the 327 MHz will be carried out as future follow up. The full resolution of our images ranges from ~6” at 610 MHz, to ~15” at 235 MHz. The sensitivity (1σ) in the final images ranges from ~ 50μJy/b at 610 MHz to ~ 0.4 mJy/b at 235 MHz. 1.Introduction Deep radio and X-ray observations of galaxy clusters and groups are providing evidence in support of the idea that the AGN activity of the central radio galaxy plays an important role in the regulation of the cooling process: the commonness and variety of bubbles, cavities and edges observed both in the radio and X-ray bands in groups and clusters provide direct evidence of the widespread presence of phenomena driven by the central AGN. Recent investigations, mainly focussed on massive galaxy clusters, indicates that the energy released by the AGN during its lifetime may be enough to counteract putative cooling flows (e.g. McNamara & Nulsen, 2007, ARAA, 45, 117). However the actual physical mechanism responsible for the heating is still unclear. We recently started a large observational radio and X-ray project which aims to extend the study of the AGN/ICM interaction to systems smaller than the well-studied rich clusters. The study of the AGN activity in groups is valuable for a number of reasons: although the scale of AGN outbursts is usually less energetic and often on a smaller scale than in clusters, in the low pressure group environment the results of the radio/ICM interactions are expected to be more apparent. More generally, the relationship between AGN and hot gas is expected to have a significant influence on the evolution of galaxies in groups, and the understanding of the group structure and evolution is important because the group environment is the locus of themajority of galaxies in the Universe. Here we present some preliminary results of the radio/X-ray analysis for a few radio galaxies and the results for the groupAWM04, which can be considered a prototypical study for our aims Preliminary results AWM04 NGC741/742 Right – Point-to point spectral index image between 4.9 GHz (VLA) and 610 MHz (GMRT) - Resolution 8”x6” Left - 610 MHz Resolution 6”x5” 235-610 MHz spectral index image (colours) with 235 MHz contours overlaid (S prop.to ν–α) GMRT 327 MHz on DSS Spectral index trend along the jets and fit Centre - 610 MHz over Chandra NGC4636– 610 MHz over Chandra NGC5044– 610 MHz on Chandra NGC507 - 610 MHz on Chandra NGC7627– 610 MHz over DSS2 Resolution 13 ” x 10 ” firs contour = 2.4 mJy/b Resolution 8” x 7” , f.c.= 1.5 mJy/b GMRT 610 MHz AWM04is at first glance a good example of arelaxed group, with no significant substructure in the X-ray surface brightness. However, spectral mapping revealed anumber of features whose temperatures and abundances diverge from the mean value of the group. Itscentral cooling time of the order of 2 Gyrsuggeststhe presence of a source of energy injection, to prevent from catastrophic cooling (O’Sullivan et al. 2005, MNRAS, 357, 1134). The group hosts acentral wide-angle tail radio galaxy, 4C+24.36, associated with NGC6051. The overall symmetry of the radio galaxy (shape and flux density of the jets) led us to estimate that it is oriented at large angle to the line of sight, i.e. ~81° - 88°. Thanks to our GMRT observations we carried out a detailed spectral analysis of the radio galaxy, which led to an estimate of its radiative age of the order of 160 Myr. Thetotal energy output of the central radio galaxyfails by about two orders of magnitude to balance the cooling. Secondary sources of heating, such as mechanical heating of bubbles and cavities, are the most plausible mechanism for feedback. The lack of cavities in AWM04 is still puzzling, but we are confident that upcoming Chandra observations may shed a light on this. Giacintucci et al.2008, ApJ, in press (arXiv:0804.1906) Resolution 6”x5” in all images – Average rms (1σ level) ~ 50 μJy/beam in all images The group hosting the cD galaxyNGC741and its companion NGC742 has avery complex and dishomogenous X-ray structure. The two galaxies are likely in mutual tidal interaction, and a striking X-ray filament is visible in the region between them (see central figure in the upper frame).A spectacular large radio sourcedominates the radio emission. It is unclear whether it is associated with NGC 741 or its companion. Two compact and relatively flat spectrum components, connected by a bright bridge of emission, are coincident with the optical nuclei of the two galaxies. Our 610 MHz images show that the radio emission of the central AGNs in galaxy groups covers a wide range of morphologies (core-dominated, twin jet, Z-shaped, typical FRI), and of linear scales (from the galactic size of few tens of kpc, to hundreds of kpc, see the images in the lower frame of the figure). The high sensitivity and appropriate resolution of our GMRT images, coupled with the high quality X-ray imaging of the intragroup gas distribution are a promising starting point for the goals of the present project.