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Radio galaxies.

Radio galaxies are strong radio sources almost exclusively associated with elliptical galaxies. Their radio emission is powered by twin jets of plasma formed during the accretion of gas in a supermassive black hole situated in the nucleus of the host galaxy. The jets, initially relativistic, propagate in opposite directions out of the host galaxy, sweeping the interstellar medium first and then the intergalactic medium. The interaction  with the external gas slow-down and deflects the jets originating a wide range of structures. Sometimes the plasma flows into roughly ellipsoidal lobes which surround the jets.  In other cases, the interaction is so strong that the jets are bent by ram pressure as the host galaxy moves through the ambient gas and the radio source assumes a characteristic head-tail morphology. Radio galaxies are impressive from many points of view. The most powerful objects can reach radio luminosities  up to  1038  watt in the radio band (100 MHz - 10 GHz) while the largest radio lobes can reach Mpc size and store up to 1053 joules of energy in the form of relativistic particles and magnetic fields. We study the morphology and the emission properties of radio galaxies with the intent to understand their evolution but also the properties of  their environment.

Main collaborators: P.Parma (INAF-IRA), R.Fanti (INAF-IRA), K.-H. Mack (INAF-IRA), H. de Ruiter (OAB)

Astrophysical  masers.

Interferometric studies indicate that extragalatic water maser emission at 22 GHz is associated either with nuclear activity in the very center of AGNs or with vigorous star formation in starburst regions of the host galaxies. While the strongest masers (the so called "megamaser", with LH2O >10 Lsun) are all located within a few parsecs from the nucleus of their parent galaxy, being related to 1) nuclear accretion disks 2) interaction between the nuclear jet(s) and ambient molecular clouds and 3) nuclear outflows, the less luminous ones (defined "kilomasers", with LH2O< 10 Lsun) are mostly found in star forming region far from the nucleus, with the exception of the masers in M51, NGC 4051, and NGC 253. Both classes of extragalactic water maser provide useful information on the molecular content, physical condition, and dynamics of the ISM in (often) obscured regions of active galaxies. Detailed studies of disk-masers yields mass estimates of the nuclear engine and trace the geometry of the disk (the best known example is NGC 4258). Jet-masers help to determine the speed of the material in the jet, while outflows masers trace the velocity and geometry of the nuclear wind. Maser related to star formation are also important because they mark the location of starbusrt regions and can be used to determine, through measurements of proper motion, distances and three dimensional velocity vectors of their host galaxies. (e.g. M33).  We perform interferometric and single-dish observations to discover and monitor masing regions both in active and starburst galaxies.

Main collaborators: C. Henkel (MPIfR), K.M. Menten (MPIfR), A. Brunthaler (MPIfR), J.A. Braatz (NRAO)
Diffuse radio sources in cluster of galaxies and beyond.

It is well known that radio emission from clusters of galaxies generally originates from individual radio emitting galaxies. In addition, some clusters of galaxies show the existence of large-scale diffuse synchrotron sources, that have no apparent connection to any individual cluster galaxy and are therefore associated with the intracluster medium. These radio sources have been classified as radio halos, relics and mini-halos depending on their morphology and location. Both halos and relics are extended (~ 1Mpc) diffuse, low surface brightness, steep-spectrum sources but while halos are located at the cluster center the relics are located at the cluster periphery. Radio halos and relics are typically found in clusters which show significant evidence for an ongoing merger. A few relaxed clusters show the presence of a diffuse synchrotron emission that extends quite far from the dominant radio galaxy at the cluster center, forming what is called a mini-halo.  These diffuse radio sources are extended on a moderate scale (~500 kpc) and, in common with large scale halos, have a steep spectrum and a very low surface brightness. All these radio sources are a direct evidence for the presence of relativistic electrons and magnetic fields in the intracluster medium. Given their low surface brightness, steep radio spectrum and large angular size, radio halos, relics and mini-halos, are very difficult sources to observe. We use the Very Large Array to obtain deep continuum and polarization images of these elusive sources with the purpose to study the origin of the relativistic electrons and the structure of the cluster magnetic fields.

 Main collaborators: L. Feretti (INAF-IRA), G.Giovannini (UniBo), M. Markevitch (SAO-CfA)

Large-scale magnetic fields in the intergalactic medium.

The existence of magnetic fields associated with the intracluster medium in clusters of galaxies is now well established through different methods of analysis. The strongest evidence for the presence of cluster magnetic fields comes from radio observations. Magnetic fields are revealed through the synchrotron emission of cluster-wide diffuse sources, and from studies of the rotation measure (RM) of polarized radio galaxies. Direct evidence for the presence of relativistic electrons and magnetic fields in clusters of galaxies comes from the detection, in an ever increasing number of galaxy clusters, of large-scale 'radio halos' or 'relics'. The presence of a magnetized plasma between an observer and a radio source changes the properties of the polarized emission from the radio source. Therefore a complementary set of information on clusters magnetic fields along the line-of-sight can be determined, in conjunction with X-ray observations of the hot gas, through the analysis of the RM of radio sources. Many high quality RM images of extended radio galaxies are now available in the literature. These data are consistent with magnetic fields of a few micro-Gauss throughout the clusters. In addition, stronger fields exist in the inner regions of strong cooling core clusters. In a few cases it has been possible to study the cluster magnetic field in more detail by sampling several extended radio galaxies located in the same cluster of galaxies. With the help of numerical simulations we are trying to interpret these radio and X-ray data with the aim to determine the strenght and structure of these large-scale magnetic fields.

 Main collaborators:  L. Feretti (INAF-IRA), G.Giovannini (UniBo), K. Dolag (MPA), G.B. Taylor (UNM)

 Radio emission from star-forming galaxies.

One of the major goals in studying spiral galaxies is to understand the relationship between the star formation process and the physical conditions in the interstellar medium. Since the discovery that stars form in molecular clouds, several efforts have been directed towards the study of the relation between the emission of the CO molecule, which traces the bulk of molecular gas, and the other star formation indicators. Global studies have revealed relationships between the radio continuum (RC), far-infrared (FIR) and CO emissions. In particular, the global correlation between the FIR and centimeter-wavelength RC emission has revealed as one of the strongest correlations in extragalactic research:  the two emissions are linearly correlated over five orders of magnitude in luminosity, with an rms scatter of less than a factor of 2. On global scales the FIR emission is well correlated with the CO emission and the CO emission is well correlated with the RC. What is most extraordinary about the FIR-RC or CO-RC correlations is that they couple emissions arising from completely different processes. The usual explanation of the FIR-RC correlation invokes massive-star formation which accounts for the thermal radio emission via ionizing stars, for the non-thermal radio luminosity via supernova events, and for the FIR luminosity by means of massive stars heating the dust. However, this basic scenario has too many steps and too many parameters to explain the tightness of the observed correlations and a "definitive'' model has not yet emerged. We conduct detailed studies of individual objects at high spatial resolution to better understand the physical processes behind the observed correlations.

Main collaborators: L.Blitz (UCB), T. Helfer (UCB), T. Wong (ATNF), R. Ekers (ATNF), L. Gregorini (UniBo)

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