EXTRAGALACTIC RADIO SOURCES

The physics of extragalactic radio sources is one of the main fields of research at the IRA. The ultimate goal is to understand the origin of the radio emission and the cosmological evolution of radio sources. Main research topics are:

RADIO GALAXIES AT CM WAVELENGHTS

Understanding the interactions of radio galaxies with their local environment is one of the key topics of the modern astrophysics. One way to study this is through the analysis of the polarization properties of radio galaxies which are modified, via Faraday effect, by the surrounding magnetized thermal medium. The IRA scientists have recently extended this kind of studies, traditionally related to galaxy clusters, by investigating a variety of environments, from rich cluster cores to very poor groups, and selecting a range of radio galaxy morphologies (elongated tails, or lobes with small axial ratios), all very extended and highly polarized. Very Large Array observations of the target sources have been analyzed in combination with high-quality X-ray data of the gas surrounding the sources from the Chandra, XMM-Newton and ROSAT satellites. A key aspect of the work is the use of 2D and 3D Monte Carlo simulations of magnetic fields to interpret the observational properties.
From these studies, it has been shown that tailed radio galaxies tend to show different properties with respect to radio galaxies with large lobes. Simulations suggest that a 2D magnetic field draped around the front end of the radio lobes is required to produce the observed structures. Moreover, rims of high depolarization have been detected at the edges of the inner radio lobes, spatially coincident with shells of enhanced X-ray surface brightness. It is in these regions, where both the field strength and the thermal gas density are likely to be increased by compression, that models like those of Bicknell et al. 1990 are most likely to apply.

Giant radio galaxies (GRG) are a remarkable subset of the radio galaxy population due to their very large linear size, in excess of ~700 kpc, up to 1.5 Mpc and beyond. Both FRI and FRII radio galaxies are represented in the GRG samples that are mainly selected from low resolution all sky surveys such as NVSS, SUMSS and WENSS. These sources are of particular interest as they represent extreme examples of radio source development, but to date their origin and evolution remain substantially unknown.
Starting from 2002, the soft gamma-ray sky has been surveyed by INTEGRAL/IBIS and subsequently by Swift/BAT at energies greater than 10 keV. As of today various all sky catalogues have been released, and their cross-check with multi-band information revealed that around 6%-8% of the high energy sources are radio loud active nuclei (AGN). Surprisingly, almost 30% of the population of gamma-ray radio loud AGN belongs to the class of GRG, suggesting a close connection between the radio and high energy properties.
An ongoing project, in collaboration with the INTEGRAL team, is addressing the nature of GRG by means of GMRT multi-resolution and multi-frequency radio observations to study the properties of their nuclei as well as of the lobe emission. The goal is to investigate if there are correlations with the high energy emission and with the environment, and to look for signatures of their special nature.


RADIO GALAXIES AT MM WAVELENGHTS

The fuelling of relativistic jets in radio-loud active galactic nuclei is still not understood. It is clear that energy is released in the vicinity of a supermassive black hole, but whether the mechanism is direct electromagnetic extraction of rotational kinetic energy or more closely related to the process of accretion remains a matter of debate. Gas accretion may indeed power the radio jets, at least in powerful AGNs associated to recent mergers. Less clear is the role of gas in "more stable" low luminosity twin-jet radio sources. The molecular gas component in nearby early-type galaxies which host a low luminosity AGN is under investigation to look for possible systematic differences between radio-loud and radio-quiet AGN. Preliminary results suggest that molecular gas does not likely set the radio-loudness of the AGN. ALMA observations in low-luminosity AGN, at the unprecedented spatial resolution of about 20 pc, will be used to test and refine scenarios of AGN feeding and feedback, and to discover new phenomena controlling gas structures and dynamics within 100 pc. These preliminary results obtained are an essential step to request even higher resolution ALMA observations in order to tackle the molecular torus below 10 pc. In addition new ALMA observations will allow us to better understand the physical properties in reaccelerating regions like those of the hot spots in FR II radio galaxies.


RADIO GALAXIES WITH HST

IRA is involved in an international project that focuses on radio galaxies studied with Hubble Space Telescope (HST) observations in the UV, optical, near-IR. Objects from z~0 up to z~2.5 are imaged in order to study the properties of the environment, from the nuclei of the host galaxy of these objects, to the scale of Mpc. Multi-wavelength information has been collected over the past 10-15 years and it is used to study the relationship between the nuclear activity (accretion onto the central black holes, production of relativistic jets) and the physical properties of the surroundings (host galaxy structure, dust distribution, galaxy mergers, star formation, and cluster environment). Recently, important results were achieved on the properties of a newly discovered sample of distant low-power radio galaxies. These objects provide a link with normal, non-active galaxies, but they are extremely important since they are associated with the most massive elliptical galaxies in the Universe and the most massive black holes, even at high redshifts. A new technique has been used to find clusters: the technique is based on a statistical treatment of photometric redshifts. It was found that these objects are better beacons for clusters of galaxies than their high-power counterparts, which have been used for decades for cluster searches.
This enables a new method of finding high-redshift galaxy clusters that will be extremely useful when data from future missions (e.g. Euclid, LSST) will become available.