EXTRAGALACTIC RADIO SOURCES AND THEIR EVOLUTION

Involved IRA Scientists and Collaborators: M. Bondi, G. Brunetti, M. Chiaberge, L. Feretti, K.-H. Mack, P. Parma, I. Prandoni, C. Stanghellini, M. Vigotti, T. Venturi, D. Dallacasa, R. Fanti, G. Giovannini, F. Govoni, L. Gregorini, F. Montenegro Montes, M. Murgia, G. Setti, S. Varano

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.
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AGEING OF RADIO GALAXIES AND RELIC RADIO GALAXIES

One of the open questions in this field is the age of individual sources which must be determined via the synchrotron theory. Taking into account the different processes of energy losses suffered by the relativistic electrons, we can interpret the radio spectra and assign a radiative age to the source. In particular the curvature of the radio spectrum can be directly connected to the age of the radio emitting electrons.  This kind of study requires observational data at as many radio frequencies as possible. If the angular resolution and sensitivity of the observations are of sufficient quality one may even determine ages in different parts of a radio source.
An example is the detailed study of NGC 326, which is an X-shaped radio source with twin jets ending in two extended radio tails. The curvature of the radio spectrum increases going from the inner parts near the radio core to the ends of the tails. This corresponds to a gradual increase of the age of the electrons as one goes outwards, with the material further out being ejected at an earlier time. An image of the source and its radio spectra measured in different parts along the radio source are shown in Figure 3.

Fig 3 - Radio image of the galaxy NGC 326 and radio spectra of selected regions in the source.  The different curvatures of the spectra indicate the different ages of the various parts of the radio galaxy: stronger curvature indicates older radio emitting material. Note that outer regions are, as expected, older than the inner parts - Courtesy of M. Murgia.

From the spectra it should be possible to recognize sources in which the activity has halted, and no more fresh electrons are produced. Indeed a small number of such ``dying'' sources is known with ages higher on average than those of normal active sources. Dying sources have very steep radio spectra and show no features typical of activity (hot spots, radio core and jet). They are more easily detected at low frequency where energy losses are even less important. The relative numbers of active and dying sources, which should be compatible with the durations predicted by the theory of synchrotron radiation, are difficult to determine due to various observational limits like for instance different flux limits at different frequencies or angular size distributions of the sources. This is the most likely reason why too few dying sources have been found up to now. Future research projects include the construction of well-defined samples of radio sources with steep spectra, based on the WENSS and NVSS surveys, which will serve as a starting point of a new search for dying sources. Results and details in: Parma et al. 2007, Jamrozy et al. 2004.

GIANT RADIO SOURCES

Multiple frequency observations are used to study the synchrotron spectra and radiative ages of relativistic particles in the lobes of giant radio galaxies (see Figure 4).

Fig 4 - VLA D-array map of J1343+3758 at 4860 MHz convolved to an angular resolution of 20"x20" and overlaid on the optical field from the Digital Sky Survey. The linear size is 3.14 Mpc (H_0 = 50 km/s/Mpc). The contour levels are spaced by factors of sqrt(2). The first contour is 40 microJy/beam. The compact 1.16 mJy radio core coincides with the parent galaxy position (from Jamrozy et al. 2005).

Giant Radio Galaxies (GRGs, linear dimensions > 1 Mpc) represent a later evolutionary stage. Because of their huge angular size, these extreme objects represent powerful tools to study physical processes in radio galaxies in general. Also their large extents, well beyond the limits of their host galaxies, probe certain characteristics of the Intergalactic Medium (IGM), like for instance its particle density. The spectral age analysis of the GRGs and the determination of properties of the IGM in which they are embedded have shown that both their old age and the low particle densities in the ambient medium (2 to 3 orders of magnitude below those in galaxy clusters) are needed for a radio galaxy to become a GRG.  An ongoing project intends to find new, more distant, GRGs. Using the unique property of these objects to probe the density of the IGM, a sample of sources at higher redshift can test theoretical predictions on the evolution of the IGM. Details and results in: Jamrozy et al. 2005; Lara et al. 2004; Sohn et al. 2003.

RADIO SOURCES WITH RECURRENT ACTIVITY

Radio galaxies with recurrent activity combine some of the characteristics of very extended radio galaxies with those of young, more compact objects. The most prominent members of this class of sources are the so-called Double-Double Radio Galaxies (DDRGs). Similar to the relic galaxies only very few objects of this kind are known. Several programs are on their way to increase the number of known sources using the large radio surveys (WENSS, NVSS, FIRST).  The possible reasons for recurrent activity have been put into focus, in particular the problem of fueling the newly active nucleus. There are several projects to search for the tracers of fuel, both in molecular and atomic form. A search for CO emission in a representative sample of recurrent sources with the IRAM 30m telescope at Pico Veleta has revealed no significantly higher detection rate than that for extended, non-recurrent radio galaxies. Details and results in: Lara et al. 2002; Jamrozy et al. 2007; Marecki et al. 2006; Saripalli & Mack 2007.

HOT SPOTS IN RADIO GALAXIES

Hot spots define the impact of the radio jets on the ambient medium in which the radio galaxy is embedded. Optical counterparts of these shock fronts are found very rarely. We had a success rate of 70\% of the sample by observing nine hot spots with ISAAC and FORS at the VLT. This study has shown the necessity of particle acceleration in situ, and also the presence of re-acceleration which can create the extended emission seen in the optical. The results indicate that the shapes of the synchrotron spectra in hot spots are predominantly determined by the strength of the local magnetic fields. In particular, the break frequency which corresponds to the age of the oldest electrons in the hot spots, increases with decreasing synchrotron power and magnetic field strength. We are currently carrying out follow-up observations of the most prominent counterparts with integral field spectroscopy (VIMOS), high-resolution optical imaging (HST-ACS) and other multi-band observations (VLA, Spitzer). Details and results in: Brunetti et al. 2003; Mack et al. 2003; Prieto et al. 2002.

HIGH-REDSHIFT QUASARS

The small statistics of Quasi-Stellar Objects (QSOs) at high redshift (z > 4) can set limits on the models of formation and evolution of galaxies and their local black holes.  Using a statistically complete sample of 13 QSOs with z > 4 the spatial density of high-luminosity high-z QSOs (i.e. those which host the most massive black holes) was determined. One of the most important results for cosmological applications is the slower decline of space density for luminous QSOs with respect to less massive and less distant (z > 2) objects.

We started a multi-frequency observational programme on radio-loud broad-absorption line (BAL) QSOs, which discovered the most luminous radio BAL QSO known so far at z = 3.377. In addition this source possesses the second highest rotation measure ever observed (19000~rad/sqm). Ongoing observations from low radio frequencies to the optical domain are being performed to check whether these extreme properties can be found also in a complete sample of radio-loud BAL QSOs. Results and details in: Carballo et al.  2006; Benn et al.  2005; Vigotti et al. 2003.

MILLIMETER STUDIES OF LOW LUMINOSITY RADIO GALAXIES

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. The detection of large masses of molecular gas in the core of radio galaxies suggests that gas accretion may indeed power the radio jets. However many questions remain open, like for instance how is the gas transported into the nuclear region, and what determines the luminosity of the AGN. We are interested in ``stable'' FRI, twin-jet radio sources rather than obvious products of recent mergers. In order to increase the number of FRI radio galaxies with CO observations and so to improve our knowledge of their molecular gas properties, we are studying a volume-limited sample of 23 nearby (z<0.03) low luminosity radio galaxies, selected from the B2 catalogue. We notice that for 16 of such objects HST imaging is available. We used the IRAM 30m telescope to search for emission in the CO(1-0) and CO(2-1) transitions in 9 B2 radio galaxies of the z<0.03 volume-limited sample. First results are presented in Prandoni et al. 2007.

THE HST VIEW OF RADIO GALAXIES

Active Galactic Nuclei are objects where very high luminosities are produced in a very small volume through physical processes which cannot be ascribed to stellar nuclear fusion. According to the most accepted scenario, the ultimate source of AGN luminosity resides in the gravitational potential of gas in the vicinity of a supermassive black hole which loses angular momentum through viscous or turbulent processes and releases energy radiatively. AGN are classified on the basis of their different characteristics, most importantly their radio-loudness. The main research line concerns the study of the nuclear regions of radio galaxies, with a large use of HST observations, in the framework of the radio-loud AGN unification model. Using HST data we have identified, for the fist time, the nuclear optical emission associated with the AGN both in Fanaroff-Riley I and II (FRI-FRII) galaxies. We have recently shown that a comparative study of the properties of the inner regions of the beamed and unbeamed populations provides fundamental information about their physical structure (i.e. the properties of accretion around the central black hole and the structure of relativistic jets). The picture which emerges from our studies is that radio-loud sources appear in two different ''flavors'' which are not directly related to their radio morphology (see for details and results: Chiaberge et al. 1999; Chiaberge et al. 2002; Chiaberge et al. 2002).
The division is instead based on the properties of the accretion process onto the central black hole. There is a population of radiatively efficient, high accretion rate quasar-type objects (most, if not all, of them showing FRII radio morphology) and a population of unobscured ''starved quasars'' that differ in many crucial aspects from other AGN. The latter class includes most of FRI and a subclass of FRII. Such a scenario has important implications for the unification model and for a better knowledge of the physical processes at work in the nuclei of these objects.  To further test this scenario, HST data in the infrared (IR) have been recently acquired. The properties of the host galaxies are also investigated.  Among other results, it was found that the isophotal propoerties of FRI and FRII galaxy hosts at low redshift are indistinguishible.

SEARCHING FOR RADIATIVELY INEFFICIENT ACCRETION DISKS IN LOW LUMINOSITY AGN

We have identified a class of Low Luminosity AGNs (LLAGN) in which it is possible, in principle, to detect emission from low radiative efficiency accretion disks and directly test the models (see for details Chiaberge et al. 2005). We found a specific object (NGC4565) for which the nuclear spectral energy distribution is unusual, lacking the signature of standard accretion onto black hole, i.e. the big blue bump and the IR (dust-reprocessed) bump. NGC4565 is the first object for which IR-optical-UV observations directly show such a behavior.  We are currently deriving the spectral energy distributions of a small sample of LLAGN, and we find that their SED is not compatible with radiatively efficient accretion, as expected in the framework of the proposed scenario. For results and details see: Chiaberge et al. 2006.