VERY LONG BASELINE INTERFEROMETRY RESEARCH

Involved IRA Scientists and Collaborators: M. Bondi, L. Feretti, M. Giroletti, K.-H. Mack, F. Mantovani, C. Stanghellini, T. Venturi, D. Dallacasa, C. Fanti, R. Fanti, G. Giovannini, E. Liuzzo, M. Orienti, A. Rossetti.

The technique of Very Long Baseline Interferometry (VLBI) provides the highest angular resolution achievable in astronomy. With the VLBI technique the signals recorded by telescopes located in different parts of the world are combined together thus allowing scientists to study the finest details of astrophysical phenomena. At the IRA, VLBI is employed to study the properties of extragalactic jets, their implication on the unification of AGN and the life cycle of radio galaxies with particular attention to the Giga-Hertz-Peaked Spectrum (GPS) and Compact Steep Spectrum (CSS) sources. The Institute operates two 32-m dishes that belong to the European VLBI Network (EVN), and participates also in observations with  global networks and Space  VLBI.

RELATIVISTIC jETS

It is well established that a super-massive black hole (SMBH, m<=10^(9) Solar mass) is responsible for the energetic phenomena that occur in AGNs. In radio loud sources, twin jets of plasma are ejected in opposite directions from the SMBH and feed extended radio-emitting lobes. The properties of these jets (composition, velocity structure, origin, collimation, acceleration mechanisms, etc.) are still poorly understood.  VLBI observations are used to address these problems. The BL Lac object Markarian 501 was observed with high sensitivity (using the High Sensitivity Array, HSA) and resolution (using Space VLBI, VSOP). The HSA observations reveal a one-sided jet (see Figure 7a) more than 300 pc long, which constrains the jet velocity (which has to be relativistic), viewing angle and magnetic field.

Fig 7a - High Sensitivity Array observations of Markarian 501, which reveal the one-sided jet out to several 100 pc (Giroletti et al. 2004).

The VSOP data have shown evidence of a ``limb-brightened'' structure in the inner jet (see Figure 7b) suggestive of a transversally resolved velocity field with a fast inner spine and a slower outer shear.

Fig 7b - A closer look  at the inner jet region using VSOP: a limb-brightened structure is well visible thanks to the higher resolution (from Giroletti et al. 2004).

Very high frequency observations (mm-VLBI), which can provide even higher resolution, are in progress. The launch of the gamma-ray missions AGILE and GLAST will provide new insights into the physics of relativistic jets. Radio observations aimed at maximizing the scientific return of these missions are also planned.  Details in: Giroletti et al. 2004.

UNIFIED MODEL

The existence of relativistic jets has been postulated to account for different observational properties of extragalactic radio sources, such as radio galaxies, radio loud quasars and BL Lac type objects. By studying the jets on parsec-scale in various classes of radio sources, it is possible to gather evidence for intrinsically similar characteristics underlying the different observational properties. For example, Giroletti et al. 2004 and 2006 have studied a sample of 30 BL Lac objects on parsec-scale, and found that their jets are relativistic and closely aligned with the viewing direction. Taking into account the corresponding Doppler factor, which measures the amplification due to relativistic beaming, these objects are consistent with a parent population composed of Fanaroff-Riley I (low power) radio galaxies. In addition, Giovannini et al. 2001 have demonstrated that low and high power radio galaxies in a sample of 27 objects have parsec-scale jets with similar velocity (see Figure 8a and Figure 8b). An extension of this sample to almost 100 radio galaxies  is currently underway. Details and results in: Giroletti et al. 2004; Giovannini et al. 2005; Giroletti et al. 2006.

Fig 8a - Relativistic parsec-scale jets in low (3C 66B) power radio galaxies from the sample of Giovannini et al. (2001).

Fig 8b - Relativistic parsec-scale jets in high (3C 452) power radio galaxies from the sample of Giovannini et al. (2001).

GIGA-HERTZ-PEAKED SPECTRUM AND COMPACT STEEP SPECTRUM SOURCES

A significant fraction of bright radio sources showing high frequency spectral steepening exhibits a compact morphology.  These sources according to their radio spectrum are classified as Compact Steep Spectrum (CSS), GigaHertz Peaked Spectrum (GPS) and High Frequency Peaked (HFP). Empirical studies of various CSS/GPS samples have shown the existence of an anti-correlation between the spectral peak and the linear size. In the evolutionary scenario this implies that the youngest (i.e. smallest) sources must be sought among those with the highest frequency spectral peak. The study of these objects is a key element to understand the evolution of the powerful extragalactic FRI and FRII radio sources. There is a general consensus that CSS and GPS sources are small (few kpc or less) because they are young, but there are still open questions about their evolution, the interaction with the interstellar medium of the host galaxy, the physics of the jets and so on.

Strong support to the youth scenario comes from the determination of both the kinematic and the radiative age.  The kinematic age can be derived by the measure of proper motion of the radio lobes in CSS sources. By looking at the advance velocity of the outermost edges of these objects, which can only be measured by observing with VLBI the tiny displacements occurring over the years, we can estimate their age (see Figure 9). Often the age is found to be very small, as small as a few hundred years. In combination with studies of the spectral ageing of the emitting particles, there is now good evidence that these sources represent the first steps in an evolutionary sequence in which they can grow to the large size of classical radio galaxies.

Fig. 9 - Contours showing the hot spot location in the western lobe of 4C31.04 at two epochs separated by five years. The ~0.8 mas displacement over five years corresponds to a source age of ~550 years (from Giroletti et al. 2003).

The radiative age is derived by the steepening of the source radio spectrum caused by radiative losses. In the lobes of CSS/GPS where the old electrons are deposited during the whole lifetime of the radio source, radiative ages of thousands of years have been determined.   These ages are well in agreement with the kinematic ages obtained by VLBI observations. Details and results in: Orienti et al. 2007; Murgia et al.  1999.

GPS/CSS EVOLUTIONARY MODELS

The youth scenario is generally accepted but a few questions are still unanswered.  Particularly, it is expected that a radio source with a constant jet-power, expanding in a medium with a King-like density distribution, increases in radio power as it grows within the core radius, and then decreases in luminosity as it gets outside the core radius. Such evolution would lead to a number of compact sources substantially lower than what is observed. This inconsistency raises questions about our current model for either the gas density distribution or the physical radio source evolution.  Starting from basic assumptions, a zeroth order analytic model has been developed to describe the main physical jet and lobe evolution parameters in powerful radio sources originating from a central AGN. In particular, the model yields the source advance speed and luminosity as a function of the source linear size for a source evolving in a King-like gas density distribution. The observational constraints on the evolutionary models come mainly from the observed source distribution in the radio power-linear size plane, because each source follows a path in this plane as it evolves. However, the samples previously used for such a statistical analysis were selected from catalogs potentially biased against very compact sources, dropped in flux limited samples because of self-absorption. New samples of double sources have been extracted from the 3CR (Jenkins et al. 1977) and from the PW (Peacock & Wall 1982) catalogs. In order to collect information on the source morphology and spectral shape new observations as well as literature data have been used. The resulting sample, spanning a range in linear sizes from 10 pc to 100 kpc, has allowed us to describe the luminosity function and the diameter distribution shapes and to re-discuss the evolutionary model in terms of the new statistical results. Details and results in: Rossetti 2003, PhD thesis; Rossetti et al. 2006.

Selection Effects: the selection of most of these radio sources is based only on their radio spectra, it is possible that other objects, such as blazars, may temporarily meet the selection criteria and, therefore, contaminate the sample ( Stanghellini et al. 2005).
During most of their lifetime blazars and young radio sources display very different characteristics: the former objects possess significant flux density variability, and the emission, often polarized, has a core-jet morphology. The latter objects do not display any variability and the emission, unpolarized, has a “Double/Triple'' structure on the parsec-scale. Simultaneous multi-frequency VLA observations performed at different epochs to check the variability and the polarization ( Orienti et al.  2007; Tinti et al. 2005) together with high-resolution VLBA observations performed to study the morphology ( Orienti et al. 2006) have been carried out for the only existing sample of 55 HFP candidates ( Dallacasa et al. 2000). Considering all the information it was found that about 45% of the sources (all the galaxies and a few quasars) match the typical characteristics of young radio sources, while 55% match the characteristics of blazars.

To obtain a more complete picture of this class of sources a complete sample of young radio sources in the local Universe has been built. This sample, called COmpact Radio sources At Low redshifts (CORALZ) is composed of 17 sources at z < 0.16 and is currently being studied in several observational programmes from the radio band to the X-ray regime. Details and results in: Snellen et al. 2004; Vink et al. 2006.
The higher sensitivity of new observations has also led to the discovery of some sources (Low Power Compact Sources) whose central engine could be too weak to form a full-grown radio galaxy (see Figure 10).

Fig 10 - NGC 4278, a Low Power Compact source with parsec-scale jets that are unable to form a large scale structure (from Giroletti et al. 2005).

Moreover there are sources that are going through intermittent phases of nuclear activity (see Figure 11).

Fig. 11 - The inner structure of  1144+35B, which could be formed in a new phase of nuclear activity, while a previous one could be responsible for the large scale emission (extending beyond the borders of this image) (from Giovannini et al. 1999).

Details and results in: Giroletti, Taylor & Giovannini 2005.

POLARIMETRIC OBSERVATIONS OF GPS/CSS SOURCES

As the source evolution strongly depends on the environmental physical properties, a polarimetric study on suitable CSS samples has been performed to obtain information on the status of the gas in which the sources expand.  Because of their small sizes, GPSs reside in the inner regions of the host galaxies and are largely confined to the narrow line region (NLR) with its relatively large column density of ionized plasma. Even for moderate magnetic field intensity in the NLR, significant Faraday Rotation takes place in these sources. If the clouds are randomly distributed and the field structure is not resolved, significant wavelength-dependent depolarization of the radiation is expected. A statistical analysis of the polarization properties has been carried out for the B3 VLA-CSS sample (Fanti et al.  2001) using multi-frequency VLA data combined with WSRT new data. A model for the external Faraday screen has been developed.

A similar analysis has been carried out for the 3CR+PW CSS sample on four-frequency low resolution 100m Effelsberg telescope data. Furthermore, high-resolution VLA data at 8.4, 15 and 23 GHz are also available for the same sample. The comparison between the high-resolution and the low-resolution data provides some clues about the gas density distribution. A sub-sample of these Young Radio Sources, mainly associated with quasars, has also been observed at mas resolution with the VLBA (see Figure 12).

Fig 12 - The VLBA image at 8.4 GHz of the high luminosity compact steep-spectrum quasars B1524-136 restored with a circular beam of 6 mas. This observation reveals well-defined radio jets on both sides of an active nucleus (from Mantovani et al. 2002).

The observations allowed us to investigate the polarization characteristics of their jets, lobes and cores, and possibly jet-cloud interactions. New data sets are under analysis.  Details and results in: Mantovani et al.  2002; Rossetti et al. 2005.

SECULAR OPTICAL BEHAVIOUR AND STRUCTURAL CHANGES OF BL LACS

VLBI observations of optically violent variable BL Lac objects are an ongoing project carried out in collaboration with people from the University of Rome.  Recently, new EVN observations of ON231 (see Figure 13) and OQ530 have allowed us to study the link between the optical variability and the nuclear radio structure. The possible scenarios for interpreting both the time evolution in the optical luminosity and the changes in the parsec scale structure are: a jet pointing very close to the observer direction and undergoing strong instability and oscillations; a slowly precessing jet, which approached the observer line of sight during the past few decades. Details in: Massaro et al. 2001; Massaro et al.  2004.

Fig 13 - VLBI map of ON 231 at 5 GHz. The contour levels are -1.5, 1.5, 3, 6, 10, 15, 20, 30, 50, 70, 100, 150 mJy. The peak brightness is 170.17 mJy/beam. The beam is 3.8 x 1.4 mas^(2) at PA=13 deg (from Massaro et al. 2001).