Daniele Dallacasa's Home Page

Postal  address:    Istituto di Radioastronomia - CNR
                               via P. Gobetti n. 101
                                I-40129 Bologna (Italy)
Telephone:            +39 51 6399386
Fax:                       +39 51 6399431

e-mail:                   ddallaca@ira.bo.cnr.it

Figure: Two tailed radio galaxies in A119 as seen at 21 cm with the VLA. The restoring beam is slightly smaller than 4", while each tail is at least 5' long. The elliptical galaxies hosting the radio sources are located  at the top of each radio source. The cluster centre is just outside this image, about 1-2' north of the source on the top part of the image. The  source closer to the cluster centre is nearly completed depolarized at this wavelengths, while the other maintains significant polarized emission at 21 cm. Note the wraps in both tails, and the presence of weak radio emission from other cluster members.

This page is still under construction. Last update Apr. 14, 1998


Extra galactic Astronomy:

                       - Compact Peaked-Spectrum radio sources

Compact Peaked-Spectrum (CPS) radio sources are characterized by sub galactic sizes (<15kpc), and are often confined to the nuclear region (<1kpc) of distant elliptical galaxies (also seen as quasars). They are quite common in flux density radio catalogues (from 15% to 30% depending on the observing frequency).Their radio spectra are steep at high frequencies, and then have a turnover at frequencies ranging from a few GHz (and the source is termed GHz-Peaked Spectrum, GPS) to a few tens of MHz (and we have a Compact Steep-Spectrun object, CSS). So far only the brightest objects have been studied, and this is related to the issue that CPS sources are very powerful in the radio domain. Moreover they are generally weakly polarized (often <0.1% at 5 GHz), with a few outstanding exceptions among quasars (e.g. 3C138, 3C286).
Due to their small angular and linear sizes, their morphology can be suitably studied by means of Very Long Baseline Interferometry (VLBI). The observations carried out in the last decade have shown that the radio structures are the scaled down version of FR2 radio sources, with a higher incidence of distorted morphologies and asymmetries. This evidence, coupled with the absence of a population of powerful, nearby CPS sources led to the development of the "evolutionary" scenario, where the CPS we observe are indeed young radio sources at the beginning of their radiative life. They will grow large to tens or hundreds of kpc in about 107 years, decreasing their radio luminosity by about one order of magnitude.
Nowadays, there is a lot of interest in the determination of new samples of much weaker CPS sources in order to have a more comprehensive and statistically stronger database; moreover the study of intrinsically weaker sources will allow to refine the evolutionary model and to extend its validity to the more numerous population of faint sources.

Papers related to this topic:  refs. 04, 05, 08, 10, 11, 12, 13, 15, 16, 17
               EVN dual frequency observations of CSS-GPS candidates  (ref. n. 16)
                Dallacasa, D., Bondi, M., Alef, W. Mantovani, F. A&AS , Vol. 129, p. 219
               (g-zipped postscript  with figures, 460k)

                       - BL Lac Objects and Unified Scheme models

BL Lac objects are a population of sources with the axis of the anisotropic emission closely aligned to the line of sight.  The anisotropic emission from relativistic particles is amplified, and the Doppler boosting enhances to high apparent fluxes sources which are intrinsically weak. Also the flux density variability is amplified, and the many BL Lac objects are also classified as BLAZARS.
In the framework of Unified Scheme models, the misaligned population was first proposed to be found in the low-power FR1 radio galaxies. However, various works done in the last few years outlined a number of inconsistencies of the proposed scenario, and there is a debate on whether some high power FR2 radio galaxies have to be  also considered,  in order to make a mixed parent population made up with both FR1 and FR2 galaxies. In the radio domain studies need to be carried out on the  unbeamed extended emission, whose power have to be compared with the typical values found for FR1 and FR2, and on the magnetic field configuration and the radio morphology of the kpc scale jets (when observed).
The jets in FR1 and FR2 have different properties: in FR1 they are two sided and contains a large fraction of the total flux density since they probably decelerate and brighten at a few kpc from the nucleus, and the magnetic field is generally perpendicular to the jet axis; on the other hand, in FR2 the jets are weak, one-sided and remain relativistic until they get in the outer edge of the source where they hit the ambient medium originating the hot-spots; the magnetic field is generally aligned with the jet axis.

Papers related to this topic:  ref. 14

                     - Intracluster magnetic field

Clusters of Galaxies are known to posses intergalactic magnetic field whose origin, strength and properties are still matter of debate. A few clusters are known to possess large haloes with sizes up to 1 Mpc, while other clusters are characterized to bring large Faraday rotation to the polarized radio emission, indicative of high ion and electron density and/or large intergalactic magnetic fields. The determination of the polarization properties in extended radio source in clusters (typically tailed radio sources) allows the identification of differential RM across different lines of sight. Results on the Coma cluster are reported in ref. 07. More data on A119 are being analysed: two tailed sources are projected in a region close to the cluster centre, and show an decrease of the Faraday rotation with increasing distance from the X-ray centroid.
Another extended radio source (3C29) at the cluster periphery suffers of no significant Faraday rotation.

Papers related to this topic: refs. 01, 02, 07, 18

                      - Radio flux density monitoring of blazars (single dish)

Extra galactic sources characterized by considerable flux density variability in many if not all observing bands are usually termed BLAZARS. The optical IDs happen generally in BL Lac objects or Flat Spectrum Quasars (FSQ). The extreme variability from the gamma rays through the radio is interpreted in terms of relativistic beaming; changes in structure and/or flux density are enhanced due to the geometry: a jet of relativistic particles is closely aligned to the line of sight, and every variation is amplified due to Doppler boosting.
In the radio domain, intrinsic flux density variability is a common phenomenon among core dominated objects, and in particular among BL Lac objects and Flat Spectrum Radio Quasars. The variability starts at higher radio frequencies and then develops at  longer wavelengths decreasing the amplitude of the phenomenon, which becomes marginal (some percent at 1.4 GHz).
At the Medicina 32m VLBI antenna, we are carrying out a single dish monitoring of the flux density of a sample of about 20 blazars. The sources were selected from a list of target sources for the Italian-Dutch X-ray satellite SAX, and the radio observations were intended to complete the information derived from the X-ray observations.
Now the program stands on its own, and monthly observations have been taken since January 1996 at 3.6 cm. During this period, we also collected flux density measures at 1.3 cm as well, ending in the summer 1997. From October 1997 we started a monthly monitoring at 6 cm as well, in order to have information on the variations of the spectral index as well.  This switch was necessary due to the to the frequent problems related to the weather conditions, seriously limiting the quality of the data at 1.3 cm.

papers related to this topic:
None as yet.

Bibliography (refereed papers only)

01. Dallacasa, D., Feretti, L., Giovannini, G., Venturi, T. 1989.
Multifrequency Observations of the Tailed Radio Source NGC4869 in the Coma Cluster
Astronomy and Astrophysics Suppl. Ser., vol. 79, p. 391.

02. Feretti, L., Dallacasa, D., Giovannini, G., Venturi, T. 1990.
Astrophysical Implications of the Study of the Tailed Radio Source NGC 4869 in the Coma Cluster
Astronomy and Astrophysics, Vol. 232, p. 337.

03. Baum S.A., O'Dea, C.P., Dallacasa D., de Bruyn, A.G., Pedlar, A., 1993.
Kiloparsec-Scale Radio Emission in Seyfert Galaxies: Evidence for Starburst-Driven Super winds?
The Astrophysical Journal, Vol. 419, p. 553.

04. Dallacasa D., Fanti C., Fanti R., Schilizzi R.T., Spencer R.E., 1995.
A Sample of Small Size Compact Steep-Spectrum Radio Sources: I. VLBI Images at 18 cm
Astronomy and Astrophysics, Vol. 295, p. 27.

05. Dallacasa D., Cotton, W.D.,Fanti C., Fanti R., Foley, A.R., Schilizzi R.T., Spencer R.E., 1995.
5 GHz VLBI polarization of 3C138
Astronomy and Astrophysics, Vol. 299, p.671.

06. Fanti C., Fanti R., Dallacasa D., Schilizzi R.T., Spencer R.E.,  Stanghellini C., 1995.
Are Compact Steep Spectrum Sources Young?
Astronomy and Astrophysics, Vol. 302, p. 317.

07. Feretti L., Dallacasa D., Giovannini G., Tagliani A., 1995.
The Magnetic Field in the Coma Cluster
Astronomy and Astrophysics, Vol. 302, p. 680.

08. Rantakyr\"o, F.T., B{\aa}{\aa}th, L.B., Dallacasa, D., Jones, D.L., Wehrle, A.E., 1996.
Multiband VLBI Observations of CTA102
Astronomy and Astrophysics, Vol. 310, p. 66.

09. Tian Wen-wu, Dallacasa, D., Sanghera, H.S., 1996.
Simulating the effects of frequency switching
Acta Astrophysica Sinica, Vol. 16, p. 154.

10. Jiang, D.R., Dallacasa, D., Schilizzi, R.T., L\"udke, E., Sanghera,  H.S., Cotton, W.D., 1996.
5 GHz EVN Polarization of 3C286
Astronomy and Astrophysics, Vol. 312, p. 380.

11. Stanghellini, C., Bondi, M., Dallacasa, D., O'Dea, C.P., Baum, S.A.,  Fanti, R., Fanti, C., 1997.
The radio source OQ208: parsec scale morphology and spectral  properties
Astronomy and Astrophysics, Vol. 318, p. 376.

12. Cotton, W. D., Fanti, C., Fanti, R., Dallacasa, D., Foley, A.R.,  Schilizzi, R.T., Spencer, R.E., 1997.
VLBA polarimetric observations of 3C286 at 5 GHz
Astronomy and Astrophysics, Vol. 325, p. 479.

13. Cotton, W. D., Dallacasa, D., Fanti, C., Fanti, R., Foley, A.R.,  Schilizzi, R.T., Spencer, R.E., 1997.
Dual frequency VLBI polarimetric observations of 3C138
Astronomy and Astrophysics, Vol. 325, p. 493

14. Stanghellini, C., Dallacasa, D., Bondi, M., Della Ceca, R., 1997.
Arcsecond scale radio polarization of BL Lacertae objects
Astronomy and Astrophysics, Vol. 325, p. 911.

15. Stanghellini, C., O'Dea, C.P., Baum, S.A., Dallacasa, D., Fanti, C.,  Fanti, R.,1997.
A VLBI study of GHz-Peaked-Spectrum radio sources. I: VLBI images at 6 cm
Astronomy and Astrophysics, Vol. 325, p. 943.

16. Dallacasa, D., Bondi, M., Alef, W., Mantovani, F., 1998
EVN dual frequency observations of CSS--GPS candidates
Astronomy and Astrophysics Suppl. Ser., Vol. 129, p. 219.

17. Stanghellini, C., O'Dea, C.P., Dallacasa, D., Baum, S.A., Fanti, R.,  Fanti, C., 1998,
Radio Observations of a complete sample of GHz-Peaked-Spectrum  radio sources
Astronomy and Astrophysics Suppl. Ser., Vol. 131, p.  303.

18. Feretti, L., Dallacasa, D., Govoni, F., Giovannini, G, Taylor, G.B.,  Klein, U., 1999,
The radiogalaxies and the magnetic field in Abell 119
 Astronomy and Astrophysics, submitted