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
RESEARCH:
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