Jet Properties and Evolution in Small and Intermediate Scale Objects.pdf

a r X i v : 0 7 0 7 .3 5 1 6 v 1 [ a s t r o - p h ] 2 4 J u l 2 0 0 7 Extragalactic Jets: Theory and Observation from Radio to Gamma Ray ASP Conference Series, Vol. **VOLUME**, **YEAR OF PUBLICATION** T. A. Rector and D. S. De Young (eds.) Jet Properties and Evolution in Small and Intermediate Scale Objects Marcello Giroletti INAF Istituto di Radioastronomia, via Gobetti 101, 40129, Bologna, Italy Abstract. Kinematic and spectral studies are improving our knowledge of the age distribution in compact radio sources, providing evidence that small sources are generally very young. The properties of jets in objects spanning the size range from a few tens of parsecs to some kiloparsecs become then of particular interest. Because of our selection criteria and of the small scales involved, the properties of jets in the population of Compact Symmetric Objects (CSO) are not well known yet. Polarization properties seem to indicate a strong influence by the interaction with the dense surrounding medium, and some objects show evidence of relativistic bulk motion. More evolved jets are present in the class of Low Power Compact (LPC) sources and a number of cases are discussed here. Since it is becoming increasingly clear that not all these sources will survive to evolve into large scale radio galaxies, the question of the final evolution of the CSO and LPC population is also discussed, with examples of candidate dying sources. 1. Introduction Despite the large range of resolutions and scales sampled by different radio telescopes and arrays, observations tend to ubiquitously reveal a common basic structure. A central compact core is at the base of two jets flowing in opposite directions, and forming diffuse lobes; bright hot spots can be present at the site of interaction between the jet and the medium confining the lobes. As Fig. 1 shows, this basic structure is found both on sources that are extended over several hundreds kiloparsecs (such as Cygnus A, left image) and i