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Astron. Astrophys. 345, 769-777 (1999)

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1. Introduction

Flux density limited radio source samples are known to contain a large portion of compact sources with high frequency spectral index [FORMULA] [FORMULA] 0.5 ([FORMULA]) and angular sizes below 2". The corresponding projected linear sizes are typically [FORMULA] 15 kpc 1. Fanti et al. (1990) have shown that the majority of these CSS sources cannot be larger sources foreshortened by projection effects, which means that their radio emission originates on sub-galactic scales.

It is obvious that one should investigate the connection between CSS and larger radio sources. Two scenarios have been proposed that would naturally explain the observed small sizes. First, they could reflect an early stage in the evolution of radio sources. This is the youth scenario (Phillips & Mutel 1982; Carvahlo 1985). The second possibility is that the unusual conditions in the interstellar medium of their host galaxies, such as a higher density and/or the presence of turbulence, inhibit the radio source from growing to larger sizes. This is the frustration scenario (van Breugel et al. 1984).

Most CSS sources exhibit double-lobed structures such as seen in classical radio galaxies. This feature is common to both quasars and galaxies. Quite a few of these are symmetric, which gave rise to the terms CSO (compact symmetric objects, with sizes [FORMULA] 0.5 kpc) and MSO (medium-size symmetric objects, with sizes [FORMULA] 0.5 kpc). These sources are considered as scaled-down versions of larger-sized double radio sources (Fanti et al. 1995). A minority of CSS sources is made up by sources with complex or highly asymmetric structures. These are mostly quasars, with most of their luminosity provided by the jet. The distortions may be caused by jet bending, which is further amplified by strong projection.

A plausible organization scheme for these different morphological classes is to identify CSO and MSO with progenitors of large doubles (`baby radio galaxies'), while the asymmetric CSS sources could represent frustrated radio sources.

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© European Southern Observatory (ESO) 1999

Online publication: April 28, 1999
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