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Astron. Astrophys. 329, 845-852 (1998)

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1. Introduction

Extragalactic radio sources are observed with a wide range of size spanning from tens of parsec up to megaparsecs and with a variety of morphological forms and spectral shapes. Different classification schemes are possible depending on which aspect is chosen to describe them. Thus, one has extended and compact sources according to their size, double, triple or core-jet according to the emission distribution in radio maps and flat or steep spectrum, simple convex, or complex spectrum. Usually one designates compact those sources that are observed at sub-arcsecond to arcsecond scale. Two classes of compact sources are of particular interest. They are Compact Steep-spectrum Sources (CSS) and Gigahertz-Peaked-Spectrum (GPS) radio sources.

The first are compact objects of sub-galactic dimensions with angular size [FORMULA] 1 arcsec. They have a steep high frequency spectrum, the spectral index being [FORMULA] 0.5, hence the name, Compact Steep-spectrum Sources (Pearson et al., 1985; Fanti et al., 1989, 1990). The morphological structure resembles that of the more extended radio sources, and on scales of tens to hundreds milliarcsec, they show a double, triple, and core-jet structure.

On yet smaller scales we find the Gigahertz-Peaked-Spectrum (GPS) radio sources of typical extent of a few tens to hundreds of parsecs. Their name derives from the fact that they have a simple convex spectrum peaking around a few GHz (Gopal-Krishna et al., 1983; Spoelstra et al., 1985; Gopal-Krishna & Spoelstra, 1993). Those associated with quasars show a complex morphology in VLBI maps while sources associated with galaxies are double or triple radio sources (Phillips & Mutel, 1980, 1981; Mutel & Hodges, 1986). They have a steep spectrum at high frequencies and their low frequency spectral turnover is believed to be due to synchrotron self absorption in a compact component with a large magnetic field (Hodges et al., 1984; Mutel et al., 1985; Baum et al., 1990; O'Dea et al., 1991). They have an extremely high luminosity of order [FORMULA] erg s-1 similar to that of many quasars. Other important properties of the GPS have been described and summarized by O'Dea et al. (1991) and recent optical observations by de Vries et al. (1995) provide identification for several GPS radio sources and in a few cases, new redshifts have been measured.

The subclass of compact sources that exhibit a double-lobed structure is called compact double (CD) and has components with similar spectral shape and flux density (Phillips & Mutel, 1982). In contrast to what happens to asymmetric sources, this symmetric structure indicates that they are not Doppler-boosted. Even so, they have luminosities comparable to the luminous, edge-brightened, extended double sources. Phillips & Mutel (1982) suggested that they are young, scaled down versions of the classical double sources of Fanaroff-Riley class II type. Although recent observations have shown that some apparent compact doubles are in fact asymmetric core-jet sources, Wilkinson et al. (1994) have confirmed the existence of a class of objects that they call Compact Symmetric Objects (CSO). They present two-sided ejection on scales of [FORMULA] pc and are intrinsically very luminous since there is no indication of relativistic beaming. The CSOs would be associated with subclass CD of the GPS objects.

Apart from the different scale size, there are many similarities between the GPS sources and CSS. It has been suggested that the CSS sources are just larger versions of the GPS sources with the latter evolving in time into the CSS (Mutel & Phillips, 1988). Other authors however believe that the CSS sources are of sub-galactic size because they become trapped near the galactic center due a high density medium that they would encounter there (Fanti et al., 1990; O'Dea et al., 1991). Recently Fanti et al. (1995) have studied a sample of double-lobed CSS with linear sizes of a few kpc, which in analogy with the name CSO, they named MSOs (Medium-sized Symmetric Objects). They conclude that these are young objects with ages of [FORMULA] years and are the precursors of the extended double radio sources.

We have presented a model (Carvalho, 1985) in which we show that the properties of the CD sources are consistent with the "youth scenario". With ages less than [FORMULA] yr and having a sub-galactic extent of [FORMULA] kpc, the CD's are very young radio sources that evolve into extended FR II radio sources on time scales of [FORMULA] yr. O'Dea et al. (1991) suggested that this scenario also would apply to most GPS sources with galaxy identifications. Mutel & Phillips (1988) used this model to show that there exists an evolutionary sequence from compact doubles (CD) to compact steep-spectrum sources (CSS) to extended classical doubles.

More recently, Readhead et al. (1996a) proposed a classification scheme for symmetric sources based on their linear size. According to this the CSO would be objects [FORMULA] kpc, the MSO would have sizes in the range 1-15 kpc while they have called the more extended sources ([FORMULA] kpc) "Large Symmetric Objects" or LSO. These three classes all have common properties, that is, high luminosity, symmetric structure and are manifestations of the same type of objects seen at different epochs of their evolution, as has already been proposed by Carvalho (1985) and Mutel & Phillips (1988). The work by Bicknell et al. (1997) adopts the view that the GPS and CSS sources and CSO are the same sort of object and proposes a single model which explain their radio spectra and optical emission.

There are, however, indications that at least a fraction of the GPS sources may not be as young as initially thought. For instance, Baum et al. (1990) detected extended emission on scales of tens of kpc from the nucleus of [FORMULA]. Other observations by Stanghellini et al. (1990) showed that about [FORMULA] of the GPS sources present a diffuse extended radio emission though more recent results indicate that this number could be lower, in the range [FORMULA] (Stanghellini et al., 1997). Baum et al. (1990) proposed that in these sources the nuclear activity is a recurrent phenomenon, the diffuse emission being the relic of a previous activity while the compact GPS source would be the product of a renewed activity of the host galaxy. Another possibility is that the galaxy has suffered a recent and sudden increase in the gas density near the center due to an interaction or merger with a gas rich companion. The radio source has been smothered by the dense and clumpy gas that keeps the source confined to the nuclear region. In that case the GPS sources would not be young objects but have ages comparable to the extended doubles.

Indeed, recently Stanghellini et al. (1993) have made CCD images of a group of GPS sources and found disturbed optical morphologies and the presence of dust, suggesting that mergers and galaxy-galaxy interaction play an important role in these objects. A more recent analysis made by O'Dea et al. (1996) of a sample of forty GPS radio galaxies shows that [FORMULA] of them have distorted isophotes. They found that a large fraction of the GPS sources exhibits evidence for interaction and/or mergers. This corroborates the hypothesis that this subclass of GPS sources is confined in the dense nuclear region. Saikia et al. (1995) have investigated the symmetry parameters of a sample of CSSs and compared with those of extended sources. They found strong evidence of intrinsic asymmetry in the environment in many sources and on all scales. The effect, though, is more pronounced in the small objects where the jets propagate through an asymmetric medium probably suffering collisions with dense clouds. The existence of such a cloudy environment has recently been well establish by de Vries et al. (1997) through the discovery of the alignment between the optical and radio emission in CSS sources.

On the other hand, De Young (1991) showed, through hydrodynamical simulations of the interaction of a jet with a dense cloud, that this can be an efficient way to decelerate the jet. Following the encounter with a heavy intergalactic cloud, the jet velocity decreases drastically while the cloud is partially destroyed by the jet. Therefore, the propagation of the GPS sources in a very cloudy medium will be slow and consequently the age will be larger than that deduced from a uniform medium (O'Dea et al., 1991). We have recently developed a very simple analytic model for the jet propagation in a clumpy medium (Carvalho, 1994). A rough estimate of the age, taking the propagation velocity of the jet to be constant, already indicates values of around [FORMULA] yr, which is of the same order as the age of the extended sources. Here we shall discuss a more detailed model taking into account the variation of the propagation velocity with the distance from the central object, different modes of interaction of the jet with the clouds and other aspects of the jet propagation. We compare three forms of motion of the jet, namely scattering by heavy clouds (Model A), drilling through the clouds (Model B) and propagation in a dense uniform medium (Model C). As we shall see, the real situation is more likely to lie between Models A and B. In Sect. 2 we study the propagation of the jet in a medium filled with clouds and describe two of the models. In Sect. 3 we give the basic expressions of the jet motion in a homogeneous medium. Sect. 4 examines the matter distribution around the center of the galaxy and in the near intergalactic medium, while in Sect. 5 the age of the GPS sources is calculated using the proposed models. Finally, in Sect. 6 we give the main conclusions.

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© European Southern Observatory (ESO) 1998

Online publication: December 16, 1997