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Astron. Astrophys. 362, 27-41 (2000)

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1. Introduction

Compact steep-spectrum sources (CSSs) are defined to be less than about 20 kpc in size (Ho = 50 km s-1 Mpc-1 and [FORMULA]), and have a steep high-frequency radio spectrum ([FORMULA]), where S[FORMULA]. This class of objects includes the more compact Gigahertz-Peaked Spectrum (GPS) sources with a turnover in their spectrum at frequencies in the GHz range. The larger CSS sources also sometimes exhibit a flattening or turn-over in the integrated spectrum, but the turnover frequency is usually in the range of a few hundred MHz. High-resolution radio observations of CSS and GPS sources (cf. Fanti et al. 1990; Sanghera et al. 1995 and references therein) reveal a variety of structures reminiscent of those seen in the more extended sources. The majority of sources have a double-lobed structure, often with a nuclear or core component. Only a small fraction of sources appear to have a highly distorted or complex structure.

A number of possible explanations have been explored to understand the nature of CSS and GPS objects (cf. O'Dea 1998). It has been argued that only a small fraction are likely to appear small because of projection effects (Fanti et al. 1990). Although some may be distorted and confined to small dimensions by a dense interstellar medium in the host galaxy (cf. Wilkinson et al. 1991a,b; Carvalho 1994, 1998), there appears to be a consensus that their small sizes are largely because they are young sources seen at an early stage of their evolution (Carvalho 1985; Fanti et al. 1995; Begelman 1996; Readhead et al. 1996a,b). The GPS sources with a double-lobed structure, the compact doubles (CD), have been proposed to be miniature versions of the classical Fanaroff-Riley class II sources, and have been suggested to evolve from a CD to a CSS and then on to a larger FRII source (Phillips & Mutel 1981; Hodges et al. 1984; Mutel & Phillips 1988; Carvalho 1985).

In this paper we present the results of an Inter-Planetary Scintillation (IPS) study of a large sample of CSS and GPS radio sources at 327 MHz using the Ooty Radio Telescope (ORT). These observations have been made with the objectives of (i) determining the sizes and prominence of hotspots to investigate the collimation of radio jets and the evolution of these sources; (ii) investigating the existence of radio halos at a low frequency on the scale of hundreds of milliarcsec in addition to the more compact structure, suggestive of earlier periods of activity; and (iii) determining the low-frequency spectra of compact components in an attempt to distinguish between different processes to explain the low-frequency turnover in the spectra of GPS sources. We describe the IPS observations, the sample of sources observed by this technique and the results of the IPS observations in Sect. 2. The dependence of the prominence of hotspots from both IPS and interferometric measurements on radio luminosity and linear size, as well as the results on the collimation of radio jets and possible constraints on evolution of radio sources are presented in Sect. 3. The evidence for the existence of possible large-scale structures on the scale of hundreds of mas is presented in Sect. 4. In Sect. 5, we present the spectra of 5 GPS sources where we have attempted to determine the spectra of the dominant component from IPS and interferometric observations, and discuss possible reasons for their spectral shape.

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© European Southern Observatory (ESO) 2000

Online publication: October 30, 19100