Astron. Astrophys. 355, 873-879 (2000)

1. Introduction

The original classification of extended radio galaxies by Fanaroff & Riley (1974) is based on a morphological criterion, i.e. edge darkened (FR I) vs edge brightened (FR II) radio structure. It was later discovered that this dichotomy corresponds to a (continuous) transition in total radio luminosity (at 178 MHz) which formally occurs at erg s^-1 Hz^-1. The presence of radio sources with intermediate morphology, in which typical FR I structures (such as extended plumes and tails) are seen together with features characteristics of FR II sources (narrow jets and hot spots) (see e.g. Parma et al. 1987; Capetti et al. 1995) argues in favour of a continuity between the two classes.

From the optical point of view both FR I and FR II are associated with various sub-classes of elliptical-like galaxies, but statistically their populations are different (Zirbel 1995). Owen (1993) found that the FR I/FR II division is also linked to the optical magnitude of the host galaxy, possibly suggesting that the environment plays an important role in producing different extended radio morphologies. Moreover, FR II are generally found in regions of lower galaxy density and are more often associated with galaxy interactions with respect to FR I (Prestage & Peacock 1988; Zirbel 1997). Differences are also observed in the optical spectra: while FR I are generally classified as weak-lined radio galaxies, strong (narrow and broad) emission lines are often found in FR II (Morganti et al. 1992; Tadhunter et al. 1993), although a sub-class of weak-lined FR II is also present (Hine & Longair 1979).

Within the unification scheme for radio-loud AGN (for a review, see Urry & Padovani 1995), FR I and FR II radio galaxies are thought to represent the parent population of BL Lac objects and radio-loud quasars, respectively (Barthel 1989). In order to explain the lack of broad lines in the "mis-oriented" (narrow-lined) FR II-type objects, obscuration by a thick torus is invoked. A combination of obscuration and beaming is therefore necessary at least for the FR II-quasars unification (e.g. Antonucci & Barvainis 1985). However, there is evidence that this simple picture is probably inadequate: some radio-selected BL Lacs - among the most powerful sources in the class - display an extended radio structure and luminosity typical of FR II (Kollgaard et al. 1992; Murphy et al. 1993) and broad - although weak - lines have been observed in some BL Lacs. Moreover, Owen et al. (1996) noted that the lack of BL Lacs in a sample of radio galaxies located in Abell clusters can be an effect of their selection criteria if the parent population of BL Lacs includes both FR I and FR II. This idea is also consistent with a recently proposed modification of the unification scheme, which claim that the weak-lined FR II are indeed associated with BL Lac objects (Jackson & Wall 1999). These observations can be however reconciled with the unification scenario once continuity between the weak and powerful radio-loud sources is allowed and thus transition objects are expected.

In Chiaberge et al. (1999, hereafter Paper I) we studied HST images of all FR I radio galaxies belonging to the 3CR catalogue, finding that unresolved nuclear sources are commonly present in these objects. A strong linear correlation is found between this optical and the radio core emission, extending over four orders of magnitude in luminosity. This, together with spectral information, strongly argues for a common non-thermal origin, and suggests that the optical cores can be identified with synchrotron radiation produced in a relativistic jet, qualitatively supporting the unifying model for FR I and BL Lacs. Furthermore, the high rate detection ( 85%) of optical cores in the complete sample indicates that a standard pc-scale geometrically thick torus is not present in these low-luminosity radio galaxies. Any absorption structure, if present, must be geometrically thin, and thus the lack of broad lines in FR I cannot be attributed to obscuration. Alternatively, thick tori are present only in a minority of FR I. Given the dominance of non-thermal emission, the optical core luminosity also represents a firm upper limit to any thermal component, suggesting that accretion might take place in a low efficiency radiative regime.

The picture which emerges from this analysis is that FR Is lack substantial thermal (disk) emission, Broad Line Regions and obscuring tori, which are usually associated with radio-quiet and powerful radio-loud AGN.

As a natural extension of Paper I, here we study the HST images of a sample of low redshift FR II radio galaxies, in order to explore how the differences in radio morphology are related to the optical nuclear properties. In particular, one of the most important questions is whether the FR I/FR II dichotomy is generated by two different manifestations of the same astrophysical phenomenon, and the transition between the two classes is indeed continuous, or instead it reflects fundamental differences in the innermost structure of the central engine.

The selection of the sample is presented and discussed in Sect. 2, while in Sect. 3 we describe the HST observations. In Sect. 4 we focus on the detection and photometry of the optical cores. Finally, in Sect. 5 we discuss our findings.

© European Southern Observatory (ESO) 2000

Online publication: March 21, 2000
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