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Astron. Astrophys. 357, L45-L48 (2000)

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1. Introduction

The radio properties of quasars with otherwise very similar optical properties can be markedly different. There is a clear dichotomy between radio-loud and radio-quiet quasars in optically selected samples. The radio-loudness is usually characterized by the radio-to-optical flux ratio. In the PG quasar sample, which is probably the best studied quasar sample in the radio and optical (Kellermann et al. 1989, Boroson & Green 1992), radio-loud and radio-weak quasars separate cleanly in two distinct populations (e.g. Kellermann et al. 1989).

It is known that radio-loud AGN almost never reside in late type, i.e. spiral galaxies (e.g. Kirhakos et al. 1997, Bahcall et al. 1995) whereas radio-quiet quasars appear both in spiral and in elliptical host galaxies. Furthermore, all relativistically boosted jets with superluminal motion and typical blazars have been detected in early type galaxies (e.g. Scarpa et al. 1999). It is still unclear, why AGN in spiral galaxies, at the same optical luminosity as their elliptical counterparts, should not be able to produce the powerful, relativistic jets seen in radio galaxies.

However, a few sources with intermediate radio-to-optical ratios appear to be neither radio-loud nor radio-quiet. They form a distinct subclass with very similar radio morphological and spectral properties. They all have a compact core at VLA scales and a flat and variable spectrum in common. These properties are very similar to the ones of radio cores in radio-loud quasars, but their low radio-to-optical ratio and their low extended steep-spectrum emission is atypical for radio-loud quasars. Miller et al. (1993) and Falcke et al. (1995, 1996a&b) have identified a number of these sources, called "radio-intermediate quasars" (RIQs), and suggested that they might be relativistically boosted radio-weak quasars or "radio-weak blazars". This would imply that most, if not all, radio-quiet quasars also have relativistic jets. In fact, VLBI observations of radio-quiet quasars already have shown high-brightness temperature radio cores and jets (Blundell & Beasly 1998). A crucial test of the relativistic jet hypothesis is the search for apparent superluminal motion in these sources. A prime candidate for detecting this is the brightest radio source in the RIQ sample, III Zw 2, which we discuss in this paper.

III Zw 2 (PG 0007+106, Mrk 1501, [FORMULA]) is one of the most extremely variable radio sources and a very unusual AGN. It was discovered by Zwicky (1967), classified as a Seyfert I galaxy (e.g., Arp 1968; Khachikian & Weedman 1974; Osterbrock 1977), and later also included in the PG quasar sample (Schmidt & Green 1983). The host galaxy was classified as a spiral (e.g. Hutchings & Campbell 1983) and a spiral arm was claimed (Hutchings 1983). A disk model was later confirmed by fitting of model isophotes to near-IR images (Taylor et al. 1996).

The most interesting property of III Zw 2, however, is its extreme variability at radio and other wavelengths with at least 20-fold increases in radio flux density within 4 years (Aller et al. 1985). The source also shows optical (Lloyd 1984) and X-ray variability (Kaastra & de Korte 1988; Pounds 1986).

III Zw 2 is a core-dominated flat-spectrum AGN with only a faint extended structure (see Unger et al. 1987). The weak extended radio emission and the host galaxy is quite typical for a Seyfert galaxy. Its [OIII ] luminosity is a mere factor three brighter than that of a bright Seyfert galaxy like Mrk 3 (e.g. Alonso-Herrero et al. 1997) which explains why it has been classified as either a Seyfert galaxy or a quasar. In this luminosity region a distinction between the two may not be of much significance.

Earlier VLBI observations of the source have only shown a high-brightness temperature core (Falcke et al. 1996b, Kellermann et al. 1998) and recent Millimeter-VLBI observations by Falcke et al. (1999) just barely resolved the source into two very compact components.

Based on its average optical-to-radio ratio of [FORMULA] and its radio properties, it was suggested that III Zw 2 could be a radio-intermediate quasar and the presence of superluminal motion in this source was predicted. Here we will discuss multi-epoch VLA and millimeter-VLBI observations of this source confirming this hypothesis.

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© European Southern Observatory (ESO) 2000

Online publication: June 5, 2000