The first possibility to consider for the interpretation of the data which are now available, is that PKS 1718-649 is a compact, flat-spectrum, core-jet radio source; the source consists of a relativistic jet of material which originates at a compact core and radiates at radio wavelengths .
The radio source is certainly very compact, as can be seen from Figures 7.1, 7.4, and 7.5. The flux density at 2.3, 4.8, and 8.4 GHz in the VLBI images presented in Figures 7.1, 7.2, and 7.3 can account for the total flux density seen with much lower resolution observations. The core dominance parameter, R (the ratio of compact to extended flux density) cannot be estimated since no radio structure on scales larger than approximately 20 mas (5 pc) has been detected. Under the core-jet interpretation this fact would make PKS 1718-649 a core dominated radio source, like a BL Lac type object or core dominated quasar.
The pc-scale structure itself consists of two components, one of which has a steep spectrum between 4.8 and 8.4 GHz ( 
), the other having a flat spectrum over the same range ( 
). The usual interpretation of this result would be that the flat-spectrum component is the active core of the radio source, coincident with the central engine and that the steep-spectrum component would be part of a jet which originates at the core. However, apart from the spectral indices, these two components do not resemble a typical core-jet structure. The two components have almost equal size and are both resolved with a 2.6 
2.3 mas beam. The two components are discrete and have major axis position angles which are misaligned with each other and the position angle joining them. This contrasts with core-jet sources in that jet components are usually much more resolved than core components; jets are elongated and are usually linear and aligned.
Many core dominated radio sources show evidence for a significant emission component of beamed synchrotron radiation at radio and other wavelengths. One line of evidence is rapid variability in flux density. From the limited radio variability data available for PKS 1718-649, there is no evidence for significant variability at low frequencies.
Another line of evidence for beamed emission is the presence of a high brightness temperature radio core (see 
4.2). The candidate core of PKS 1718-649, the south-east component, has a brightness temperature of only 2 10 
K, two orders of magnitude below the nominal limit for synchrotron emission. Thus, beaming is not required to explain the pc-scale radio emission.
For PKS 1718-649, the non thermal emission component at optical wavelengths is very small, only approximately 6% of the continuum at 
5460 [Filippenko 1985]. This can be compared to approximately 91% for the lobe dominated radio source PKS 0518-458 (Pictor A) [Filippenko 1985]. For a core dominated source like PKS 1514-241 (AP Libræ), the non thermal emission completely dominates at optical wavelengths [Visvanathan & Griersmith 1977].
The radio spectrum of PKS 1718-649 is inverted between 408 MHz and 4.8 GHz, typical for a core dominated radio source. Between 4.8 and 22 GHz the spectrum steepens so that the spectral index corresponds to optically thin synchrotron emission, 
. Typically, for a core dominated radio source the spectrum will remain inverted or flat in this region, due to a strong flat-spectrum core component.
The environment of PKS 1718-649 is also unusual. It is rare to find a strong flat or complex radio spectrum source associated with a galaxy, especially a galaxy that appears to have a spiral structure. Fosbury et al. [1977] cites PKS 1934-649 as a more extreme example of complex radio spectrum source in a galaxy, but the host of PKS 1934-638 is an elliptical and the radio source itself is not a typical core-jet radio source, but a GHz Peaked-Spectrum (GPS) radio source. The powerful optical emission lines of PKS 1718-649 are also reminiscent of PKS 1934-638 [Fosbury et al. 1977].
PKS 1718-649 has some of the features expected of the core-jet paradigm, compact structure and a candidate flat-spectrum core. However, the morphology of the radio source and the galaxy, and the optical and radio spectra do not strongly support the interpretation of PKS 1718-649 as a core dominated core-jet radio source.