6 IR-radio alignment effect

We give in Tables 4 and 5 the radio and IR position angles along with the differences between them for the USS and GPS subsamples of radio galaxies. In case of USS sources we used the VLA radio maps provided us from Rengelink (private communication) in order to measure the radio position angles. After selecting the objects with definite identification, we have not found evidence for an IR alignment effect: 3 objects, although resolved, show an IR round morphology; 4 objects are too faint in the IR to say if they are elongated; for 5 objects we have measured radio to IR position angle differences clearly not biased toward zero.

These results, although obtained on a small number of objects, suggest that the IR alignment in our sample of USS sources is not as strong as detected in the optical on powerful distant radio sources (e.g. McCarthy 1993). Furthermore, we are unable to confirm on our sample the claim for a precise IR-radio alignment obtained by Dunlop & Peacock (1993) for 3CR sources at $z \sim$ 1. We suggest that the lack of alignment in our sample may be connected with its lower radio power. It seems in fact plausible that the alignment effect, if it is a manifestation of the nuclear activity on the optical/IR morphology, would decrease with radio power which is a measure of nuclear activity.

In the case of GPS sources we have defined the radio position angles using the analysis of multifrequency VLBI observations carried out by Snellen (PhD thesis) for our sample of faint objects. In all cases, we chose the lowest frequency maps to enhance the importance of the extended parts. The IR position angles of GPS galaxies are in general not aligned with the radio axis (see Table 5). The only exception (1942+721) could well happen by chance. Furthermore, 3 objects show an IR round morphology and 1 object is too faint in the IR to measure any orientation. The lack of optical-radio alignment in GPS sources has also been noticed by Snellen et al. (1996).

 

Table 4: Position angles in the USS sources. For those galaxies indicated by a question mark in Col. 2, we have only one radio component. The numbers with colon are uncertain

3pt

 

Table 5: Position angles in the GPS sources. The numbers with colon are uncertain

 

Table 6: Observing parameters of the gravitational lens system 1600+434

5pt

^*Lensing galaxy only.