4 Concluding remarks

Excepting the quasar RC 2036+0451, the present galaxies do not have measured redshifts as yet. Hence, it is interesting to ask whether optical morphology provides information on the redshift distribution of the sample. Other properties, for example the Hubble diagram of the RC/USS sample, suggest (Sect. 1.3) that it contains galaxies with .

Of the 22 observed objects, three are extremely faint and three others are either faint or have several possible identifications. Of the remaining 16 objects, 5 are unresolved. This is roughly the same fraction of point sources which Röttgering et al. (1995) found, but slightly more than Lu et al. (1996) found from their "distant'' sample ($S_{1.4\,{\rm GHz}}\gt 35$ mJy), which does not have radio spectral index selection criteria. Typically RC/USS objects have a multicomponent structure with extended emission. This compares with HST images which have shown that about 30% of intermediate redshift 3CR galaxies have distorted morphology (De Koff et al. 1996). More distant ($z\sim1$) 3CR galaxies have typically multicomponent structure with diffuse extended emission (Best et al. 1996). The ellipticity (e) of the current sample (11 objects) ranges from 0.05 to 0.57, with a mean of 0.25. Taking into account the measurement errors these values agree with studies by Rigler et al. (1992) for 3C galaxies (e=0.19) and by Röttgering et al. (1995) USS sample (e=0.33).

Visual inspection of the images in Fig.  3 suggests that about half of the objects have a companion with comparable brightness within 10$\hbox{$^{\prime\prime}$}$.We examined the excess of companion galaxies along the radio axis suggested by Röttgering et al. (1996). Our sample has 7 resolved objects with 3$\hbox{$^{\prime\prime}$}<{\rm LAS}<20\hbox{$^{\prime\prime}$}$. Only two of these have companion galaxies (RC 1152+0449 and RC 1510+0438). RC 1152+0449 has one companion almost exactly on the extension of the radio axis. RC 1510+0438 has two companions, the brighter one being along the radio axis and the fainter one being perpendicular. It is also interesting to note that both of these "aligned'' companions are about 1 magnitude fainter than the object. Four out of 16 objects have an apparent excess of companions and hence may be situated in a cluster of galaxies.

We can conclude that the morphology of the present weak radio flux USS population is close to that observed by e.g HST in high-z radio galaxies. The results of this study make it imperative to measure spectroscopic redshifts for the RC/USS galaxies. Especially, the fainter flux limit makes it very interesting to see where in the Hubble diagram (both R and K, the latter magnitudes still lacking) the RC-sources are found (cf. Eales et al. 1997). Naturally, the number of well observed RC/USS galaxies is still small. A more conclusive discussion of morphological features must wait until we complete the observations in the range $22^{\rm h}<{\rm RA}<4^{\rm h}$ for which 6 m telescope identifications are now available. Also, we intend to reobserve the cases of poor seeing in the present sample.

Acknowledgements

The authors thank the anonymous referee for useful comments. This work has been partially supported by grants from the Russian Foundation of Basic Research 95-02-03783 and 96-02-16597 and the "Astronomy'' programme project 2-296 and 1.2.1.2., 1.2.2.4. Yu.V.B. acknowledges the support of the Russian "Integration'' project No. 578. TP acknowledges financial support from the Wihuri foundation. This work has been supported by the Academy of Finland (project Cosmology in the local galaxy Universe). This research has made use of the NASA/IPAC Extragalactic Database (NED) which is operated by the Jet Propulsion Laboratory, California Institute of Technology, under contract with the National Aeronautics and Space Administration. The National Radio Astronomy Observatory is a facility of the National Science Foundation operated under cooperative agreement by Associated Universities, Inc.