Nuclear bars and blue nuclei within barred spiral galaxies

Multicolour near IR photometry for a sample of 32 large barred spiralgalaxies is presented. By applying ellipse fitting techniques, we identifysignificant isophote twists with respect to the primary bar axis in the nuclearregions of $\sim$70 \%\ of the sample. These twists are identified in galaxiesas late as SBbc and are clearly distinguishable from spiral arm morphology. Atmost seven of the galaxies with isophote twists are inferred to possesssecondary (nuclear) bars, the axis ratios of which appear to correlate withmorphological type. The remainder may result from triaxial bulges, or fromoblate bulges misaligned with the primary bar. The near IR colour distributionsin these data show evidence for (red) circumnuclear star forming rings in 4galaxies. The majority of the sample (19) also possess striking blue nuclearregions, bluer than typical old stellar populations by $\sim$0.3 mag. in (J--H)and $\sim$0.23 mag. in (H--K). Such blue colours do not appear to correlatewith the presence of nuclear rings or pseudo--rings, nor with the activity ofthe host galaxy (as determined from emission--line spectroscopiccharacteristics). Several mechanisms to explain this blue colour areconsidered.Comment: 24 pages plain LaTex( including table captions), 5 tables and 18 figures. Accepted for publication in MNRAS. Paper and tables available via anonymous ftp://143.167.4.101/pub/papers as sqiid-paper.tex and table1,2,3.tex, table4.ps, table5.ps. Figures available as postcript upon request to first Autho