Nuclear bars and blue nuclei within barred spiral galaxies

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Abstract
Multicolour near-IR photometry for a sample of 32 large barred spiral galaxies is presented. By applying ellipse-fitting techniques, we identify significant isophote twists with respect to the primary bar axis in the nuclear regions of ∼ 70 per cent of the sample. These twists are identified in galaxies as late as SBbc, and are clearly distinguishable from spiral-arm structure. At most, seven of the galaxies with isophote twists are inferred to possess secondary (nuclear) bars, the axial ratios of which appear to correlate with morphological type. The remainder may result from triaxial bulges, or from oblate bulges misaligned with the primary bar. The near-IR colour distributions in these data show evidence for (red) circumnuclear star-forming rings in four galaxies. The majority of the sample (19) also possess striking blue nuclear regions, bluer than typical old stellar populations by ∼ 0.3 mag in (J–H) and ∼ 0.23 mag in (H–K). Such blue colours do not appear to correlate with the presence of nuclear rings or pseudo-rings, nor with the activity of the host galaxy (as determined from emission-line spectroscopic characteristics). Several mechanisms to explain this blue colour are considered.
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