On the nature of the galaxy NGC 5666

We present B and R CCD images and optical spectroscopy in the range 3700–8900 Å for the galaxy NGC 5666, which until now was considered as a low-luminosity elliptical. The high-resolution images show a conspicuous spiral pattern which is limited to the inner region (, or 1/6 of the isophotal radius) and a faint (presumably tidal) outer feature dotted with small knots resembling star clusters or tidal dwarf galaxies. The patchy spiral structure and the luminosity profiles suggest a late-type morphological type, but the disk is of high central surface brightness and the bulge weak and very extended. The nuclear spectrum reveals typical emission lines found in late-type galaxies, but the underlying nuclear stellar population and continuum only match that of an elliptical galaxy combined with a young (<1 Gyr) stellar population. These and other properties of the galaxy suggest that this could be a minor merger, at an intermediate stage, between a gas-rich dwarf and a small early-type disk galaxy. If this is the case, the galaxy NGC 5666 will provide important constraints to the theory of hierarchical galaxy formation.