Nuclear stellar discs in early-type galaxies - I. HST and WHT observations

We present multi-colour (U, V and I) photometry obtained with the second Wide Field and Planetary Camera (WFPC2) on the Hubble Space Telescope (HST), and spectra taken with the William Herschel Telescope (WHT) and the HST Faint Object Spectrograph (FOS), for the early-type galaxies NGC 4342 and 4570. These galaxies are intermediate between ellipticals and lenticulars, and they both have a small nuclear stellar disc in addition to their main outer disc. Colour images reveal no colour differences between the nuclear discs and the bulges. Comparison of the U - V and V - I colours with stellar population models indicates that the central regions of both galaxies are of intermediate age (similar to 8 Gyr) and of high metallicity. For NGC 4342 this is consistent with the values of the line strengths in the central region derived from the FOS spectra. For NGC 4570, an unusually large H beta line strength may suggest recent star formation. The long-slit WHT spectra have a high signal-to-noise ratio (S/N) and a spatial resolution of similar to 1 arcsec. They are used to determine the stellar rotation velocities V, the velocity dispersions sigma, and the deviations of the line-of-sight velocity profiles from a Gaussian shape, as quantified by the Gauss-Hermite moments h(3) and h(4). Both galaxies are rapidly rotating, and they both have velocity dispersions that increase strongly towards the centre. The V and h(3) profiles clearly reflect radial changes in the relative contributions of the different structural components identified photometrically. The FOS was used to obtain spectra with the 0.26 arcsec diameter circular aperture at seven different positions in the central region of each galaxy. Measurements of V and a yield the stellar kinematics at four times higher spatial resolution than available from the WHT spectra. The FOS spectra of NGC 4342 indicate a central velocity dispersion of similar to 420 km s(-1), higher than the similar to 320 km s(-1) measured from the WHT spectra. Also, the nuclear rotation gradient measured with the FOS is steeper than that measured with the WHT; it reaches V-rot similar to 200 km s(-1) at 0.25 arcsec. The rapid stellar motions seen in the centre of NGC 4342 suggest a large central mass concentration, possibly a massive black hole. The kinematics of the more massive NGC 4570 are less spectacular, with a central velocity dispersion of similar to 250 km s(-1) and a central rotation curve that reaches only similar to 60 km s(-1) at 0.25 arcsec. The stellar kinematical measurements for both galaxies will be interpreted quantitatively with detailed dynamical models in a series of companion papers.