NUCLEAR STELLAR DISCS IN EARLY-TYPE GALAXIES - I - HST AND WHT OBSERVATIONS

We present multi-colour (U, V and I) photometry obtained with the seco nd Wide Field and Planetary Camera (WFPC2) on the Hubble Space Telesco pe (HST), and spectra taken with the William Herschel Telescope (WHT) and the HST Faint Object Spectrograph (FOS), for the early-type galaxi es NGC 4342 and 4570. These galaxies are intermediate between elliptic als and lenticulars, and they both have a small nuclear stellar disc i n addition to their main outer disc. Colour images reveal no colour di fferences 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 wit h 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 h igh 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-s ight velocity profiles from a Gaussian shape, as quantified by the Gau ss-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 ide ntified 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 s tellar kinematics at four times higher spatial resolution than availab le from the WHT spectra. The FOS spectra of NGC 4342 indicate a centra l velocity dispersion of similar to 420 km s(-1), higher than the simi lar to 320 km s(-1) measured from the WHT spectra. Also, the nuclear r otation gradient measured with the FOS is steeper than that measured w ith 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 lar ge central mass concentration, possibly a massive black hole. The kine matics of the more massive NGC 4570 are less spectacular, with a centr al velocity dispersion of similar to 250 km s(-1) and a central rotati on 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 companio n papers.