Precision photometry of early-type galaxies in the Coma and Virgo clusters : a test of the universality of the colour-magnitude relation - I. The data.

We have undertaken a comprehensive study of a large sample of early-type galaxies in the Virgo and Coma clusters. In the first of two papers, we present accurate UVJK photometry for a total of 94 elliptical and S0 galaxies. All U- and V-band measurements are based on CCD observations made with the 2.5-m Isaac Newton Telescope. J- and K-band observations for the Coma cluster were made with a chopping photometer at the United Kingdom Infrared Telescope (UKIRT). Virgo J-and K-band measurements have been taken from Persson, Frogel & Aaronson. The rms internal scatter of these data are respectively: 0.025, 0.015, 0.03 and 0.03 mag for Virgo in, V, J and K (60-arcsec aperture) measurements; 0.030, 0.021, 0.026 and 0.027 mag for coma U, V(13-arcsec aperture), J and K (17-arcsec aperture) measurements. We have been careful to ensure that the data set is internally homogeneous. We estimate that the photometric zero-points in the two clusters agree to better than 0.01 mag for U and V, and 0.02 mag for J and K. From these measurements, we have derived U - V, V - K and J - K colours corrected for redshift, galactic extinction and aperture effects. After allowing for the systematic uncertainties in these corrections, we estimate that the U - V, V - K and J - K colour systems in both clusters are matched to better than 0.02, 0.03 and 0.04 mag respectively. This data set is completed by photometric diameter parameters derived from our own V-band data, total V-band magnitudes derived from a combination of our measurements and those of Godwin & Peach and Michard, and velocity dispersions and morphological types taken from the literature. A detailed analysis of colour-magnitude correlations of these galaxies will be presented in Paper II (this issue).