GROUPS OF GALAXIES IN THE ROSAT NORTH ECLIPTIC POLE SURVEY

We present the first measurement of the ensemble properties of X-ray-selected groups of galaxies. Their X-ray luminosity and temperature functions are a smooth extrapolation from rich clusters of galaxies. The X-ray-selected groups have a spiral fraction which is about a factor of 2 smaller than optically selected groups, possibly because the X-ray selection produces a more dynamically evolved sample. The binding masses of these groups and their contribution to the density of the universe is typical of those found from analyses of optically selected samples, but our estimates were derived from completely different selection criteria and mass measurements.