GROUPS OF GALAXIES IN THE ROSAT NORTH ECLIPTIC POLE SURVEY

We present the first measurement of the ensemble properties of X-ray-s elected groups of galaxies. Their X-ray luminosity and temperature fun ctions are a smooth extrapolation from rich clusters of galaxies. The X-ray-selected groups have a spiral fraction which is about a factor o f 2 smaller than optically selected groups, possibly because the X-ray selection produces a more dynamically evolved sample. The binding mas ses of these groups and their contribution to the density of the unive rse is typical of those found from analyses of optically selected samp les, but our estimates were derived from completely different selectio n criteria and mass measurements.