HI Observations of Early-Type Galaxies

Abstract

We present high resolution (15") HI observations of the S0 galaxy NGC 404. We derive an HI mass of MHi = 6.7 x 10???, in good agreement with previous measurements. The HI is distributed in a broad annulus (a doughnut) and extends out to a diameter of 9', well beyond the optical diameter of 6'. The velocity field is regular and shows, surprisingly, a declining rotation curve. This decline is purely Keplerian, strongly suggesting that all the mass is contained within the inner 200".