Distribution and motions of atomic hydrogen in lenticular galaxies. X. The blue SO galaxy NGC 5102

We have mapped the blue gas-rich S0 galaxy NGC 5102 in the 21-cm HI line with a spatial resolution of 34'' x 37'' (DELTAalpha x DELTAdelta) and a velocity resolution of 12 km s-1. Optically NGC 5102 is a peculiar S0 galaxy, in the sense that it has unusually blue colours, and shows evidence for a burst of star formation a few times 10(8) years ago in its nucleus. Also the inner bulge and parts of its disk show signs of low-level star formation activity. It further has a 'supershell' of 1.7 kpc diameter seen in Halpha around its central regions, a unique feature for a lenticular galaxy.

The HI distribution has a pronounced central depression of 1.9 kpc radius, and most of the HI is concentrated in a 3.6 kpc wide ring with an average radius of 3.7 kpc (= 0.7 R25), assuming a distance of 4 Mpc for NGC 5102. The maximum azimuthally averaged HI surface density in the ring is 1.4 M. pc-2, comparable to that found in other SO galaxies. The extent of the HI gas is not much larger than the optical de Vaucouleurs' radius (R25) of die galaxy, contrary to what is found in other gas-rich S0's, which often have large outer HI rings at R approximately 2 R25. The HI velocity field is quite regular, showing no evidence for large-scale deviations from circular rotation, and the HI is found to rotate in the plane of the stellar disk. The rotation curve is essentially flat at V(rot) approximately 95 km s-1 out to R = 6 kpc (1.1 R25). Assuming a simple spherical mass model, we find a total mass-to-light ratio M(T)/L(B)0 = 4.3 M./L. within R = 6 kpc.

In contrast to other SO galaxies studied in this series of papers, both the HI mass/blue luminosity ratio and the radial HI distribution are similar to those in early-type spirals. The HI may be an old, ''smouldering'', disk or it may have been acquired through capture of a gas-rich smaller galaxy. The recent starburst in the nuclear region, which gave the galaxy its blue colour, may have been caused by partial radial collapse of the gas disk, or by infall of a gas-rich dwarf galaxy.