DISTRIBUTION AND MOTIONS OF H(I) IN THE SA GALAXIES NGC-1169 AND NGC-3898

The Sa-type galaxies NGC 1169 and NGC 3898 were mapped in the 21-cm HI line at Westerbork with a spatial resolution of about 30'' and a velocity resolution of 40 km s-1. NGC 1169, classified as SBa(r) I, has M(HI)/L(B)0=0.12. M./L.,B. The HI distribution of NGC 1169 shows a central hole, 2.5 times as large as the bar, and a ring-like structure with a radius of 13 kpc [H-0=100 km s-1 Mpc-1], or 0.8 times the optical de Vaucouleurs' radius R25, and a width of about 7 kpc. The azimuthally averaged HI surface density distribution has a maximum of sigma(HI) approximately 3.6 M.pc-2 at R approximately 0.8 R25. The HI velocity field does not show clear deviations from circular rotation in a flat disc, and the rotation curve is essentially flat at V(rot) approximately 265 km s-1. Assuming a simple spherical mass model, we derive a total mass M(T)=4.5 10(11) M. and M(T)/L(B)=10 M./L.,B within R=29 kpc (=7 disc scalelengths). We also obtained V and R band CCD images of NGC 11 69, which show a disc scalelength of 4.2 kpc.

NGC 3898 is a quite gas-rich SaI type galaxy with M(HI)/L(B)0=0.19 M./L.,B. Its HI distribution shows an extended disc with a radius of 20 kpc(=1.8 R25) at a level of sigma(HI)=l M.pc-2. Local HI maxima occur at the radius of the outer spiral arms (R approximately 6.5 kpc, where the maximum sigma(HI) approximately 3.1.M.pC-2) and at R approximately 18 kpc. The velocity field of NGC 3898 is regular, and the derived rotation curve is essentially flat at V(rot) approximately 240 km s-1. Using a spherical mass model, we find M(T)=2.5 10(11) M. and M(T)/L(B)0 approximately 18 M./L.,B within R=19 kpc (=6.5 optical disc scalelengths). Fitting a disc/bulge/dark halo mass model to the surface photometry and the combined Halpha/H I rotation curve of NGC 3898 shows the presence of a halo, which is about as massive as the luminous matter within R=19 kpc.

We also detected H I in the small irregular galaxy UGC 6816 which lies 26' away from NGC 3898 and has a 284 km s-1 lower systemic velocity. Its H I content (M(HI)/L(B)=0.9 M./L.,B) is normal for its morphological type. Its H I distribution is somewhat larger than the optical D25 size, without a clear correlation between the locations of the H I emission and the star forming regions.

A comparison of the H I distributions of the 3 Sa galaxies mapped by us (NGC 1169 and 3898, and IC 5267) with those of 6 SO/a and RSB0/a, and 9 Sb-type spirals shows no clear difference between the mean H I surface density peak values of galaxies of types SO/a, Sa and Sb, and a distinct difference between the H I morphologies of barred and non-barred objects, the former showing pronounced central holes in their H I distributions. In all three Sa galaxies the H I surface density stays well below the threshold gas surface density for star formation, estimated with a simple model for disc stability against large-scale density perturbations. Even if we add the estimated H-2 surface density in NGC 3898, which was detected in the CO(I-0) line, the total gas surface density probably stays below the threshold value everywhere in the galaxy. Therefore the wide-spread star formation presently occurring in Sa galaxies (though at a drastically reduced level compared to the past) must be due to local mechanisms, rather than to spontaneous large-scale gravitational collapse.