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 H I line at Westerbork with a spatial resolution of about 30" and a velocity resolution of 40 km/s. NGC 1169, classified as SBa(r) I, has M_HI_/L_B_^0^=0.12 M_sun_/L_sun,B_. The H I 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/Mpc], or 0.8 times the optical de Vaucouleurs' radius R_25_, and a width of about 7 kpc. The azimuthally averaged H I surface density distribution has a maximum of σ_HI_~3.6 M_sun_pc^-2^ at R~ 0.8 R_25_. The H I velocity field does not show clear deviations from circular rotation in a flat disc, and the rotation curve is essentially flat at V_rot_~265 km/s. Assuming a simple spherical mass model, we derive a total mass M_T_=4.5 10^11^ M_sun_ and M_T_/L_B_ =10 M_sun_/L_sun,B_ within R=29 kpc (=7 disc scalelengths). We also obtained V and R band CCD images of NGC 1169, which show a disc scalelength of 4.2 kpc. NGC 3898 is a quite gas-rich Sa I type galaxy with M_HI_/L_B_^0^=0.19 M_sun_/L_sun,B_. Its H I distribution shows an extended disc with a radius of 20 kpc (=1.8 R_25_) at a level of σ_HI_=1 M_sun_pc^-2^. Local H I maxima occur at the radius of the outer spiral arms (R~6.5 kpc, where the maximum σ_HI_~ 3.1 M_sun_pc^-2^) and at R~18 kpc. The velocity field of NGC 3898 is regular, and the derived rotation curve is essentially flat at V_rot_~240 km/s. Using a spherical mass model, we find M_T_=2.5 10^11^ M_sun_ and M_T_/L_B_^0^~18 M_sun_/L_sun,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 Hα/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 lower systemic velocity. Its H I content (M_HI_/L_B_=0.9 M_sun_/L_sun,B_) is normal for its morphological type. Its H I distribution is somewhat larger than the optical D_25_ 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 S0/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 S0/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(1-0) 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.