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

DISTRIBUTION AND MOTIONS OF HI IN THE RINGED GALAXY NGC-4736

The ringed Sab-type galaxy NGC 4736 was mapped in the 21 cm H i line at Westerbork with a resolution of 13'' x 20'' (alpha x delta) and a velocity resolution of 33 km s 1. Detailed maps of H I distribution and motions were constructed for the entire galaxy, including the outer ring (R almost-equal-to 6'). The loci of Halpha, H I line and 21 cm continuum radiation from the inner ring (R almost-equal-to 50'') are found to be generally coincident. Only towards the S and SE the H I line radiation comes from regions some 8'' to 17'' farther away from the nucleus than the Ha and the 21 cm continuum. The velocity field can be modelled well assuming circular rotation of the gas in a plane with a constant inclination of 40-degrees, but with a position angle of the kinematical major axis varying with radius by 20-degrees. The rotation velocity falls, in this model, from ca. 175 km s-1 at a radius of 50'' to ca. 125 km s-1 at 500''. The H I kinematics show evidence for the presence of a slight warp or an oval distortion. In a future paper this will be modelled in detail. The outer ring is found to be a smooth continuation of the H I disc in its kinematics. A total H I mass of (9.9 +/- 0.3) 10(8) M. is found. Assuming a distance of 6.6 Mpc, this implies a M(HI)/L(B)0 ratio of 0.04 in solar units. A total mass of 5.1 10(10) M. is found from the rotation curve. This implies a global M/L ratio of 2.0 in solar (blue) units, which is exceptionally low. Since the galaxy has a normal total blue luminosity its total mass, including a possible dark halo, must be lower than normal for this type of object