SIMBAD references

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_B⁰=0.12 M_☉/L_☉,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 [H₀=100 km/s/Mpc], or 0.8 times the optical de Vaucouleurs' radius R₂₅, and a width of about 7 kpc. The azimuthally averaged H I surface density distribution has a maximum of σ_HI∼3.6 M_☉pc^–2 at R∼ 0.8 R₂₅. 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 10¹¹ 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 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_B⁰=0.19 M_☉/L_☉,B. Its H I distribution shows an extended disc with a radius of 20 kpc (=1.8 R₂₅) at a level of σ_HI=1 M_☉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_☉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 10¹¹ M_☉ and M_T/L_B⁰∼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 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_☉/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 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 H₂ 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.