| Abstract | The ringed galaxies NGC 2273, 4826, and 6217 were mapped in the 21 cm H I line at Westerbork with a velocity resolution of 40, 20 and 20 km s ^-1^, and a spatial resolution of 23" x 27", 28" x 60" and 13" x 14" (α x σ), respectively. Here we use H_0_ = 100 km s^-1^Mpc^-1^. NGC 2273 (=Mk620) is a type (R')SB(rs)ab galaxy with an inner and two outer pseudo-rings, formed by separate sets of spiral arms. The morphology is atypical compared to most outer-ringed galaxies. NGC 2273 is not very gas-rich for its morphological type (M_H I_/L_B_= 0.13 M_sun_/L_sun_). We find the H I to be concentrated in the outer rings at a radius of about 10 kpc, with a maximum average H I column density (corrected to face-on) of σ_H I_~3 M_sun_pc^-2^CO(1-0) line observations indicate that NGC 2273 has a massive "star formation efficiency" similar to that of starburst galaxies, i.e. a L_FIR_/M_H2_ ratio of 10-25 L_sun_/M_sun_ . From the undisturbed H I velocity field we derive an almost flat rotation curve, implying a total mass M_T_~1.3 10^11^M_sun_ and a total mass-to-light ratio M_T_/L_B_~12 M_sun_/L_sun_ if we assume circular rotation and a simple spherical mass model. NGC 4826 (= M64), type (R)SA(rs)ab, has an optical inner ring and a weak outer ring. The galaxy is quite gas-poor (M_H_I_/L_B_=0.020M_sun_/L_sun_) ). NGC 4826 has a normal massive star formation efficiency for its morphological type, 6 L_sun_/M_sun_ . The H I is distributed in an asymmetric disk which extends far beyond the optical de Vaucouleurs radius R_25_; the highest column densities occur at R~3 kpc in or just outside the optical outer pseudoring. Another ring-like H I structure is evident around R~9 kpc as well. The average H I column density σ_H I_, is smaller than 1M_sun_pc^-2^. The H I velocity field looks undisturbed and the derived rotation curve is flat out to R=10.5 kpc, or 11 optical disk scale lengths, implying the presence of a substantial dark halo. We derive M_T_~0.6 10^11^M_sun_ and a total mass-to-light ratio M_T_/L_B_~10 M_sun_ /L_sun_ within R = 10.5 kpc. We resolved the radio continuum source in the centre of the galaxy. NGC 6217, type (R)SB(rs)bc, shows an inner and outer optical pseudoring. The H I mass-to-light ratio, M_H I_/L_B_=0.41M_sun_/L_sun_, is normal for the galaxy's morphological type. Its star formation efficiency, L_FIR_/M_H2_=8 L_sun_/M_sun_ is not abnormally high for its morphological type. The H I is concentrated in the inner and outer pseudorings and beyond ,and the H I column density reaches a maximum of ~6M_sun_ pc^-2^ near the centre. From the regular H I velocity field we derive a rotation curve which is flat out to R = 15 kpc, or 11 times the (uncertain) optical disk scale-length, implying M_T_~0.6 10^11^M_sun_ and a normal total mass-to- light ratio for a spiral, M_T_/L_B_~9 M_sun_/L_sun_ within R = 15 kpc. Possible resonances associated with the rings of each of these galaxies are discussed. Only NGC 6217 fits easily into recent work on the dynamics of ringed galaxies. |
