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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.