Dynamical modeling of two nearby disc galaxies

The morphology and kinematics of interstellar gas in the nearby disc galaxies NGC 4736 (= Messier 94) and NGC 3310 (= Arp 217), derived from observations in the 21 cm line of neutral hydrogen and in the H alpha line, are compared to the results of simulations of the dynamics of an ensemble of gas clouds in an external potential. The form of the potential was derived from broadband optical photometry. For NGC 4736, good fits are obtained, for HI and H alpha data alike, to both the morphology and the kinematics of the inner ring of HII regions (R approximate to 50('')) when using a potential which is mildly non-axisymmetric at intermediate radii (b/a = 0.8 for R between 16('') and 180('')) further assuming an inclination of 35 degrees and setting the major-axis position angle to 115 degrees +/- 3 degrees. Residual velocity differences between the best-fitting model and observed velocity fields are at most 2 times the observational velocity resolutions. For NGC 3310, the potential was derived from a wide-field I-band image, assuming a reasonable run of mass-to-light ratio with radius, such that the rotation curve measured in HI was reproduced. While not as precise as those for NGC 4736, reasonable fits to the observed gas morphology and kinematics could be found. The pattern speed of the m = 2 perturbation is found in the modeling process. In both cases, the corotation is located at half the outer radius of the simulations of the optical disc