NGC 4414 : a flocculent galaxy with a high gas surface density.

^12^CO(1-0), ^12^CO(2-1), ^13^CO(1-0), ^13^CO(2-1), and HI observations with comparable angular resolution ( ~22") are presented for the flocculent Sc-type galaxy NGC 4414. The neutral gas surface density in the disk is one of the highest known, about 50 M_sun_pc^-2^, comparable to that of the grand design galaxy M51. The large-scale conditions do not appear to change from point to point as the ^12/13^CO(1-0) emission ratio is equal to 9 and the ^12^CO(_1-0_^2-1^) line ratio equal to 0.7 throughout the disk. The variation in the ^12^/^13^CO(1-0) ratio is much lower than has yet been observed. We believe that this is largely due to the quality of the data. For the points near the center where the ^13^CO(_1-0_^2-1^> line ratio is well determined, it is also equal to 0.7 and the ^12/13^CO(2-1) ratio is about 9. We note that despite the huge difference in optical depth ^12^CO(_1-0_^2-1^) ~ ^13^CO(_1-0_^2-1^) as well in several other galaxies, although at different values. This behavior is not easy to explain. We have determined the ^12/13^CO(1-0) ratio as a function of velocity and find that the ^12/13^CO ratio is about 8 throughout most of the velocity space for most spectra but that the low-velocity line wing of each side of the galaxy has a higher ^12/13^CO ratio. This likely implies a lower optical depth for these clouds. The Hα picture reveals a central hole in star-formation, of 8" diameter. The molecular line data is not conclusive about the presence of a central hole. No compact radio continuum source is present and the 1.4 GHz continuum appears entirely associated with star-formation in the disk. Despite the high gas surface density, NGC 4414 is not a "starburst" galaxy, a fact that could be related to its isolation. The HI disc extends far beyond the optical outlines of the galaxy, and it is quite asymmetric at low column densities. A combined CO(2-1) and HI rotation curve shows that the rotational velocities start declining outside the optical disc, a phenomenon found only in compact galaxies with high rotational velocities. Therefore, no direct evidence for a massive dark halo in NGC 4414 is found from the rotation curve, a rare case for an Sc spiral.