NGC-4414 - A FLOCCULANT GALAXY WITH A HIGH GAS-SURFACE DENSITY

(CO)-C-12(1-0), (CO)-C-12(2-1), (CO)-C-13(1-0), (CO)-C-13(2-1), and Hi observations with comparable angular resolution (almost-equal-to 22'' ) are presented for the flocculent Sc-type galaxy NGC 4414. The neutra l gas surface density in the disk is one of the highest known, about 5 0 M.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 ( CO)-C-12/13(1-0) emission ratio is equal to 9 and the (CO)-C-12(2-1/1- 0) line ratio equal to 0.7 throughout the disk. The variation in the ( CO)-12/13(1-0) ratio is much lower than has yet been observed. We beli eve that this is largely due to the quality of the data. For the point s near the center where the (CO)-C-13(2-1/1-0) line ratio is well dete rmined, it is also equal to 0.7 and the (CO)-12/13(2-1) ratio is about 9. We note that despite the huge difference in optical depth (CO)-C-1 2(2-1/1-0) almost-equal-to (CO)-C-13(2-1/1-0) as well in several other galaxies, although at different values. This behavior is not easy to explain. We have determined the (CO)-12/13(1-0) ratio as a function of velocity and find that the (CO)-12/13 ratio is about 8 throughout mos t of the velocity space for most spectra but that the low-velocity lin e wing of each side of the galaxy has a higher (CO)-12/13 ratio. This likely implies a lower optical depth for these clouds. The Halpha pict ure reveals a central hole in star-formation, of 8'' diameter. The mol ecular line data is not conclusive about the presence of a central hol e. No compact radio continuum source is present and the 1.4 GHz contin uum appears entirely associated with star-formation in the disk. Despi te the high gas surface density, NGC 4414 is not a ''starburst'' galax y, a fact that could be related to its isolation. The H I disc extends far beyond the optical outlines of the galaxy, and it is quite asymme tric at low column densities. A combined CO(2-1) and H I rotation curv e shows that the rotational velocities start declining outside the opt ical disc, a phenomenon found only in compact galaxies with high rotat ional velocities. Therefore, no direct evidence for a massive dark hal o in NGC 4414 is found from the rotation curve, a rare case for an Sc spiral.