Extragalactic CO - Gas distributions which follow the light in IC 342 and NGC 6946

Abstract

The CO emission in the two Scd galaxies, IC 342 and NGC 6946, has been mapped using the 14 m FCRAO telescope (HPBW = 50''). In each galaxy, the radial distribution of CO out to Rroughly-equal10 kpc exhibits a falloff which follows the exponential luminosity profile from the stellar disk. Neither galaxy shows a breach in the molecular gas like that found in our Galaxy between the nucleus and the molecular cloud annulus. The occurrence of this gap in our own Galaxy, but in neither of the external galaxies, suggests a possible link between the deficiency of gas and the presence of either a nuclear bulge or inner Lindblad resonance (ILR). From its rotation curve, the Milky Way has a large nuclear bulge and also has an ILR expected in the vicinity of the hole in the CO distribution. The CO rotation curves for IC 342 and NGC 6946 agree well with previous HI observations; at the present resolution these galaxies appear to undergo nearly rigid rotation inside R = 5 kpc and therefore have no ILRs and have small nuclear bulges. Further support for the association of the gap with ILR or bulge is provided by brief observations reportedmore » here of NGC 4321, a more distant Sc galaxy with a rotation curve similar to the Milky Way; the CO distribution appears like that in our own Galaxy with coarser resolution.« less