  NGC 4258 is a well known spiral galaxy with a peculiar large scale jet flow
detected in the radio and in H-alpha. Due to the special geometry of the
galaxy, the jets emerge from the nuclear region through the galactic disk. Also
the distribution of molecular gas looks different from that in other spiral
galaxies: [12]CO(1-0)emission has only been detected in the center and along
the jets and only up to distances of about 50 arcsec (1.8 kpc) from the
nucleus. The reason for the CO concentration along the inner jets in NGC 4258
was not understood and is the motivation for the observations presented here.
Using the IRAM interferometer, we mapped the [12]CO(1-0) emission of the
central part of NGC 4258 along the nuclear jet direction in the inner 3 kpc. We
detected two parallel CO ridges along a position angle of -25 degr with a total
length of about 80 arcsec (2.8 kpc), separated by a CO-depleted funnel with a
width of about 5 arcsec (175 pc). The H-alpha emission is more extended and
broader than the CO emission with its maximum just in between the two CO
ridges. In CO we see a peculiar velocity distribution in the iso-velocity map
and p-v diagrams. We discuss different scenarios for an interpretation and
present a model which can explain the observational results consistently. We
propose here that the concentration of CO along the ridges is due to
interaction of the rotating gas clouds with the jet's magnetic field by
ambipolar diffusion. This magnetic interaction is thought to increase the time
the molecular clouds reside near the jet thus leading to the quasi-static CO
ridge.
