Magnetic interaction of jets and molecular clouds in NGC 4258

Abstract

Context. NGC 4258 is a well known spiral galaxy with a peculiar large scale jet flow detected in the radio and in H α . Due to the special geometry of the galaxy, the jets emerge from the nuclear region through the galactic disk – at least in the inner region. Aims. 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 $50\hbox{$^{\prime\prime}$}$ (1.8 kpc) from the nucleus. This concentration of CO along the jets is similar to what is expected as fuel for jet-induced star formation in more distant objects. 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. Methods. Using the IRAM interferometer at Plateau de Bure, 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. In order to get a properly positioned overlay with H α we observed NGC 4258 in H α at the Hoher List Observatory of the University of Bonn. Results. We detected two parallel CO ridges along a position angle of $-25^\circ$ with a total length of about $80\hbox{$^{\prime\prime}$}$ (2.8 kpc), separated by a CO depleted funnel with a width of about $5\hbox{$^{\prime\prime}$}$ (175 pc). The H α emission is more extended and broader than the CO emission with its maximum just in between the two CO ridges. It seems to be mixed in location and in velocity with the CO emission. 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. Conclusions. 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 (ion-neutral drift). This magnetic interaction is thought to increase the time the molecular clouds reside near the jet thus leading to the quasi-static CO ridge.