SIMBAD references

NGC 4631 is an edge-on galaxy well-known for its prominent radio continuum halo. We present sensitive CO observations of this galaxy in the ¹²CO(1-0), ¹³CO(1-0), ¹²CO(2-1) and ¹³CO(2-1) transitions. Its central region is completely mapped in ¹²CO whereas the observations of the outer disk concentrate on the major axis. The central and total molecular mass are estimated to be 0.9x10⁹M_☉ and 1.2x10⁹M_☉, respectively. The dynamical mass of the central region is 8.6x10⁹M_☉. Thus the molecular mass comprises ∼10% of the dynamical mass. The central molecular gas is not distributed on a well-defined plane, but has a disturbed, asymmetric appearance. It is more consistent with a disturbed molecular gas disk with a position angle of ∼83deg, the position angle of the less disturbed western disk of NGC 4631. Several rotation curves and channel maps reveal the details of the rotation of the galaxy. Besides the overall rigid rotation in the central region (r≤2.4kpc) and differential rotation with v∼140km/s in the outer disk some indications for non-circular motions were found in the central region. They are probably caused by deviations of the gas distribution from axisymmetry induced by the gravitational interaction. Line ratios are derived in the central region. The global central ¹²CO(2-1)/(1-0) line ratio is 0.76±0.12. The ¹³CO to ¹²CO line ratios show variations between ∼6 and ∼17.4. They are in agreement with values found in other normal galaxies. The star forming efficiency (SFE) is deduced from the FIR luminosity and the molecular mass. It is ∼11L_☉/M_☉, consistent with the SFE of a disturbed, but not strongly interacting galaxy. The line ratios as well as the moderate SFE classify NGC 4631 as a galaxy with only a mild central starburst. A comparison of the properties of the CO gas in NGC 4631 with two other edge-on galaxies, NGC 891 and M82, is presented. Despite their different CO properties and star formation activity all three galaxies exhibit disk-halo interaction.