NGC 5218 - A starburst driven LINER galaxy? High resolution CO, radio continuum and H i-absorption

Aims. We investigate the distribution of molecular and atomic gas and the nature of the power source of the LINER (Low-Ionisation Nuclear Emission-line Region) activity in NGC 5218. Methods. We performed a multi wavelength study of the barred interacting LINER galaxy NGC 5218. We used the Owens Valley Radio Observatory to obtain interferometer data of CO 1-0, the Multi Element Radio Linked Interferometer for 1.4 GHz continuum and Hi-absorption, and the Onsala Space Observatory to obtain single dish data of CO 1-0, HCN 1-0 and HCO + 1-0. Results. Toward the center of the galaxy, we have detected a double CO peak. The peaks are separated by 2" (380 pc). The observed peaks appear to be caused by an almost edge-on ring of molecular gas with a radius of 470 pc and a rotational velocity of 140 kms-1. We see no kinematical signs of a compact nuclear disc. However, there are kinematical signs of an expanding shell of molecular gas at the eastern side of the ring. The mass of the gas involved in the expansion is 7 x 107 M⊙, and we suggest that the expansion is driven by a burst of supernovae that took place some 5 x 106 years ago. We estimated that 10000 supernovae were required to drive the expansion and that the corresponding number of supernovae per year was 0.002 over the age of the expanding shell. The radio continuum peak agrees well in space with the expanding molecular shell, and the Hi-absorption agrees well in both space and velocity, and supports the notion of an expanding shell. We find that the radio flux density and the bulk of the FIR are associated with nuclear, but slightly off-centre, star formation in the central R = 2" (380 pc). Conclusions. We suggest a scenario where shock fronts of the expanding shell shock heat the surrounding gas, and give rise to a LINER like spectrum. We conclude that the LINER activity observed in NGC 5218 is probably due to nuclear starburst activity, and not to AGN-activity. A fraction of the molecular gas in the bar, outside of the central region, appears to be in a different, gravitationally unbound phase, possibly on X2 orbits to the large scale optical bar. © ESO 2007.