Winds of change - a molecular outflow in NGC 1377? The anatomy of an extreme FIR-excess galaxy

Abstract

We use high (0."65 x 0."52,(65x52pc)) resolution SubMillimeter Array (SMA) observations to image the CO and 13CO 2-1 line emission of the extreme FIR-excess galaxy NGC 1377. We find bright, complex CO 2-1 line emission in the inner 400 pc of the galaxy. The CO 2-1 line has wings that are tracing a kinematical component which appears perpendicular to that of the line core. Together with an intriguing X-shape of the integrated intensity and dispersion maps, this suggests that the molecular emission of NGC 1377 consists of a disk-outflow system. Lower limits to the molecular mass and outflow rate are M_out(H2)>1e7 Msun and dM/dt>8 Msun/yr. The age of the proposed outflow is estimated to 1.4Myrs, the extent to 200pc and the outflow speed to 140 km/s. The total molecular mass in the SMA map is estimated to M_tot(H2)=1.5e8 Msun (on a scale of 400 pc) while in the inner r=29 pc the molecular mass is M_core(H2)=1.7e7 Msun with a corresponding H2 column density of N(H2)=3.4e23 cm-2 and an average CO 2-1 brightness temperature of 19K. Observing the molecular properties of the FIR-excess galaxy NGC 1377 allows us to probe the early stages of nuclear activity and the onset of feedback in active galaxies. The age of the outflow supports the notion that the current nuclear activity is young - a few Myrs. The outflow may be powered by radiation pressure from a compact, dust enshrouded nucleus, but other driving mechanisms are possible. The buried source may be an AGN or an extremely young (1Myr) compact starburst. Limitations on size and mass lead us to favour the AGN scenario, but further studies are required to settle the issue. In either case, the wind with its implied mass outflow rate will quench the nuclear power source within a very short time of 5-25 Myrs. It is however possible that the gas is unable to escape the galaxy and may eventually fall back onto NGC 1377 again.