Winds of change – a molecular outflow in NGC 1377?
The anatomy of an extreme FIR-excess galaxy
S. Aalto1, S. Muller1, K. Sakamoto2, J. S. Gallagher3, S. Martín4 and F. Costagliola1
1 Department of Earth and Space
SciencesChalmers University of Technology, Onsala
Observatory, 43994 Onsala, Sweden
2 Institute of Astronomy and Astrophysics, Academia Sinica, PO Box 23-141, 10617 Taipei, Taiwan
3 Department of Astronomy, University of Wisconsin-Madison, 5534 Sterling, 475 North Charter Street, Madison, WI 53706, USA
4 European Southern Observatory, Alonso de Córdova 3107, Vitacura, Casilla, 19001, Santiago 19, Chile
Accepted: 13 June 2012
Aims. Our goal was to investigate the molecular gas distribution and kinematics in the extreme far-infrared (FIR) excess galaxy NGC 1377 and to address the nature and evolutionary status of the buried source.
Methods. We used high- (0''65 × 0''52, (65 × 52 pc)) and low- (4''88 × 2''93) resolution SubMillimeter Array (SMA) observations to image the 12CO and 13CO 2−1 line emission.
Results. We find bright, complex 12CO 2−1 line emission in the inner 400 pc of NGC 1377. The 12CO 2−1 line has wings that are tracing a kinematical component that appears to be perpendicular to the component traced by 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 Mout(H2) > 1 × 107 M⊙ and Ṁ > 8 M⊙ yr-1. The age of the proposed outflow is estimated to be 1.4 Myr, the extent to be 200 pc and the outflow speed to be Vout = 140 km s-1. The total molecular mass in the SMA map is estimated to Mtot(H2) = 1.5 × 108 M⊙ (on a scale of 400 pc) while in the inner r = 29 pc the molecular mass is Mcore(H2) = 1.7 × 107 M⊙ with a corresponding H2 column density of N(H2) = 3.4 × 1023 cm-2 and an average 12CO 2−1 brightness temperature of 19 K. 13CO 2−1 emission is found at a factor 10 fainter than 12CO in the low-resolution map while C18O 2−1 remains undetected. We find weak 1 mm continuum emission of 2.4 mJy with spatial extent less than 400 pc.
Conclusions. 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 Myr. 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 active galactic nucleus (AGN) or an extremely young (1 Myr) compact starburst. Limitations on size and mass lead us to favor the AGN scenario, but additional studies are required to settle this question. In either case, the wind with its implied mass outflow rate will quench the nuclear power source within the very short time of 5−25 Myr. It is possible, however, that the gas is unable to escape the galaxy and may eventually fall back onto NGC 1377 again.
Key words: galaxies: evolution / galaxies: individual: NGC 1377 / galaxies: active / galaxies: starburst / radio lines: ISM / ISM: molecules
© ESO, 2012