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
e-mail: saalto@chalmers.se
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

Received: 8 September 2011
Accepted: 13 June 2012

Abstract

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 M_out(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 V_out = 140 km   s^-1. The total molecular mass in the SMA map is estimated to M_tot(H₂) = 1.5 × 10⁸ M_⊙ (on a scale of 400 pc) while in the inner r = 29 pc the molecular mass is M_core(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.