Near-infrared observations of the gas structure and kinematics in the circumnuclear region of NGC 1672

Abstract

We present our near-infrared, integral-field spectroscopy observations of the central 550 × 550 pc2 (10″ × 10″) of the local galaxy NGC 1672. Our aim is to investigate the morphology and kinematics of gas and stars to probe evidence for feeding and feedback phenomena. For this purpose the near-infrared wavelength regime and integral-field unit technique provide a vast amount of useful information. The narrow emission line ratios suggest that the galaxy hosts a low-luminosity active galactic nucleus. From the MBH − σ* relation, we find a black hole mass of a few 107 M⊙. The total reservoir of hot molecular and ionised gas is estimated, using flux of H2 and Brγ emission lines, to be 258 M⊙ and 2.3 × 106 M⊙ respectively. The most striking feature in the emission line flux maps is the star-forming ring at a radius of ∼3″. The stellar and gaseous kinematics both show a rotation pattern in a disc that is blue-shifted in the north-west and red-shifted in the south-east. However, unlike the stellar kinematics, gas kinematics show various complexities, such as higher amplitudes in the central region (within the star-forming ring). The molecular gas velocity-field map shows deviations from pure disc motion in the form of an S-shaped zero-velocity line. In particular the position angle of the zero-velocity line changes in the central arcsecond, resembling the decoupled nuclear disc or molecular torus found in CO(3–2). There is an offset of 0.″5 between the nuclear continuum barycentre and kinematic centre of molecular gas towards the north-west. The velocity field of Brγ emitting gas on the other hand, is highly red-shifted in the centre and offset from the continuum emission peak by about 1″. Investigating the spectra of this region we detect a doubled-horn Brγ emission line in the nucleus and north-west. This region is also luminous in radio 3 cm emission connecting the nucleus to the star-forming ring.