ALMA-backed NIR high resolution integral field spectroscopy of the NUGA galaxy NGC 1433

Abstract

Aims: We present the results of near-infrared (NIR) H- and K-band European Southern Observatory SINFONI integral field spectroscopy (IFS) of the Seyfert 2 galaxy NGC 1433. We investigate the central 500 pc of this nearby galaxy, concentrating on excitation conditions, morphology, and stellar content. NGC 1433 was selected from our extended NUGA(-south) sample, which was additionally observed with the Atacama Large Millimeter/submillimeter Array (ALMA). NGC 1433 is a ringed, spiral galaxy with a main stellar bar in roughly east-west direction (PA 94°) and a secondary bar in the nuclear region (PA 31°). Several dusty filaments are detected in the nuclear region with the Hubble Space Telescope. ALMA detects molecular CO emission coinciding with these filaments. The active galactic nucleus is not strong and the galaxy is also classified as a low-ionization emission-line region (LINER).
Methods: The NIR is less affected by dust extinction than optical light and is sensitive to the mass-dominating stellar populations. SINFONI integral field spectroscopy combines NIR imaging and spectroscopy, allowing us to analyse several emission and absorption lines to investigate the stellar populations and ionization mechanisms over the 10″ × 10″ field of view (FOV).
Results: We present emission and absorption line measurements in the central kpc of NGC 1433. We detect a narrow Balmer line and several H₂ lines. We find that the stellar continuum peaks in the optical and NIR in the same position, indicating that there is no covering of the center by a nuclear dust lane. A strong velocity gradient is detected in all emission lines at that position. The position angle of this gradient is at 155° whereas the galactic rotation is at a position angle of 201°. Our measures of the molecular hydrogen lines, hydrogen recombination lines, and [Fe ii] indicate that the excitation at the nucleus is caused by thermal excitation, i.e., shocks that can be associated with active galactic nuclei emission, supernovae, or outflows. The line ratios [Fe ii]/Paβ and H₂/Brγ show a Seyfert to LINER identification of the nucleus. We do not detect high star formation rates in our FOV. The stellar continuum is dominated by spectral signatures of red-giant M stars. The stellar line-of-sight velocity follows the galactic field whereas the light continuum follows the nuclear bar.
Conclusions: The dynamical center of NGC 1433 coincides with the optical and NIR center of the galaxy and the black hole position. Within the central arcsecond, the molecular hydrogen and the ¹²CO(3-2) emissions - observed in the NIR and in the submillimeter with SINFONI and ALMA, respectively - are indicative for a nuclear outflow originating from the galaxy's SMBH. A small circum-nuclear disk cannot be fully excluded. Derived gravitational torques show that the nuclear bar is able to drive gas inward to scales where viscosity torques and dynamical friction become important. The black hole mass, derived using stellar velocity dispersion, is ~10⁷M_⊙.

Based on the ESO-VLT proposal ID: 090.B-0657(A) and on observations carried out with ALMA in cycle 0.Appendix A is available in electronic form at http://www.aanda.org