Gemini/GMOS integral field unit stellar kinematics of the nuclear region of six nearby active galaxies

Abstract

We present 2D mapping of the stellar velocity field within the inner 5 arcsec of six nearby active galaxies, using spectra obtained with the Integral Field Unit (IFU) of the Gemini Multi-Object Spectrograph instrument at the Gemini North telescope. The sampling of the observations is 0.2 arcsec, corresponding at the galaxies to spatial extents ranging from 10 to 30 pc. The spatial resolution range from 20 to about 180 pc, and the observed field of view covers a few hundred parsecs around the nuclei. The Ca II triplet absorption features at ≈ 8500Å were used to measure the stellar radial velocities and velocity dispersions. The radial velocity fields are dominated by rotation in all galaxies. A simple kinematical model assuming a purely rotating system with circular orbits in a plane was fitted to the radial velocity data. The turnover of the rotation curve is at only ≈ 50 pc for NGC4051 and between 200 and 700 pc for the other five galaxies. The velocity dispersion (σ) maps show the largest values (100 ≥ σ ≥ 150 km s-ˡ) at the centre. In the cases of NGC2273 and 3227, there is a decrease to σ ≈ 70–80 km s-ˡ at ≈ 200–300 pc from the nucleus, delineating partial rings of low-σ values. A similar broken ring seems to be present at ≈ 400 pc from the nucleus also in NGC4593. We interpret these low-σ rings as traces of recently formed stars that partially keep the cold kinematics of the original gas from which they have formed. In NGC3516 there is a decrease of σ outwards with the steepest gradient observed along the direction of the galaxy major axis, where σ reaches ≈ 80–90 km s-ˡ at ≈ 400 pc from the nucleus. The main novelty of the present work is the unprecedented spatial resolution reached by a 2D study of stellar kinematics of Seyfert galaxies using an IFU. The few similar IFU studies available in the literature for Seyfert galaxies have a much poorer spatial resolution and/or are restricted to the study of emission-line kinematics.