Mrk 609: resolving the circumnuclear structure with near-infrared integral field spectroscopy

Aims. We present first results of near-infrared (NIR) J and H + K ESO-SINFONI integral field spectroscopy of the composite starburst/Seyfert 1.8 galaxy Mrk 609. The data were taken during the science verification period of SINFONI. We aim to investigate the morphology and excitation conditions within the central 2 kpc. Additional Nobeyama 45 m CO( 1-0) data are presented, which we used to estimate the molecular gas mass. The source was selected from a sample of SDSS/ROSAT-based, X-ray bright AGN with redshifts of 0.03 < z < 1 that are suitable for adaptive optics observations. This sample allows for a detailed study of the NIR properties of the nuclear and host environments with high spectral and spatial resolution. Methods. Integral field spectroscopy with SINFONI delivers simultaneous spatial and spectral coverage of the circumnuclear environment. The NIR light is influenced less by dust extinction than by optical light and is sensitive to mass-dominating stellar populations. Furthermore, several NIR emission lines allow us to distinguish between Seyfert and starburst activities. Results. Our NIR data reveal a complex emission-line morphology that is possibly associated with a nuclear bar seen in the reconstructed continuum images. The detections of [Si VI] and a broad Pa alpha component are clear indicators of the presence of an accreting super-massive black hole at the center of Mrk 609. In agreement with previous observations, we find that the circumnuclear emission is not significantly extincted. Analysis of the high angular-resolution, molecular hydrogen emission and [FeII] emission reveals the LINER character of the nucleus. The large H-2 gas mass deduced from the CO(1-0) observation provides the fuel needed to feed the starburst and Seyfert activity in Mrk 609. Conclusions. High angular resolution imaging spectroscopy provides an ideal tool for resolving the nuclear and starburst contributions in active galaxies. We show that Mrk 609 exhibits LINER features that appear to be hidden in visible/NIR spectra with larger apertures.