What produces the extended LINER-type emission in the NUGA galaxy NGC 5850?

Abstract

Context. The role of low ionization nuclear emission region (LINER) galaxies within the picture of active galactic nuclei (AGN) has been controversial. It is still not clear whether they host an AGN in a low accretion mode or whether they are not active at all but are instead dominated by alternative ionization mechanisms, namely shocks, winds/outflows, or photoionization by a post-asymptotic giant branch (p-AGB) stellar population. The detection of extended LINER-like emission was often taken as evidence of ionization by stellar components, but this has not been undisputed. Aims. Using optical spectroscopy, we examine the possible ionization mechanisms responsible for the extended LINER-like emission in the central ~4 kpc of NGC 5850. Methods. We performed integral field spectroscopic observations using VIMOS at the VLT, which provides spatially-resolved spectra for the gas emission and the stellar continuum. We subtract the underlying stellar continuum from the galaxy spectra and fit the emission lines. With these methods, we derive and analyze emission line and kinematic maps. Emission line ratio maps are examined by means of diagnostic diagrams. Results. The central few kpc of NGC 5850 are dominated by extended LINER-like emission. The emission-line ratios that are sensitive to the ionization parameter increase with radial distance to the nucleus. The LINER-like region is surrounded by emission that is classed as "composite" in terms of diagnostic diagrams. Two star-forming (SF) regions are present in the 21″ × 19″ field of view. One of them is located approximately in the ring, surrounding the kinematically decoupled core. The second one is close to the nucleus and is the origin of a region of decreased emission line ratios oriented radially outwards. We find the interstellar gas to have a complex kinematic morphology and to have areas of steep velocity gradients. Conclusions. The extended LINER-like emission in NGC 5850 is dominated by ionization from distributed ionization sources, probably by stars on the p-AGB. The extended "composite" emission is likely due to a mixture of a LINER-like ionization pattern and photoionization by low-level star formation. With the extended region of decreased emission line ratios, we possibly observe enhanced SF or a region that is shielded from the central LINER-like emission by the central H ii-region. The peculiar gas kinematics are probably caused by the lopsided (m = 1) distribution of the gas and make the inflow of gas toward the center appear possible. © ESO, 2013.