The molecular H2 and stellar discs in the nuclear region of NGC 4258

Abstract

NGC 4258 is an SABbc Seyfert galaxy, located at a distance of $7.2 \pm 0.3$ Mpc. This object is well known by its nearly edge-on molecular nuclear disc, located between 0.16 and 0.28 pc from the nucleus, whose H$_2$O maser emission allows a very precise measurement of the central supermassive black hole mass ($M_{\bullet}(maser) = 3.78 \pm 0.01 \times 10^7$ Mo). We analyse the emission line properties and the stellar kinematics in a data cube of the central region of NGC 4258, obtained, in the K band, with the Near-Infrared Integral Field Spectrograph, at the Gemini-north telescope. The nuclear spectrum, after the starlight subtraction, shows only the H$_2 \lambda 2.1218$ $\mu$m and Br$\gamma$ emission lines, the latter having a broad component with FWHM$_{Br\gamma}(broad) = 1600 \pm 29$ km s$^{-1}$. The spatial morphology and kinematics of the H$_2 \lambda 2.1218$ $\mu$m line are consistent with a rotating molecular disc around the supermassive black hole, with an upper limit for its diameter of 15.7 pc. The inner radio jet in this object is, in projection, almost perpendicular to the H$_2$ emitting disc detected in this work, and also to the H$_2$O maser emitting disc. The main features of the maps of the stellar kinematic parameters are well reproduced by a model of a thin rotating stellar circular disc. The supermassive black hole mass provided by this dynamical modelling ($M_{\bullet}(disc) = 2.8 \pm 1.0 \times 10^7$ Mo) is compatible, at 1$\sigma$ level, with the precise measurement resulting from the H$_2$O maser emission.