2013MNRAS.428.2389M

We present the first results of an analysis of the properties of the molecular gas in the nuclear regions (r ≲ 300 pc) of a sample of six nearby galaxies, based on new high-spatial-resolution observations obtained in the K-band with the near-infrared integral field spectrograph SINFONI at the Very Large Telescope. We derive 2D distributions of the warm molecular and ionized gas from the H2, Brγ and Hei emission lines present in the spectra of the galaxies. We find a range of morphologies, including bar- and ring-like distributions and either centrally peaked or off-centre emission. The morphologies of the molecular and the ionized gas are not necessarily coincident. The observed emission-line ratios point towards thermal processes as the principal mechanism responsible for the H2 excitation in the nuclear and circumnuclear regions of the galaxies, independently of the presence of an active nucleus. We find that a rescaling of the H2 2.12 µm emission-line luminosity by a factor β ≃ 1200 gives a good estimate (within a factor of 2) of the total (cold) molecular gas mass. The galaxies of the sample contain large quantities of molecular gas in their centres, with total masses in the ∼ 105-108M☉ range. Nevertheless, these masses correspond to less than 3 per cent of the stellar masses derived for the galaxies in these regions, indicating that the presence of gas should not affect black hole mass estimates based on the dynamical modelling of the stars. The high spatial resolution provided by the SINFONI data allowed us to resolve a circumnuclear ring (with a radius of ∼ 270 pc) in the galaxy NGC 4536. The measured values of the Brγ equivalent width and the Hei/Brγ emission-line ratio suggest that bursts of star formation occurred throughout this ring as recently as 6.5 Myr ago.