A catalogue of nuclear stellar velocity dispersions of nearby galaxies from Hα STIS spectra to constrain supermassive black hole masses

We present new measurements for the nuclear stellar velocity dispersion σ* within sub-arcsecond apertures for 28 nearby galaxies. Our data consist of Space Telescope Imaging Spectrograph (STIS) long-slit spectra obtained with the G750M grating centred on the Hα spectral range. We fit the spectra using a library of single stellar population models and Gaussian emission lines, while constraining in most cases the stellar-population content from an initial fit to G430L STIS spectra. We illustrate how these σ* measurements can be useful for constraining the mass M• of supermassive black holes (SBHs) by concentrating on the cases of the lenticular galaxies NGC 4435 and NGC 4459. These are characterized by similar ground-based half-light radii stellar velocity dispersion σe values but remarkably different M• as obtained from modelling their central ionized-gas kinematics, where NGC 4435 appears to host a significantly undermassive SBH compared to what is expected from the M• - σe relation. For both galaxies, we build Jeans axisymmetric dynamical models to match the ground-based stellar kinematics obtained with Spectrographic Areal Unit for Research on Optical Nebulae integral-field spectrograph, including an SBH with M• value as predicted by the M• - σe relation and using high-resolution HST images taken with the Advanced Camera for Surveys to construct the stellar-mass model. By mimicking the HST observing conditions we use such reference models to make a prediction for the nuclear σ* value. Whereas this was found to agree with our nuclear σ* measurement for NGC 4459, for NGC 4435 the observed σ* is remarkably smaller than the predicted one, which further suggests that this galaxy could host an undermassive SBH.