NGC 307 and the effects of dark-matter haloes on measuring supermassive black holes in disc galaxies

We present stellar-dynamical measurements of the central supermassive black hole (SMBH) in the S0 galaxy NGC 307, using adaptive-optics IFU data from VLT-SINFONI. We investigate the effects of including dark-matter haloes as well as multiple stellar components with different mass-to-light (M/L) ratios in the dynamical modelling. Models with no halo and a single stellar component yield a relatively poor fit with a low value for the SMBH mass [(7.0 +/- 1.0) x 10(7) M-circle dot] and a high stellarM/L ratio (gamma(K) = 1.3 +/- 0.1). Adding a halo produces a much better fit, with a significantly larger SMBH mass [(2.0 +/- 0.5) x 10(8) M-circle dot] and a lower M/L ratio (gamma(K) = 1.1 +/- 0.1). A model with no halo but with separate bulge and disc components produces a similarly good fit, with a slightly larger SMBH mass [(3.0 +/- 0.5) x 10(8) M-circle dot] and an identical M/L ratio for the bulge component, though the disc M/L ratio is biased high (gamma(K), disc = 1.9 +/- 0.1). Adding a halo to the two-stellar-component model results in a much more plausible disc M/L ratio of 1.0 +/- 0.1, but has only a modest effect on the SMBH mass [(2.2 +/- 0.6) x 10(8) M-circle dot] and leaves the bulge M/L ratio unchanged. This suggests that measuring SMBH masses in disc galaxies using just a single stellar component and no halo has the same drawbacks as it does for elliptical galaxies, but also that reasonably accurate SMBH masses and bulge M/L ratios can be recovered (without the added computational expense of modelling haloes) by using separate bulge and disc components.