The V_c-sigma_c relation in high and low surface brightness galaxies

We investigated the correlation between the circular velocity V_c and the central velocity dispersion of the spheroidal component sigma_c. We measured V_c and sigma_c for a sample of 42 high surface brightness disc galaxies (hereafter HSB), 11 low surface brightness spiral galaxies (hereafter LSB), and 24 elliptical galaxies characterized by flat rotation curves. We find that the V_c- sigma_c correlation for the HSB and elliptical galaxies is consistent with a linear law out to velocity dispersions as low as sigma_c ≈50 km/s . We analyzed the location of the elliptical galaxies with V_c based on H I data and LSB galaxies in the V_c- sigma_c plane. These types of galaxies were not considered in previous studies. The data points corresponding to the elliptical galaxies with H I -based V_c follow the same relation as the HSB galaxies and the elliptical galaxies with V_c based on dynamical models. On the contrary, the LSB galaxies follow a different V_c- sigma_c relation, since most of them show either higher V_c (or lower sigma_c) with respect to their HSB counterparts. If the V_c- sigma_c relation is equivalent to one between the mass of central supermassive black hole and that of the dark matter halo, these results suggest that the LSB galaxies host a supermassive black hole with a smaller mass compared to HSB galaxies. Moreover, the finding that at a given dark matter mass (as traced by V_c) the central sigma_c of LSB galaxies is smaller than in their HSB counterparts, would argue against the relevance of baryon collapse in the radial density profile of the dark matter haloes of LSB galaxies.