  Based on the modeling of the central emission-line width measured over
sub-arcsecond apertures with the Hubble Space Telescope, we present stringent
upper bounds on the mass of the central supermassive black hole, MBH, for a
sample of 105 nearby galaxies (D<100Mpc) spanning a wide range of Hubble types
(E-Sc) and values of the central stellar velocity dispersion, sigma
(58-419km/s). For the vast majority of the objects the derived MBH upper limits
run parallel and above the well-known MBH-sigma relation independently of the
galaxy distance, suggesting that our nebular line-width measurements trace
rather well the nuclear gravitational potential. For values of sigma between 90
and 220km/s the 68% of our upper limits falls immediately above the MBH-sigma
relation without exceeding the expected MBH values by more than a factor 4.1.
No systematic trends or offsets are observed in this sigma range as a function
of the galaxy Hubble type or with respect to the presence of a bar. For 6 of
our 12 MBH upper limits with sigma<90km/s our line-width measurements are more
sensitive to the stellar contribution to the gravitational potential, either
due to the presence of a nuclear stellar cluster or because of a greater
distance compared to the other galaxies at the low-sigma end of the MBH-sigma
relation. Conversely, our MBH upper bounds appear to lie closer to the expected
MBH in the most massive elliptical galaxies with values of sigma above 220km/s.
Such a flattening of the MBH-sigma relation at its high-sigma end would appear
consistent with a coevolution of supermassive black holes and galaxies driven
by dry mergers, although better and more consistent measurements for sigma and
K-band luminosity are needed for these kind of objects before systematic
effects can be ruled out.
