  Stellar dynamical modeling is a powerful method to determine the mass of
black holes in quiescent galaxies. However, in previous work the presence of a
dark matter halo has been ignored in the modeling. Gebhardt & Thomas (2009)
showed that accounting for a dark matter halo increased the black hole mass of
the massive galaxy M87 by a factor of two. We used a sample of 12 galaxies to
investigate the effect of accounting for a dark matter halo in the dynamical
modeling in more detail, and also updated the masses using improved modeling.
The sample of galaxies possesses Hubble Space Telescope and ground based
observations of stellar kinematics. Their black hole masses have been presented
before, but without including a dark matter halo in the models. Without a dark
halo, we find a mean increase in the estimated mass of 1.5 for the whole sample
compared to previous results. We attribute this change to using a more complete
orbit library. When we include a dark matter halo, along with the updated
models, we find an additional increase in black hole mass by a factor of 1.2 in
the mean, much less than for M87. We attribute the smaller discrepancy in black
hole mass to using data that better resolves the black hole's sphere of
influence. We redetermined the M-sigma and M-L relationships using our updated
black hole masses and found a slight increase in both normalization and
intrinsic scatter.
