The Effect of Spatial Gradients in Stellar Mass-to-Light Ratio on Black
  Hole Mass Measurements
  We have tested the effect of spatial gradients in stellar mass-to-light ratio
(Y) on measurements of black hole masses (MBH) derived from stellar orbit
superposition models. Such models construct a static gravitational potential
for a galaxy and its central black hole, but typically assume spatially uniform
Y. We have modeled three giant elliptical galaxies with gradients alpha = d(log
Y)/d(log r) from -0.2 to +0.1. Color and line strength gradients suggest mildly
negative alpha in these galaxies. Introducing a negative (positive) gradient in
Y increases (decreases) the enclosed stellar mass near the center of the galaxy
and leads to systematically smaller (larger) MBH measurements. For models with
alpha = -0.2, the best-fit values of MBH are 28%, 27%, and 17% lower than the
constant-Y case, in NGC 3842, NGC 6086, and NGC 7768, respectively. For alpha =
+0.1, MBH are 14%, 22%, and 17% higher than the constant-Y case for the three
respective galaxies. For NGC 3842 and NGC 6086, this bias is comparable to the
statistical errors from individual modeling trials. At larger radii, negative
(positive) gradients in Y cause the total stellar mass to decrease (increase)
and the dark matter fraction within one effective radius to increase
(decrease).
