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 (gamma) on measurements of black hole masses (M-) 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 gamma. We have modeled three giant elliptical galaxies with gradients alpha d log(gamma)/d log(r) from -0.2 to + 0.1. Color and line strength gradients suggest mildly negative a in these galaxies. Introducing a negative (positive) gradient in gamma increases (decreases) the enclosed stellar mass near the center of the galaxy and leads to systematically smaller (larger) M- measurements. For models with alpha = -0.2, the best-fit values of M-* are 28%, 27%, and 17% lower than the constant-gamma case, in NGC 3842, NGC 6086, and NGC 7768, respectively. For alpha = + 0.1, M-* are 14%, 22%, and 17% higher than the constant-gamma 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 gamma cause the total stellar mass to decrease (increase) and the dark matter fraction within one effective radius to increase (decrease).