The Effect of Spatial Gradients in Stellar Mass-to-Light Ratio on Black Hole Mass Measurements

TL;DR

This study investigates how spatial variations in stellar mass-to-light ratio affect black hole mass estimates in galaxies, revealing that gradients can cause significant biases in measurements.

Contribution

It introduces a method to account for spatial gradients in stellar mass-to-light ratio in black hole mass modeling, highlighting their impact on measurement accuracy.

Findings

Negative gradients decrease MBH estimates by up to 28%.

Positive gradients increase MBH estimates by up to 22%.

Biases are comparable to statistical errors in some cases.

Abstract

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).