Effect of a dark matter halo on the determination of black hole masses

TL;DR

This study investigates how including dark matter halos in stellar dynamical models affects black hole mass estimates, revealing that accounting for dark matter generally increases mass estimates and slightly alters galaxy scaling relations.

Contribution

It provides a detailed analysis of dark matter halo effects on black hole mass measurements across 12 galaxies with improved modeling techniques.

Findings

Including dark matter halos increases black hole mass estimates by an average of 20%.

Using better-resolved data reduces the discrepancy in mass estimates compared to previous models.

Updated black hole masses slightly increase the normalization and scatter of the M-sigma and M-L relations.

Abstract

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.