The Demography of Massive Dark Objects in Galaxy Centers

TLDR

We predict the second‑moment profiles that should be observed at HST resolution for the 32 galaxies that our models describe well. We construct axisymmetric dynamical models for 36 nearby galaxies using HST photometry and ground‑based kinematics, assuming a two‑integral distribution function, arbitrary inclination, a constant stellar mass‑to‑light ratio, and a central massive dark object of arbitrary mass. The models fit 32 of 36 galaxies, yielding mass‑to‑light ratios consistent with the fundamental‑plane relation and requiring a massive dark object of mass ≈0.006 times the bulge mass in all but six galaxies, while the remaining six are consistent with zero MDO or have stronger upper limits.

Abstract

We construct dynamical models for a sample of 36 nearby galaxies with Hubble Space Telescope (HST) photometry and ground-based kinematics. The models assume that each galaxy is axisymmetric, with a two-integral distribution function, arbitrary inclination angle, a position-independent stellar mass-to-light ratio Υ, and a central massive dark object (MDO) of arbitrary mass M•. They provide acceptable fits to 32 of the galaxies for some value of M• and Υ; the four galaxies that cannot be fitted have kinematically decoupled cores. The mass-to-light ratios inferred for the 32 well-fitted galaxies are consistent with the fundamental-plane correlation Υ ∝ L0.2, where L is galaxy luminosity. In all but six galaxies the models require at the 95% confidence level an MDO of mass M• ∼ 0.006Mbulge ≡ 0.006ΥL. Five of the six galaxies consistent with M• = 0 are also consistent with this correlation. The other (NGC 7332) has a much stronger upper limit on M•. We predict the second-moment profiles that should be observed at HST resolution for the 32 galaxies that our models describe well.