Abstract
We use numerical simulations to study the evolution of triaxial elliptical galaxies with central black holes. In contrast to earlier studies which used galaxy models with central density cores, our galaxies have steep central cusps, as observed in real ellipticals. As a black hole grows in these cuspy triaxial galaxies, the inner regions become rounder owing to chaos induced in the orbital families that populate the model. At larger radii, however, the models maintain their triaxiality, and orbital analyses show that centrophilic orbits there resist stochasticity over many dynamical times. While black hole-induced evolution is strong in the inner regions of these galaxies and reaches out beyond the nominal sphere of influence of a black hole, our simulations do not show evidence for a rapid global transformation of the host. The triaxiality of observed elliptical galaxies is therefore not inconsistent with the presence of supermassive black holes at their centers.
Highlights
Observations indicate that most, and perhaps all elliptical galaxies harbor supermassive black holes at their centers (e.g. Gebhardt et al 2000; Richstone et al 1998; but see Gebhardt et al 2001b)
We present a study of black hole growth in triaxial elliptical galaxies with central density cusps, using the “adiabatic squeezing” technique described in Holley-Bockelmann et al (2001; hereafter, “Paper 1”) to generate models with prescribed shapes and cusps that are stable for many dynamical times
What is less clear is the specific agent driving the chaos in the system — is it scattering by the central black hole itself, the effects of the steepened stellar density cusp, or something else entirely? One way to check if the stellar cusp is responsible for the chaos is to remove the black hole and categorize the orbits that result from the γ ∼ 2 frozen stellar potential
Summary
Observations indicate that most, and perhaps all elliptical galaxies harbor supermassive black holes at their centers (e.g. Gebhardt et al 2000; Richstone et al 1998; but see Gebhardt et al 2001b). Best-fit models of black hole demography indicate that roughly 97% of ellipticals contain such black holes (Magorrian et al 1998). There is apparently a trend between black hole mass and galaxy velocity dispersion, implying that there is a Fundamental Plane even in the four-dimensional space defined by [log MBH, log L, log σe, log Re] (Gebhardt et al 2001a; Ferrarese & Merritt 2001). These correlations suggest that galaxy formation and the formation of central black holes are deeply connected. Black hole binaries created during galaxy mergers may explain the flat density profiles seen in the inner regions of the largest ellipticals (Makino & Ebisuzaki 1996; Quinlan & Hernquist 1997)
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