Black holes in barred galaxies: the Rosetta Stones of secular galaxy evolution

Regents Of The University Of Michigan - Ann Arbor, Ann Arbor MI

Investigators

Abstract

Studies have been showing for some years that the size of a supermassive black hole in a galaxy is correlated with the size of the galaxy itself, perhaps unsurprisingly, but also with the appearance (the 'type') of the galaxy. However, the black hole mass is normally calculated by studying the motions of stars in the galaxy, and recent work suggests that the presence of a bar in the galaxy causes a significant perturbation in the stellar speeds, which has not been taken into account. This leads to a bias that over-estimates the mass and can hide the effect of galaxy type. This award supports development of a new numerical method that does not suffer from this problem. Comparison with other methods and careful checks throughout the work will lay the foundations for future better understanding of the co-evolution of galaxies and their central black holes. This project will work to correct this confounding factor in understanding how the co-evolution of black holes (BH) and galaxies depends upon the host galaxy's morphological type. Recent studies show that the unique orbital structure of a bar makes significant changes in line-of-sight velocity profiles, but although nearly 60% of spiral galaxies with existing dynamical BH mass measurements contain bars, all of those estimates were derived using axisymmetric models that do not account for the bar. The resultant systematic over-estimation bias can mask dependencies on host galaxy morphology. This study will (a) develop the first stellar dynamical orbit superposition code capable of accurately determining BH masses in barred disk galaxies; (b) run high resolution self-consistent N-body simulations of bars and generate mock kinematic datasets for validating the new code; (c) improve the BH mass measurements in several barred galaxies; and (d) obtain the first stellar dynamical BH mass measurements for two nearby galaxies with independent reverberation mapping (RM) estimates. Building a new code is crucial because: (1) the predicted over-estimate is comparable to the scatter and thus can mask genuine differences in BH growth mechanisms; (2) using stellar dynamics to confirm RM BH estimates is important for future high redshift estimates where only RM is possible; and (3) accurate BH masses for specific galaxies are critical for modeling the accretion physics of X-ray emitting gas. This multi-faceted project, involving N-body simulations, code development, and the modeling of real data, will give the graduate student involved the broad and strong technical skills necessary for a successful career. Other activities will provide future elementary school teachers with strong STEM (Science, Technology, Engineering, Mathematics) foundations through the university's school of education, involve non-science major undergraduates through a newly-developed course, and use a summer camp to inspire high school students to pursue STEM disciplines in college.