Raising the bar for black hole mass measurements in lower mass galaxies

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

Investigators

Abstract

It is now known that supermassive black holes live at the centers of all galaxies. Over the past decade scientists have found that the masses of these monster black holes depend strongly on the properties of their host galaxies. Supermassive black holes, some of the most intriguing objects in the universe, also play a transformative role in how their parent galaxies evolve. While they are gigantic (between a million and a ten billion times the mass of the sun), supermassive black holes are only a fraction of a percent of the total mass of the galaxy they live in. Much is still not understood about how these black holes grew or how they were able to so dramatically influence galaxy evolution. To understand how black holes grow, it is first necessary to measure their masses, both in nearby galaxies and in distant galaxies. The ‘gold standard’ method for measuring black hole masses in nearby galaxies uses the motions of stars that orbit close to the black hole. In the nearby Universe most of the black holes that are still growing today (the so-called Active Galactic Nuclei) live in disk galaxies like our own Milky Way. The black holes in disk galaxies are less massive, and therefore are harder to study since they have weaker effects on the motion of stars around them. The team will develop training tools to allow other scientists to use their new modeling software. Several undergraduate research students will participate in this research adding to their scientific and technical training and better equipping them for future pursuits. The accurate determination of supermassive black hole masses in galaxies, both at low and high redshifts, underpins almost all efforts to understand galaxy evolution. Black hole mass measurements that use the dynamics of stars in galactic nuclei are the current ‘gold standard’. Black hole masses are found to be tightly correlated with the properties of their host galaxies over a very wide range of black hole masses (from 100 million to 10 billion times the mass of the sun). At lower black holes masses, found in predominantly disk-like galaxies, the correlations are weaker. Spiral galaxies like our Milky Way, that contain a stellar bar (a rapidly rotating cigar-like central feature) show the most scatter. The team recently found that black hole mass measurements in barred galaxies can be overestimated when the bar is ignored, especially as the galaxy is viewed more face-on. Since 65% of disk galaxies have stellar bars, this bias primarily affects black hole masses in galaxies at the low-end of the mass function. It also impacts the black hole mass scaling relations - the correlations between the host galaxy properties and black hole masses. The new black hole mass measurements resulting from this project will better constrain the statistical uncertainties on the black hole masses at the low end of the black hole mass range, where it is most uncertain. They will also aid in determining the “reverberation mapping” mass scale a relationship that is crucial for future measurements of black hole masses at high redshift. The modeling will yield detailed three-dimensional distributions of the orbits of stars in the host galactic nuclei. The open source software that will be developed during this project will have broad applicability beyond black hole mass determination. The team will also make available tutorials for using the software for modeling galaxies and involve several undergraduates in the research allowing them to apply state-of- the-art galactic dynamics modeling tools to real data. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.