Current Research

Supermassive Black Holes 

One of the biggest puzzles facing contemporary astronomers is “Why are supermassive black holes (SMBH) found in every massive galaxy, and why are their masses correlated with the properties of their host galaxies, (e.g. the total mass in stars)?” Over the past 20 years I have developed tools to accurately determine the masses of these SMBH by observing their influence on the motions of stars in their vicinity. Currently, I’m focused on understanding the effects of supermassive black holes on bars in spiral galaxies and how the presence of a bar affects our ability to measure the mass of a SMBH.  Eugene Vasiliev (Cambridge) and I recently released FORSTAND: a new dynamical modeling code for modeling any type of galaxy including, for the first time, galaxies with bars. This is a publicly available and flexible code that we hope will make measuring black hole masses more efficient. This article on AAS Nova talks about this code in terms that are accessible to the public. We are applying our new  code for black hole mass measurement to a special class of galaxies (“reverberation mapping” AGN) where an independent method to measure black hole masses already exists, serving as a powerful self-consistency check.

Recent studies have shown that correlations between black holes and their host galaxies break down in spiral galaxies with “pseudo-bulges” and /or central “bars”. These stellar features arise from secular evolution in the disk and are not thought to have been produced by mergers (as in the case of bulges).

In recent years my group has advanced our understanding of bars, which exist in 60% of spiral galaxies, by creating “mock data” from simulations of barred galaxies. Using these simulations we showed that a bar significantly alters both the estimated  central velocity dispersions  and black hole mass, significantly affecting the  scaling relations between black hole masses and the observed properties of the host galaxy (Brown et al. 2013, Hartmann et al. 2014).  We found that bars can cause 40% to 100% errors in the measurements of black hole and host galaxy properties.

We have argued that (Valluri et al. 2016) that the textbook view of how stars move in bars is incomplete, especially when an SMBH is present.  My revised view of the motions of stars in bars makes it clear that although they are rapidly rotating triaxial (cigar shaped) structures, their orbital structure is  very similar to  the properties of large (triaxial) elliptical galaxies.

These advances in the dynamics of bars, are crucial to uncovering the origin of correlations between SMBHs and their host galaxies, and for uncovering the very processes by which the SMBH in our own barred galaxy, the Milky Way, was formed.

The Dark Matter Halos of  Milky Way like Galaxies

Another big puzzle in astronomy is the question of what type of particle constitutes the ubiquitous “dark matter halos” in which galaxies are embedded. My work over the past decade has explored the orbits of dark matter particles in cosmological simulations. This work has led to insights into why the shapes of dark matter halos are altered by the infall of gas and the formation of stars. My group is also trying to measure the shape of the Milky Way’s dark matter halo  in order to distinguish between the predictions of leading “cold dark matter” paradigm and a competing models e.g.   MOND which requires modification of Newton’s laws of motion. In the dominant scientific paradigm, dark matter halos are slightly flattened close to their centers, but football shaped at larger radii.  Our goal is to  model the motions of millions of randomly distributed halo stars, soon to become available from the European Space Agency’s “Billion Star Surveyor” Gaia satellite. Here’s a nice public news article that describes what Gaia will do.

We are building tools to be able to measure the mass distribution of the dark matter in the Milky Way with upcoming data from the Dark Energy Spectroscopic Survey (DESI). DESI  will soon get spectra for 10-50 million stars in the Gaia catalog giving us information on the 3 dimensional motions of these stars.

We are also applying clustering algorithms to stars in cosmological simulations of Milky Way-like galaxies to assess how far back in time one can detect the merger events that made up the stellar halo and developing new ways to measure figure rotation or tumbling of the Milky Way’s dark matter halo.