Supermassive black holes in galaxy centers

Observations show that large galaxies contain black holes at their centers, having masses of millions to billions of solar masses, but we still lack a full understanding of the demographics of black holes and their relationship to their host galaxies.  Our understanding of the black hole population is particularly uncertain at the very top end of the mass range, above around a billion solar masses, and at the bottom end, below a few million solar masses (comparable to the mass of the black hole in the center of the Milky Way Galaxy).  Our group uses the ALMA Observatory, the Hubble Space Telescope, and the Keck Observatory to carry out direct measurements of black hole masses in the centers of nearby galaxies. Other current projects involve studies of the central compact star clusters in low-mass spiral galaxies, which may be homes of intermediate-mass black holes having masses below 1 million solar masses.

Reverberation mapping of active galactic nuclei

Active galactic nuclei (AGNs) and quasars have strong, broad emission lines from ionized gas orbiting very close to the black hole. Understanding the structure and dynamics of this “broad-line region” is an important goal but these regions are so small and so distant that we have no way to image their structure directly.  Instead, we can use time variability of the quasar emission to produce an “echo map” of the ionized gas within just a few light days of black holes that are hundreds of millions of light-years away.  These reverberation mapping observations are a key to understanding the masses and growth rates of supermassive black holes in active galaxies. Our group has led large observing campaigns at Lick Observatory to monitor temporal changes in the brightness of the emission from active galaxies in order to study their inner structure and determine the masses of their black holes.  Our Lick AGN Monitoring Project collaboration involves astronomers from five UC campuses and several other institutions. Our group also participates in the AGN Key Project of Las Cumbres Observatory.

Links to my complete publication list are on the publications page.