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Black Hole Masses in Nearby Brightest Cluster Galaxies


The most massive galaxies in the Universe live at the centers of galaxy clusters and exhibit a number of extreme properties. Although their evolution broadly resembles that of normal elliptical galaxies, with early gas quenching and gradual assembly from smaller stellar systems, their unique cosmic environments may have offered additional pathways for growth. The extreme stellar mass growth of BCGs is clearly demonstrated by their overall luminosities, but the growth histories and present-day masses of their central black holes are not well known. A key body of evidence for the evolutionary connections between galaxies and supermassive black holes is the set of scaling relations between black hole masses (MBH) and the stellar velocity dispersions (σ), luminosities (L), or bulge masses (Mbulge) of their host galaxies. However, these scaling relations are poorly sampled for BCGs. Populating the relations with direct measurements of MBH could offer new insights to the growth of black holes and stellar systems at the hearts of galaxy clusters.

Along with collaborators, I have undertaken a series of observations of the centers of BCGs, using integral-field spectrographs on the Keck, Gemini, and Harlan J. Smith telescopes. In this dissertation, I describe the measurement and analysis of stellar kinematics at the centers of five BCGs, and measurements of their black hole masses using stellar orbit models. The most notable result is the measurement of black holes with approximately 10 billion solar masses in NGC 3842 and NGC 4889. These are the largest black hole masses ever directly measured, and they significantly exceed predictions from both the MBH-σ and MBH-L relations. Their masses are comparable to the biggest black holes powering high-redshift quasars, suggesting a tantalizing link between early sites of prolific black hole growth and rich galaxy clusters today. In contrast, I find that NGC 6086 and NGC 7768 host black holes with only a few billion solar masses. These measurements, as well as my upper limit for MBH in NGC 2832, are more consistent with the existing black hole scaling relations.

Recent measurements by my team and others have reshaped the sample of well-measured black hole masses, introducing significant updates to previous compilations. I present a sample of 65 dynamical black hole mass measurements, compiled from published literature through May 2012. In addition to previously reported values of σ and L, I have compiled an updated sample of bulge masses for 34 galaxies. The updated sample yields a steeper MBH-σ relation than previous versions, while the MBH-L and MBH-Mbulge relations experience relatively small changes. I have examined the black hole scaling relations for a variety of galaxy subsamples and find noteworthy variations in the MBH-σ relation for early- versus late-type galaxies and core-profile versus power-law galaxies.

Using the new sample, I have measured the empirical scatter in MBH and have attempted to measure the intrinsic scatter for multiple intervals in σ, L, and Mbulge. This is an important step forward from previous studies, which have only measured the intrinsic scatter over the full range of a given host galaxy property. Several models of black hole growth over cosmic time have predicted decreasing scatter in MBH as galaxy mass increases, reflecting the influence of hierarchical mergers driving galaxies and black holes toward an average MBH/Mbulge ratio. In contrast, I find nearly constant scatter in MBH over a wide range of galaxy luminosities and bulge masses.

My investigations thus far have contributed to a gradual change in astronomers' understanding of the black hole scaling relations. The present-day relations are not as tight as previously reported versions, and evidence is mounting against a universal process for co-evolution between black holes and galaxies. I will use observations of a larger sample of BCGs and massive group galaxies to explore the effects of environment on the growth of individual black holes and on cosmic scatter in MBH.

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